Osteology of Fukuivenator paradoxus:
a bizarre maniraptoran theropod from the Early Cretaceous of Fukui, Japan
Soki HATTORI, Soichiro KAWABE, Takuya IMAI, Masateru SHIBATA, Kazunori MIYATA,
Xing XU and Yoichi AZUMA

Reprinted from
Memoir of the Fukui Prefectural Dinosaur Museum 20: 1–82, December 2021

ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 20: 1–82 (2021)© by the Fukui Prefectural Dinosaur Museum

OSTEOLOGY OF FUKUIVENATOR PARADOXUS:
A BIZARRE MANIRAPTORAN THEROPOD FROM THE EARLY CRETACEOUS OF FUKUI, JAPANSoki HATTORI , Soichiro KAWABE , Takuya IMAI , Masateru SHIBATA ,
1, 2

1, 2

1, 2

1, 2

Kazunori MIYATA2 , Xing XU 3 and Yoichi AZUMA1,2

1

Institute of Dinosaur Research, Fukui Prefectural University, 4-1-1 Matsuoka Kenjojima, Eiheiji, Fukui 910-1195, Japan
2
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, Japan
3
Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology,
Chinese Academy of Sciences, Beijing 100044, China
ABSTRACT

A bizarre coelurosaurian theropod Fukuivenator paradoxus is known only from the holotype specimen preserving majority ofthe skeleton from the Kitadani Dinosaur Quarry of the Lower Cretaceous Kitadani Formation, Tetori Group, Fukui, Japan. With
aids of computed tomography techniques, a re-examination of the holotype specimen reveals additional features of Fukuivenator
which was unobservable in the original description, such as the presence of parietals and a quadrate, and the fusion of theposteriormost caudal vertebrae. The thorough description in this study results in the emendation of diagnosis including theretraction of the large promaxillary fenestra subequal in size to maxillary fenestra, and the addition of the large maxillary fenestra
expanded well dorsally above the suprantral strut. Expansion of morphological information elaborates the phylogenetic dataset,
resulting in locating Fukuivenator as an unambiguous member of Maniraptora at the basalmost position of Therizinosauria. Thisphylogenetic position of Fukuivenator is supported by several therizinosaurian synapomorphies such as the subotic recess on thebraincase, 11 cervical vertebrae some of which having two pneumatic foramina, and distal articular condyles on the anterior surfaceof the humerus. Among numerous diagnostic features, eight characters shared with some non-maniraptoran coelurosaurs and five
shared with different clades within Maniraptora, highlighting the notably mosaic condition of Fukuivenator proposed in the originaldescription. The combination of characters for herbivorous and carnivorous diets suggests the omnivory of Fukuivenator,
projecting the dietary shift in the earliest evolutionary stage of Therizinosauria. Also, the large olfactory ratio revealed by therevised brain endocast highlights the unusually high olfactory acuity further developed than the plesiomorphic condition, implyingthat the acute sense of smell might be a characteristic of therizinosaurian theropods.
Key words: Maniraptora, Coelurosauria, Theropoda, Early Cretaceous, Kitadani Formation, Tetori Group, Fukui, Japan
服部創紀・河部壮一郎・今井拓哉・柴田正輝・宮田和周・徐星・東 洋一（2021）Fukuivenator paradoxus の骨学：
福井県の下部白亜系から産出した特異なマニラプトル類．福井県立恐竜博物館紀要 20：1‒82.
Fukuivenator paradoxus は，北谷恐竜化石発掘現場に露出する下部白亜系手取層群北谷層から発見された
Coelurosauria の一種である．そのホロタイプについて CT スキャンを行ったところ，原記載では見出されなかった部位や特徴が明らかとなったため，改めて記載を行った．その結果，複数の標徴形質の見直しを行う必要性が生じた．また，様々な形質状態の再評価及び拡充を行い，改めて系統解析をしたところ，本種を Maniraptora の一種とし，さらに Therizinosauria の基盤に位置づける結果が得られた．ホロタイプには多数の固有派生形質が認められたが，様々なクレードとの同形形質が含まれていた．また，2 通りの食性を示す特徴の組み合わせは，本種の雑食性を示すとともに，Therizinosauria の進化史初期における食性の変化を物語っている．さらに，脳エンドキャストの形状から，本種の嗅球が他の獣脚類に比べて非常に大きいことがわかった．このことから，Therizinosauria には共通して鋭い嗅覚が備わっていたことが示唆される．

INTRODUCTION

dinosaur species recovered in the Kitadani Dinosaur Quarry
following Fukuiraptor kitadaniensis (Azuma and Currie, 2000),
Fukuisaurus tetoriensis (Kobayashi and Azuma, 2003), Fukuititan
nipponensis (Azuma and Shibata, 2010) and Koshisaurus
katsuyama (Shibata and Azuma, 2015), increasing the known
diversity of dinosaur fauna of the formation. The Kitadani
Dinosaur Quarry is located in the bank of the Sugiyama River in
the northern part of Katsuyama, Fukui (36 ° 7’ 17.9” N, 136 ° 32’
41.4” E). The formation cropping out in the quarry represents
alternating beds of coarse to fine sandstone and siltstone, which

Fukuivenator paradoxus is a small-sized theropod dinosaur
from the Lower Cretaceous Kitadani Formation of the Tetori
Group, Fukui, Japan (Azuma et al., 2016). It represents the fifth
Received June 30, 2021. Accepted October 11, 2021.
Corresponding author―Soki HATTORI
E-mail: s-hattori＊fpu.ac.jp
1

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 1. Locality and horizon for Fukuivenator paradoxus. A, regional map for the location of Fukui in Japan, and the Kitadani Dinosaur Quarry in
Fukui; B, photograph of the Kitadani Dinosaur Quarry in 2007 with an arrowhead indicating the locality for FPDM-V-8461; C, stratigraphic section of thepart of the Kitadani Formation in the Kitadani Dinosaur Quarry, representing the stratigraphic position of FPDM-V-8461; D, occurrence of
FPDM-V-8461 in the field with arrowheads indicating exposed elements.

yield abundant plant, invertebrate, vertebrate, and trace fossils(Goto et al., 2002; Azuma, 2003; Shibata and Goto, 2008; Fig.
1). The Aptian age is inferred for the formation based on theco-occurrence of multiple species of charophyte gyrogonites(Kubota, 2005; Sano, 2015) as well as on the suggested
stratigraphic correlations of the formations within the Tetori
Group (Sano and Yabe, 2017). Furthermore, the tuff beds
zircons U-Pb age of 121.2±1.1 Ma from the directly underlying
Akaiwa Formation (Sakai et al., 2019) narrows the inferred age
for the Kitadani Formation into middle to late Aptian.
While the holotype specimen of Fukuivenator paradoxus(FPDM-V-8461) was collected within a 50 by 50 cm area
without any material identifiable to other animals and has been
described in Azuma et al. (2016), subsequent preparations
supplemented by computed tomography (CT) techniques has
enabled us to observe additional morphological features andpreviously-unrecognized elements. Here, we provide a complete

description of the holotype specimen and conduct aphylogenetic analysis taking previously unrecognizedmorphological features and elements into account. The present
study confirms a peculiar set of features of Fukuivenator, assuggested by Azuma et al. (2016), and suggests an alternative
phylogenetic position of the taxon as a basal maniraptoran.
MATERIAL
The material of Fukuivenator paradoxus described in thiswork is the holotype, FPDM-V-8461.
Institutional abbreviations: FPDM, Fukui Prefectural
Dinosaur Museum, Fukui, Japan; IGM, Mongolian Institute ofGeology, Ulaanbaatar, Mongolia; IPFUB, Institut für
Geologische Wissenschaften, Fachrichtung Paläontologie, Freie
Universität, Berlin, Germany; UMNH, Natural History Museumof Utah, Salt Lake City, Utah, USA.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

METHODS
Computed tomography analyses
Individual skeletal elements of FPDM-V-8461 were analyzed
using a veterinary X-ray CT, Latheta LCT-200 (Hitachi, Ltd,Tokyo, Japan) with the following parameters: voltage of 80 kV,
current of 500 µA, and voxel size of 0.048–0.08 mm (x- andy-axes) and 0.096–0.16 mm (z-axis) at Institute of Dinosaur
Research, Fukui Prefectural University. The complex including
frontals, left parietal, left ectopterygoid, right atlantal
neurapophysis, axis and fifth cervical vertebrae was alsoscanned using an industrial microfocus CT, TXS320-ACTIS(TESCO Co., Yokohama, Japan), at FPDM with the following
parameters: voltage of 270 kV, current of 250 µA, and voxel
size of 0.05 mm (x- and y-axes) and 0.05 mm (z-axis). We
subsequently segmented all analyzed elements from the acquired
CT images and exported polygon meshes using Amira (v2020.1, Thermo Fisher Scientific; Waltham, MA, USA).
Polygon meshes were then imported into Meshmixer (Autodesk,
San Rafael, CA, USA) to observe, measure and capture images
for figures under the orthographic projection view.
Anatomical terminologies
The anatomical terminologies used in this study are
essentially based on the frequency of use in the referred
literatures. However, some terminologies coined in previous
studies are applied as follows; Hendrickx and Mateus (2014) forthe maxilla, Hendrickx et al. (2015) for the dentition, Wilson(1999, 2012) for laminae of vertebrae, Wilson et al. (2011) for
fossae of vertebrae, and Nicholls and Russell (1985) for theorientation of the pectoral girdle.
Phylogenetic analyses
To reassess the phylogenetic affinity of Fukuivenatorparadoxus, FPDM-V-8461 was scored into the recent version of
Theropod Working Group (TWiG) dataset (Brusatte et al.,2014), which is a large expansion of the one previously updated
by Turner et al. (2012) and more recently updated by Nesbitt etal. (2019), with an additional set of morphological featuresdescribed in this study. Although this specimen was previously
scored for an older version of this dataset (Turner et al., 2012)by Azuma et al. (2016), scores on 106 characters were modified
as a result of the present study. In addition, the derived state ofcharacter 90 was modified from “12 or more” to “11 or more”
following Maryańska et al. (2002; character 98) so that
Fukuivenator and other potential operational taxonomic units(OTUs) having 11 cervical vertebrae could be scored for thischaracter. In addition, some scores for other OTUs on character
27, 28 and 318 were modified based on previously publishedliteratures (see “Remarks on maniraptoran anatomy” in
DISCUSSION for details). The final dataset compiled with3

Mesquite 3.6 (Maddison and Maddison, 2019) includes 155 taxa
covering an extensive sample of coelurosaurs and outgroups
scored for 853 discrete anatomical characters. The following
characters were ordered following previous studies (Turner etal., 2012; Brusatte et al., 2014; Nesbitt et al., 2019): 3, 16, 17,
18, 19, 24, 25, 27, 38, 39, 40, 45, 63, 66, 72, 74, 89, 103, 108,
111, 114, 117, 119, 121, 123, 130, 146, 148, 152, 154, 161, 163,
166, 168, 169, 171, 175, 178, 179, 181, 195, 197, 200, 217, 222,
232, 234, 235, 238, 242, 250, 252, 255, 256, 261, 262, 265, 268,
270, 279, 287, 292, 299, 309, 316, 319, 321, 326, 328, 347, 351,
359, 364, 379, 384, 385, 387, 393, 397, 401, 409, 413, 415, 417,
420, 425, 426, 427, 428, 433, 434, 443, 445, 446, 459, 462, 475,
482, 485, 488, 494, 499, 520, 540, 545, 556, 557, 560, 578, 579,
605, 613, 622, 628, 631, 632, 635, 642, 648, 651, 665, 669, 670,
684, 694, 698, 702, 711, 712, 715, 727, 728, 763, 780, 805, 806,
807, 818, 823, 842 and 843. The dataset was analyzed withequally weighted parsimony in TNT 1.5 (Goloboff and
Catalano, 2016). Following TWiG protocols, Allosaurus wasused as an outgroup to root the tree. At first, the dataset was
analyzed under the “New Technology” search options, using
sectorial search, ratchet, tree drift and tree fuse options withdefault settings, and the minimum tree length was found in 10
replicates to recover as many tree islands as possible.
Subsequently, the results were exposed to the branch-swapping
algorithm of Tree Bisection Reconnection (TBR). Reduced strict
consensus of MPTs was once constructed with a posteriori pruningof five wildcard taxa: Kinnareemimus, Epidendrosaurus,
Pyroraptor, Hesperonychus and Limenavis, following previous
studies (Turner et al., 2012; Brusatte et al., 2014; Nesbitt et al.,2019). The supporting values for each node were calculated as
Bremer support (with retaining trees suboptimal by five steps)
and bootstrapping score (with 10,000 replicates).
SYSTEMATIC PALEONTOLOGYDINOSAURIA Owen, 1842
THEROPODA Marsh, 1881
COELUROSAURIA von Huene, 1914
MANIRAPTORA Gauthier, 1986
Fukuivenator paradoxus Azuma, Xu, Shibata, Kawabe, Miyata
and Imai, 2016
Holotype
FPDM-V-8461, a disarticulated and closely associatedskeleton composed of the following elements (Fig. 2): twoisolated premaxillary teeth, both maxillae with four intact and
one isolated teeth, both lacrimals, right jugal, both frontals, bothparietals, braincase without laterosphenoid, both ectopterygoids,
right palatine, posterior part of right dentary with two intact
dentary teeth, 9 cervical, 11 dorsal, 5 sacral and 31 caudal
vertebrae, several cervical and dorsal ribs, gastralia andchevrons, most of both forelimbs, right pubis, and nearlycomplete hindlimbs.
Modifications from the original description by Azuma et al.

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 2. Reconstructed skeleton of Fukuivenator paradoxus. Elements missing in FPDM-V-8461 are illustrated in grey. Artwork by Genya Masukawa.

(2016) are summarized below. Re-identified elements: rightmaxilla, originally identified as right premaxilla; right lacrimal,
originally identified as right postorbital; atlantal neurapophyses,
originally identified as axial ribs; partial neural arch of fifth
dorsal vertebra, originally identified as left squamosal; right
manual phalanx III-1, originally identified as right pedal phalanx
I-1; right pubis, originally identified as left pubis; left
metatarsals II and V, originally identified as right metatarsals IV
and V, respectively; left pedal phalanges II-1, II-2, III-2, IV-1
and IV-2, originally identified as right pedal phalanges IV-1,
III-1, IV-2, II-1 and II-2, respectively. Additionally identified
elements: right quadrate; both parietals; right ectopterygoid;
centrum of 2nd dorsal vertebra; neural spine of 9th dorsal
vertebra; partial neural arch of 4th caudal vertebra;
prezygapophyses of 18th caudal vertebra; 29th caudal vertebra;
several rib elements; distal end of right humerus; left manualphalanx I-2; left femoral head; distal part of right fibula; right
distal tarsal III; left metatarsal III; right metatarsal IV; left pedal
phalanx I-2. Withdrawn elements: left pterygoid; posteriorcaudal vertebra; left ischium.

and additional deep sockets; 7. anterior cervical vertebrae
with intraprezygapophyseal, postzygodiapophyseal,
prezygodiapophyseal, and intrapostzygapophyseal laminae
connecting to each other to form an extensive platform; 8.
middle and posterior cervical vertebrae with transversely bifid
neural spines; 9. dorsal, sacral, and anterior caudal vertebrae
with strongly laterally curved hyposphene andcentropostzygapophyseal laminae that, together with thepostzygapophyseal facet, form a socket-like structure forreceiving the prezygapophysis; 10. dorsoventrally bifid sacral
ribs; 11. caudal zygapophyseal facets expanded to be
substantially wider than the zygapophyseal processes; and 12.
middle caudal vertebrae with transversely and distally bifid
prezygapophyses. The autapomorphies 2 and 5–12 follow
Azuma et al. (2016). See also Emendation of diagnosis in
DISCUSSION.

Emended diagnosis

MaxillaThe left maxilla is nearly complete, only missing the distal
end of the dorsal ramus (Fig. 4A–F). On the other hand, theright maxilla, which was originally identified as the rightpremaxilla (Azuma et al., 2016) due to incomplete preparation,
lacks the anterior ramus and most of the posterior ramus (Fig.
4G–L). The distal portion of the right dorsal ramus is preservedin contact with the lacrimal (Fig. 6G).
In lateral view, the anterior end of the maxillary body forms
an acute angle, about 45 degrees (Fig. 4C). A small subnarial
foramen is present on the anterior margin at the level of theventral margin of antorbital fossa. The ascending ramus projects
posterodorsally and is slightly recurved to enclose the antorbital
fenestra ventral to it. Distally, the ascending ramus bears a

A maniraptoran theropod with following autapomorphies(Fig. 3): 1. large maxillary fenestra expanded well dorsally
above the suprantral strut; 2. large oval lacrimal foramen on
antorbital fossa medial wall; 3. jugal anterior ramus with thickand rounded dorsal margin continuous with the lateral surface;
4. bifid posterior end of ectopterygoid for contact withpterygoid; 5. highly heterodont dentition featuring robust
unserrated teeth including small spatulate anterior teeth, large
and posteriorly curved middle teeth, and small and nearly
symmetrical posterior teeth; 6. mid-posterior cervical vertebrae
with a complex lamina system surrounding the neural canalresulting in deep and wide grooves for interspinous ligaments

DESCRIPTION
Skull

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

5

FIGURE 3. Selected elements of FPDM-V-8461 illustrating diagnostic features of Fukuivenator paradoxus. Mesialmost (A) and mesial (B) teeth, left
maxilla (C), mirrored right lacrimal (D), mirrored right ectopterygoid (E), mirrored right jugal (F, G), 6th (H), 8th (I) and 9th (J) cervical vertebrae, 10th
dorsal vertebra (K), 5th sacral vertebra (L) and 14th caudal vertebra (M) in mesial (A), labial (B), lateral (C–F, L, M), dorsal (G, H), anterior (I, J) andposterior (K) views. See Emended diagnosis in SYSTEMATIC PALEONTOLOGY for the numbers corresponding to each diagnostic feature.

concave dorsal surface for the contact with the lacrimal (Fig.
4G). The lacrimal contact surface bears an opening of a small
pneumatic foramen present at the preserved distal end of theascending ramus. The antorbital fossa is deeply excavated andextends to the anterodorsal margin of the ascending ramus,
suggesting its possible expansion to the nasal (Fig. 4C) as inallosauroids (Rauhut, 2003) and avialans (Turner et al., 2012).

The lateral wall of the antorbital fossa bears a large maxillaryfenestra anteriorly and a large pneumatic excavation dorsal to it.
Anterior to the antorbital fossa, several small foramina areobservable on the lateral surface. Ventrally, the lateral surface
exhibits a series of subtle and discrete foramina along theventral margin approximately above the 5th–10th alveoli.
In the original description, a large fenestra in the antorbital

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 4. Maxillae of FPDM-V-8461. Left (A–F) and right (G–L) maxillae in dorsal (A, G), anterior (B, I), lateral (C, H), posterior (D, K), medial (E,J) and ventral (F, L) views. Abbreviations: 2–10, 2nd to 10th erupted teeth; ap, anteromedial process; er, epiantral recess; lc, lacrimal contact; ma,maxillary antrum; mf, maxillary fenestra; pe, pneumatic excavation; pf, promaxillary fenestra; pmf, posteromedial maxillary fenestra; pnf, pneumaticforamen; pr, promaxillary recess; rt, replacement tooth; sf, subnarial foramen; ss, suprantral strut.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

7

FIGURE 5. Jugal of FPDM-V-8461. Right jugal in dorsal (A), anterior (B), medial (C), posterior (D), lateral (E) and ventral (F) views. Abbreviations:anr, anterior ramus; dp, dorsal prong; fo, foramen; itf, infratemporal fenestra; o, orbit; pc, pneumatic cavity; qjc, quadratojugal contact; vp, ventral prong.

fossa is described as separated dorsoventrally into the maxillary
and promaxillary fenestrae (Azuma et al., 2016). However, therendered polygon mesh derived from CT images reveals that afaint bony bridge between them is situated medially to thelateral wall of the antorbital fossa (Fig. 4C, E). This bridge
probably corresponds to the suprantral strut based on its
continuous contact with the postantral strut, resulting in a single
large maxillary fenestra expanded further dorsally above thesuprantral strut, unlike that of Allosaurus (Hendrickx and
Mateus, 2014). Although the maxillary fenestra abuts theanterior margin of the antorbital fossa as in Tsaagan (Norell etal., 2006; Turner et al., 2012), it never extends to theanteroventral corner unlike that of Linheraptor (Xu et al., 2015).
The maxillary fenestra is expanded posteriorly to occupyslightly more than the half the length between the anterior
margin of the antorbital fossa and that of the antorbital fenestra,
as in troodontids (Currie and Varricchio, 2004). The maxillaryfenestra is expanded dorsoventrally as well, while separated
from the ventral margin of the antorbital fossa. Anteroventral tothe maxillary fenestra, a true, small promaxillary fenestra
excavates the anterior margin of the antorbital fossa
anteromedially, slightly above its anteroventral corner.
In medial view, the jugal ramus and maxillary body arelargely occupied by the medial shelf extending along the ventral
margin (Fig. 4E). Anteriorly, the shelf becomes inflated to be abar-like projection which extends to the short anteromedial
process occupying only the area above to the first two alveoli(Fig. 4E) unlike more elongated ones of other coelurosaurs(Brusatte et al., 2014). The maxillary antrum is delimited
ventrally by the medial shelf and dorsally by the suprantral strut.

The postantral strut demarcates the posterior margin of themaxillary antrum and the anterior margin of the posteromedial
maxillary fenestra (foramen in maxillary pila sensu Choiniere etal., 2014) with both antra connected through a canal lateral to
the strut. Posterodorsal to the suprantral strut is the epiantral
recess excavating the interfenestral strut (the area between the
maxillary and antorbital fenestrae) posteriorly (Fig. 4E, I, J) as
in allosauroids and tyrannosauroids (Hendrickx and Mateus,2014). The area anterior to the maxillary antrum is severely
damaged, while the promaxillary recess is clearly present near
the anterodorsal margin of the maxillary body (Fig. 4E).
Thirteen alveoli are discernible in the maxilla with four
erupted teeth preserved in the second, fifth, eighth and 10th
alveoli (Fig. 4E, F). Each of the 5th to 7th, 9th and 10th alveoli
has an opening dorsally on the medial shelf. The CT images
reveal one more alveolus contra to the original description(Azuma et al., 2016). Each of the 5th, 8th and 10th maxillaryteeth is followed by an unerupted replacement tooth
mediodorsally.
Jugal
The right jugal is partitioned into the anterior and posterior
parts by the breakage along the midline of the ascending ramus(Fig. 5C, E). The anterior part lacks the anterior end, and theascending ramus is shifted ventrally by the breakage, which iscorrectly positioned in Fig. 5 and Table 1. In lateral view, thejugal is triradiate with a nearly straight ventral margin as inother coelurosaurs, while bending anterodorsally below thejunction of the ascending and anterior rami (Fig. 5E). The jugal
is thinned ventrally to form a keen edge on the ventral margin,
and the ventral part of the element below the ascending ramus is

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 6. Lacrimals of FPDM-V-8461. Left lacrimal (A–F), right lacrimal incorporating the posterodorsal part of the right maxilla (shaded) as
preserved (G), and right lacrimal (H–L) in dorsal (A, G), anterior (B, K), lateral (C, J), posterior (D, I), medial (E, H) and ventral (F, L) views.
Abbreviations: af, antorbital fossa fc, frontal contact; fnc, frontal or nasal contact; lf, acrimal foramen; ml, medial lamina; nlc, nasolacrimal canal; or,orbitonasal ridge; pc, postorbital contact; pnf, pneumatic foramen; pp, posterodorsal process.

curved ventromedially to form a broad, concave medial surface(Fig. 5C).
The anterior ramus is inflated by a large pneumatic cavity
within it so that the lateral surface is rounded and continuouswith the dorsal surface forming an unusually thick dorsal margin
to border the ventral rim of the orbit (Fig. 5A). Similarly, themedial surface of the anterior ramus is mostly convex (Fig. 5C).
The breakage at the posterodorsal part of the ramus indicates thepresence of the pneumatic cavity (Fig. 5B, C). In lateral view, atleast two small foramina are present along the ventral margin of
the anterior ramus (Fig. 5E). Both foramina continue to theinternal pneumatic cavity.
The ascending ramus projects posterodorsally in lateral view.
The contact surface for the postorbital is present as a shallow
depression along the anterior margin of the dorsal two-thirds ofthe ascending ramus. The posterior ramus is slightly shorter than
the ascending ramus as preserved and bears dorsal and ventral
prongs projected posteriorly to contact with the quadratojugal.
In medial view, the cleft between both prongs continues
anteriorly as a groove on the medial surface of the jugal below
the ascending ramus, probably for the quadratojugal contact(Fig. 5C).
Lacrimal
The elements originally identified as the right postorbital andthe left lacrimal by Azuma et al. (2016) is re-identified as theleft (Fig. 6A–F) and right (Fig. 6G–L) lacrimals, respectively.
The right lacrimal is nearly complete and preserved inarticulation with the distalmost part of the dorsal ramus of theright maxilla (Fig. 6G). The left lacrimal, on the other hand,lacks the anterior end and most of the ventral ramus. The
anterior extent of the right lacrimal only slightly exceeds thepreserved anterior margin of the frontal when in articulation,
indicating that the contact between the lacrimal and the nasal
was faint to absent in Fukuivenator.
In lateral view, the medial lamina is visible in the ventral halfof the ventral ramus (Fig. 6J). In medial view, the orbitonasal
ridge runs anteroventrally from the ventral margin of theposterodorsal process to meet the medial lamina just above themiddle of the ventral ramus (Fig. 6D, E, H). The laterodorsal
margin of the lacrimal forms a stout ridge which does not extend
posteriorly to the level of the orbit (Fig. 6C, J). The anteriorramus is narrowed anteriorly, and its dorsal surface bears a
depression (Fig. 6A, G). This depression deepens anteriorly tobear a dorsal opening at the anterior margin of the nasolacrimal
canal perforating the lacrimal posteroventrally. This canal isanteriorly continuous with the one within the ascending ramusof the maxilla through the contact surface between the lacrimaland maxilla. The canal has another opening just posterior to theorbitonasal ridge on the medial surface. A deep anteroposteriorgroove is present on the mediodorsal margin of the anterior

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

9

FIGURE 7. Frontals of FPDM-V-8461. Left (A–F) and right (G–L) frontals in dorsal (A, G), anterior (B, H), lateral (C, K), posterior (D, J), medial (E, I)
and ventral (F, L) views. Abbreviations: lpc, lacrimal and prefrontal contact; or, orbitonasal ridge; pac, parietal contact; poc, postorbital contact: pp,
postorbital process; sf, supratemporal fossa

ramus and probably represents a contact surface for the frontal
or nasal (Fig. 6E, H).
The posterodorsal process forms a conical process set into a
deep pit on the lateral surface of the frontal in lateral view (Fig.
6C, J). The medial surface of the process is slightly concave to
fit with the swelling within a pit on the anterolateral margin of
the frontal (Fig. 6E, H). The posteromedial margin of theprocess exhibits a narrow, rough surface probably for thecontact with the prefrontal (Fig. 6D, I). The lacrimal body (inwhich the anterior and ventral rami meet) and a shortposterodorsal process are excavated by a large lacrimal recesswith a single, moderately sized foramen opening laterally (Fig.
6C, J). The lacrimal recess does not inflate the anterior ramus
unlike those of some tyrannosaurines (Carr, 2005) andoviraptorosaurs (Gold et al., 2013). The subcutaneous surface

anterior to the lacrimal foramen is not demarcated from thelateral surface of the antorbital fossa; they rather blend into eachother as in some tyrannosaurines (Carr, 2005).
FrontalBoth frontals are preserved while the anterior margins arebroken, hindering their conjoined morphology (Fig. 7). The
posterior part of the element expands laterally, and its lateral
margin exhibits a smooth transition from the orbital margin to
the postorbital process (Fig. 7A, G). The dorsal surface of theposterior expansion is mostly occupied by the supratemporal
fossa with strongly sinusoidal anterior margin that is mediallyformed by a blunt ridge and laterally by a sharp ridge along the
postorbital contact. The posterior margin exhibits a nearlystraight and somewhat indented suture for the parietal at thelevel slightly posterior to the apex of the postorbital process.

10

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 8. Parietals of FPDM-V-8461. Assembled (A, C, E, G), right (B, H) and left (D, F) parietals in dorsal (A), lateral (B, D), anterior (C), posterior(E), medial (F, H) and ventral (G) views. Abbreviations: fc, frontal contact; lc, laterosphenoid contact; nc, nuchal crest; pac, parietal contact; sc, sagittal
crest; sf, supratemporal fossa; soc, supraoccipital contact.

FIGURE 9. Quadrate of FPDM-V-8461. Right quadrate in dorsal (A),posterior (B), lateral (C), anterior (D), medial (E) and ventral (F) views.
Abbreviations: ec, ectocondyle; en, entocondyle; lp, lateral process; pf,
pterygoid flange; ppr, posterior pneumatic recess; qf, quadrate foramen;qh, quadrate head; vqc, ventral quadratojugal contact.

In lateral view, a discrete longitudinal notch for the contact
with the lacrimal and prefrontal is observable anteriorly (Fig.
7C, K). This contact surface gradually projects laterally toward
its anterior end so that it is also visible in dorsal view (Fig. 7A,
G). The projection matches with the medial surface of theposterodorsal process of the lacrimal (Fig. 6E, H). Therefore, theremaining posterior part of the notch is likely for the contact
with the prefrontal. Posteriorly, the lateral (orbital) margin
between the prefrontal and postorbital contacts exhibits anarrow, slightly rough surface.
Posterior to the orbital margin, the postorbital contact forms alarge, deep and posterolaterally-directed notch (Fig. 7C, K) as insome troodontids (Currie, 1985; Turner et al., 2012). In addition,
this contact is partially visible in the dorsal view (Fig. 7A, G).
Although such contacts are also visible in dorsal view in
Stenonychosaurus (van der Reest and Currie, 2017) and
Latenivenatrix (van der Reest and Currie, 2017:fig. 5), they arelocated posteriorly along the postorbital process, in contrast to
the one in Fukuivenator in which it is situated anteriorly.
Parietal
Both parietals are completely preserved (Fig. 8). The parietal
exhibits a square shape in dorsal view with a concave lateral
margin demarcating the supratemporal fenestra (Fig. 8A). The
anterior margin is somewhat indented to fit with the posterior
margin of the frontal, while they are not fused with each other.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

11

FIGURE 10. Ectopterygoids of FPDM-V-8461. Left (A–G) and right (H–M) ectopterygoids in dorsal (A, H), anterior (B, L), lateral (C, K), posterior (D,J), medial (E, G, I) and ventral (F, M) views. Open square within E indicates the area magnified in G. Abbreviations: jp, jugal process; pf, posterior flange;
sf, subtemporal fenestra; ipr, opening of the internal pneumatic recess; mpa, m. pterygoideus attachment.

Similarly, the medial margin is nearly straight and somewhat
indented in dorsal view, and the medial surface does not exhibit
a trace for fusion with another parietal (Fig. 8F, H). The dorsal
surface is occupied by a convex, laterodorsally-facing
supratemporal fossa except for the sagittal and nuchal crests
along the medial and posterior margins, respectively (Fig. 8A,
B, D). The sagittal crest is represented by a flat, slightly elevatedskull table occupying about the medial fourth of the dorsal
surface. The nuchal crest emerges from the posteroventral
margin of the skull table and is sloped lateroventrally inposterior view (Fig. 8E). Additionally, it forms a broad occipitalsurface facing posterodorsally with a slight concavity in its
medial third for the contact with the supraoccipital. The
supraoccipital also contacts the posteroventral part of theparietal, forming a narrow, medioventrally facing surface on thelatter (Fig. 8F, H). In ventral view, a narrow concavity anterior
to the supraoccipital contact is facing ventrally most likely tocontact with the laterosphenoid (Fig. 8G). The remaining
anterior half of the lateroventral margin bears a lateroventrally
facing surface probably representing another contact for thelaterosphenoid.
Quadrate
Only the right quadrate is preserved and lacks most of thepterygoid flange and lateral process (Fig. 9). The quadrate head
has a single articular surface for the squamosal dorsally (Fig.
9A, D). From the posterior margin of the quadrate head, thequadrate ridge emerges and extends ventrally toward the middleof the posteromedial margin of the entocondyle (Fig. 9E). The
ridge is bowed laterally and bears a slight depression in theproximal third in medial view. Just lateral to the quadrate ridge,
an oval fossa is present in the midheight of the posterior surface(Fig. 9B), probably corresponding to the posterior pneumatic
recess (Eddy and Clarke, 2011). Although the lateral margin is
incomplete in the dorsal half, the medioventral margin of thequadrate foramen is preserved above the ventral quadratojugal
contact (Fig. 9D). Above the ento- and ectocondyles, the base of
the pterygoid flange projects anteriorly to bear a shallow fossa
ventrally (Fig. 9E, F), while lacking the deep recess unlike
Falcarius (Zanno, 2010) and pantyrannosaurs (Brusatte et al.,2014). The ventral quadratojugal contact faces posterolaterally
to the quadratojugal process and only partially invades into thelateral surface of the ectocondyle (Fig. 9C). The medial marginof the ventral quadratojugal contact directs vertically inposterior view (Fig. 9B).
Ectopterygoid
Both ectopterygoids are preserved, with the left one being
more complete (Fig. 10). The anterior part is stout andcomposed of the medially-hollowed main body and the lateralprojection, namely the jugal process. The posterior part forms
the lateromedially thin, dorsoventrally tall flange. In dorsal
view, the jugal process and the posterior flange constitutes theanterior and lateral rims of the subtemporal fenestra (Fig. 10A,
H). In medial view, the ectopterygoid body is concave, and its
deepest portion bears a very small opening of the internal
pneumatic recess (Fig. 10E, G), although the body is not inflated
unlike those of tyrannosaurines (Holtz, 2001; Brusatte et al.,2010; Gold et al., 2013). The anterior surface of the jugal ramus
is slightly concave to form a shallow groove directed laterally

12

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 11. Palatine of FPDM-V-8461. Right palatine in dorsal (A), medial (B), posterior (C), lateral (D), anterior (E) and ventral (F) views.
Abbreviations: in, internal naris; jp, jugal process; mc, maxilla contact; pf, pneumatic foramen; pp, pterygoid process; vp, vomeropterygoid process.

(Fig. 10B, L). Ventrally on the jugal ramus, another concave
surface partially invades the anterolateral margin of the posteriorflange (Fig. 10K, M). This concavity probably represents theattachment of m. pterygoideus as in Deinonychus (Ostrom,1969). The contact for the jugal on the lateral end of the jugal
process is ambiguous. The posterior margin of the main body isbifurcated by a notch in lateral view, possibly for the contact
with the pterygoid (Fig. 10E, I).
Palatine
Right palatine is nearly complete (Fig. 11). In lateral view, thevomeropterygoid process is projected anterodorsally (Fig. 11D).
The contact for the maxilla is present as a narrow longitudinalsurface along the ventral border. Dorsally along the contact, ashallow longitudinal groove is present, extending to the anterior
region of the jugal process. A pneumatic recess within the main
body exhibits a primary opening at the central part of thelaterodorsal surface and the secondary opening in the middle ofthe shallow longitudinal groove above the maxillary contact(Fig. 11A, D).
Braincase (supraoccipital, otoccipital, basioccipital,
basisphenoid, prootic, orbitosphenoid and parasphenoid)
The braincase is preserved well except for the laterosphenoids(Fig. 12). Parietals are also present and not in contact with thebraincase. The basisphenoid is partly disarticulated from theprootic and basioccipital.
Although the supraoccipital exhibits its dorsal margin flat inposterior view (Fig. 12D), it represents a postmortem artifact
because it does not fill the contact surface on parietals. The

occipital surface faces posterodorsally when the lateralsemicircular canal is oriented horizontally (Fig. 12C). The
occipital surface of the supraoccipital exhibits a triangularoutline lacking its apex and a blunt medial swelling extending
dorsoventrally in contrast to the prominent keel seen in
Incisivosaurus (Xu et al., 2002a), Linheraptor (Xu et al., 2015)
and Zanabazar (Turner et al., 2012). Lateral to the swelling, apair of semi-circular depressions is present along the sutureagainst the parietals (Fig. 12D). The dorsoventral margin of each
depression is pierced as an exit of the caudal middle cerebral
vein canal. The depression accompanying the caudal middle
cerebral vein canal is also present in Alioramus (Bever et al.,2013:fig. 6) and Linheraptor (Xu et al., 2015:fig. 3), while notin Falcarius (Smith et al., 2011:figs. 1, 2).
The neck of the occipital condyle is not constricted in lateralwidth, whereas it is slightly constricted in dorsoventral height bya ventral excavation (Fig. 12A, C) as in Falcarius (Smith et al.,
2011:fig. 1) and Velociraptor (Norell et al., 2004:fig. 5.1), in
which the excavation is shallower than in Dromaeosaurus(Currie, 1995:fig. 4). The foramen magnum is sub-triangular,slightly wider (9.9 mm) than tall (9.3 mm) in posterior view(Fig. 12D), and is larger than the occipital condyle as in mostmaniraptorans except for troodontids (Maryańska et al., 2002;
Zanno et al., 2009). The contribution of each bone to theforamen magnum is hardly discernable due to their complete
fusion.
In posterior view, the paroccipital process formed by theotoccipital (coossified exoccipital and opisthotic) is straight,

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

mediolaterally long and dorsoventrally narrow unlike short andwide ones in pennaraptorans (Turner et al., 2012). Distally, theparoccipital process is widening slightly dorsoventrally to forma squared end (Fig. 12D) as in tyrannosauroids (Bever et al.,2013; Brochu, 2003:figs. 3–5), Nqwebasaurus (Choiniere et al.,2012:fig. 7) and dromaeosaurids (Colbert and Russell, 1969;
Norell et al., 2004). Although the paroccipital process is also
elongated in late-diverging ornithomimosaurs (Osmólska et al.,
1972:fig. 3; Kobayashi and Barsbold, 2005a), Haplocheirus(Choiniere et al., 2014) and therizinosaurs (Smith et al., 2011;
Lautenschlager et al., 2014), it is tapered distally to form arounded end. The paroccipital process projects posterolaterally
in dorsal view (Fig. 12A) and its distal end is twisted to faceposterodorsally as in most dromaeosaurids (Colbert and Russell,
1969; Norell et al., 2004). On the posterior surface of the base of
the paroccipital process, a mediolaterally elongate fossa
excavates the process laterodorsally (Fig. 12D). The ventral
margin of the paroccipital process is continuous with the metotic
strut, which is short and robust as in most other theropods(Turner et al., 2012). Mediolateral width across opposing struts
is extensive so that is greater than one half of the dorsoventral
depth of the braincase.
In posterior view, three exits for cranial nerves (CNs) arelocated together within a shallow depression immediately lateral
to the occipital condyle and medial to the metotic strut (Fig.
12D) as in some dromaeosaurids (Norell et al., 2004; Norell etal., 2006). The lateral one is for the glossopharyngeal, vagus andaccessory nerves (CNs IX–XI), the dorso- and ventromedial
ones are for the hypoglossal nerve (CN XII). The basal tubera
are separated by a shallow concavity on the posteroventral
margin of the basioccipital. The conjoined width of the basal
tubera is slightly greater than that of the occipital condyle unlike
the reduced ones of alvarezsaurs and most troodontids(Makovicky et al., 2003; Turner et al., 2012). On the other hand,the dorsoventral depth of the basal tubera does not exceed thatof the occipital condyle.
The prootic and orbitosphenoid are completely fused to form
the lateroventral wall of the brain and bear exits for thetrigeminal and facial nerves (CNs V, VII) and the otic recess on
the lateral surface (Fig. 12C, E). The exit for trigeminal nerve isincompletely divided into the dorsal and ventral foramina, which
are completely separated in Erlikosaurus (Lautenschlager et al.,2014). The dorsal foramen probably corresponds to the exit forthe maxillary branch (CN V2) and lacks the dorsal margin
originally formed by the missing laterosphenoid. If this is thecase, the ventral foramen should correspond to an exit for themandibular branch (CN V3). Within the otic recess, two fossae
are separated dorsoventrally by the crista interfenestralis, where
the dorsal region forms the fenestra ovalis (or vestibularis) andthe ventral region forms the fenestra pseudorotundum (orcochlearis). A stapedial groove is oriented posterolaterally fromthe fenestra pseudorotundum, as well as the posterior tympanic
recess about the half the length of the groove as in Falcarius(Smith et al., 2011).
The basipterygoid process is well projected lateroventrally

13

from the ventral margin of the basisphenoid. The lateral surfaceof the basisphenoid is excavated by a large fossa comprising
three recesses (Fig. 12C, E). The basipterygoid recess is present
immediately above the basipterygoid processes and followed
posteriorly by the anterior tympanic recess, which is subdivided
dorsoventrally by a thin lamina into the dorsal region
corresponding to the prootic recess and the ventral region
corresponding to the subotic recess. The presence of subotic
recess is known in ornithomimosaurs (Osmólska et al., 1972:fig.
5A; Makovicky and Norell, 1998; Tahara and Larsson, 2011),
therizinosaurs (Smith et al., 2011, 2018) and some troodontids(Currie and Zhao, 1993b; Norell et al., 2000, 2009; Yin et al.,2018) and also reported in an alvarezsaur Xiyunykus (Xu et al.,2018) and an oviraptorosaur Conchoraptor (Kundrát andJanáček, 2007). Among them, the subotic recess of
Fukuivenator is especially similar to that of Falcarius due to its
position anteroventral to the otic recess, anterior to the basal
tuber and posteroventral to the prootic recess (Fig. 12C; Smithet al., 2011:fig. 2C). Left and right prootic recesses excavate thebone deeply to connect with each other. The anterodorsal
margin of the prootic recess is demarcated by a distinct crest,
namely the otosphenoidal crest. The dorsal surface of thebasisphenoid is laterally narrow and is excavated by the pituitary(hypophyseal) fossa (Fig. 12A). The deepest part of the pituitary
fossa bears a narrow canal for the internal carotid passage (Fig.
12B). Ventrally, this passage is bifurcated and directed
lateroventrally to exit just below the anteroventral margins ofotosphenoidal crests (Fig. 12C, E). The anterior surface of thebasisphenoid also has a significant fossa continuous with theventral groove of the cultriform process (Fig. 12B, F). Ventrally,
the basisphenoid recess is present as a broad fossa facing
posteroventrally as in early-diverging tyrannosauroids (Brusatte
et al., 2010) and spanning the basisphenoid and basioccipital(Fig. 12F). In the basioccipital, the basisphenoid recess is deep
and further divided into two by an anteroposteriorly-directed
septum. The basicranium defined by positions of basal tubera
and basipterygoid processes faces posteroventrally and isanteroposteriorly longer than mediolaterally wide.
The parasphenoids form an anteroposteriorly-elongatedcultriform process (parasphenoid rostrum) with deep ventral
concavity continuous with the fossa present anteriorly on thebasisphenoids (Fig. 12F). Laterally, the base of the cultriform
process is slightly pneumatized by a shallow fossa (Fig. 12C, E)
and lacks apparent parasphenoid bulla unlike those of highly
pneumatized taxa such as late-diverging troodontids (Barsbold,
1974; Currie, 1985) and ornithomimosaurs (Osmólska et al.,
1972; Barsbold, 1981; Pérez-Moreno et al., 1994; Kobayashi
and Lü, 2003; Tahara and Larsson, 2011).
Cranial Endocast
Computed tomography images of frontals, parietals, andpartial braincase allow reconstruction of nearly complete cranial
endocast (Fig. 13). However, the degree of development of the
optic lobes and the positional relationship between the forebrain
and midbrain are unknown because the cranial portion covering
the midbrain area is missing (Fig. 12).

14

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 12. Braincase of FPDM-V-8461. Braincase in dorsal (A), anterior (B), left lateral (C), posterior (D), right lateral (E) and ventral (F) views.
Abbreviations: atr, anterior tympanic recess; bp, basipterygoid process; bpr, basipterygoid recess; bsr, basisphenoid recess; bt, basal tuber; ci, crista
interfenestralis; cp, cultriform process; cvcmc, caudal middle cerebral vein canal; dtr, dorsal tympanic recess; fm, foramen magnum; fo, fenestra ovalis; fp,
fenestra pseudorotundum; icp, internal carotid passage; lc, laterosphenoid contact; ms, metotic strut; oc, occipital condyle; or, otic recess; osc,
otosphenoidal crest; pc, parietal contact; pf, pituitary fossa; pp, paroccipital process; pr, prootic recess; ptr, posterior tympanic recess; scd, semi-circulardepression; sg, stapedial groove; sr, subotic recess; V2, exit for maxillary branch of trigeminal nerve ;V3, exit for mandibular branch of trigeminal nerve;
VII, exit for facial nerve; IX–XI, exit for glossopharyngeal, vagus and accessory nerves; XII; exit for hypoglossal nerve.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

15

FIGURE 13. Cranial endocast of FPDM-V-8461. Cranial endocast in dorsal (A), anterior (B), left lateral (C), posterior (D), right lateral (E) and ventral(F) views. Midbrain area is reconstructed as the hatched area in lateral views. Abbreviations: car, cerebral carotid artery canal; cer, cerebral hemisphere;
cvcm, caudal middle cerebral vein; crc, crus communis; fl, floccular lobe; fo, cast of fenestra ovalis; fp, cast of fenestra pseudorotundum; if,interhemispherical fissure; lab, endosseous labyrinth; p, pone; pg, pituitary gland; ob, olfactory bulb; otc, olfactory tract; V 2, canal for maxillary branch oftrigeminal nerve; V3, canal for mandibular branch of trigeminal nerve; VII, canal for facial nerve; IX–XI, shared canal for glossopharyngeal, vagus andspinal accessory nerve; XII, canal for hypoglossal nerve.

16

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 14. Dentary of FPDM-V-8461. Right dentary in dorsal (A), anterior (B), medial (C), posterior (D), lateral (E) and ventral (F) views.
Abbreviations: dg, dentary groove; et, erupted tooth; ip, interdental plate; mf, meckelian fossa; mg, meckelian groove; nf, neurovascular foramen.

TABLE 1. Measurements on the skull of FPDM-V-8461.

*Specimen incomplete in measured dimension.

The olfactory bulbs appear relatively short, with the olfactory
tracts being as long as the bulbs (Fig. 13A). Generally, longolfactory tracts are a primitive trait in theropods and they tend tobe shorter in those closer to the crown Aves (Witmer andRidgely, 2009). Therefore, the olfactory condition in
Fukuivenator represents a late-diverging tetanuran feature. The
cerebrum lacks its posteroventral part, whereas a partial pituitary
gland and the carotid artery are present in its ventral part (Fig.
13C, E, F). The canal for CN V is observed on the posterodorsalside of the pituitary gland, and the route of the mandibular
branch of CN V, in particular, is preserved. The canal for CN
VII presents anteriorly to the cochlear duct, and the cast of thefenestra pseudorotundum and the canals for CNs IX–XI and XII
are present posteriorly to the duct. While the ventral part ismissing, the cerebrum region appears caudolaterally expanded(Fig. 13A), resembling that of Erlikosaurus (Lautenschlager etal., 2012). The cerebral hemispheres are partitioned dorsally bya deep interhemispheric fissure.
The cranial endocast of Fukuivenator bears well-marked
cerebral and pontine flexures (Fig. 13C, E). The posterior part ofthe cerebellum slopes steeply toward the foramen magnum. Thisconfiguration is found in small theropods including
Archaeopteryx (Domínguez Alonso et al., 2004; Kundrát, 2007;
Kundrát et al., 2020). The midbrain to hindbrain morphology isrelatively similar to that of Falcarius among therizinosaurs,
while differing largely from that of Nothronychus and
Erlikosaurus which exhibit an overall linear structure(Lautenschlager et al., 2012). A moderately developed duralpeak is found in the cerebellar region, and the pone is elongatedmediolaterally, resembling therizinosaurian endocasts(Lautenschlager et al., 2012). The cerebellar floccular lobes are
expanded laterally toward the level of the lateral semicircular
canals (Fig. 13B). They are much more developed than othermaniraptoriforms (Balanoff et al., 2018) and fill most of thespace formed by the rostral semicircular canal. SuchOSTEOLOGY OF FUKUIVENATOR PARADOXUS

17

FIGURE 15. Teeth of FPDM-V-8461. Mesialmost tooth tentatively oriented as right premaxillary tooth (A–D), mesial tooth tentatively oriented as left
premaxillary tooth (E–H), and second (I, J), fifth (K, L) and eighth (M, N) maxillary teeth in labial (A, F), mesial (B, E, I, K, M), lingual (C, H), distal(D, G, J, L, N) views. I–N are only roughly adapted to the scale. Abbreviations: cc, cervix constriction; dc, distal carina; mc, mesial carina.

well-developed cerebellar floccular lobes are shared with
Erlikosaurus (Lautenschlager et al., 2012), Nothronychus(Smith et al., 2018) and Falcarius (Smith, 2014), and can beconsidered a unique feature of Therizinosauria shared with
Fukuivenator.
Endosseous Labyrinth
The vestibular system including the semicircular canal andthe ventral cochlear duct are preserved in the braincase (Fig.

13). The cochlear duct is relatively long, and its distal part isslightly medially oriented (Azuma et al., 2016; Fig. 13B). On
the other hand, the cochlear duct is shorter than those of
therizinosaurs Erlikosaurus and Nothronychus, while
comparable to Falcarius (Lautenschlager et al., 2012). The
vestibular system is well developed with the rostral semicircular
canal being elongated dorsally (Azuma et al., 2016; Fig. 13C,
E). Overall, the inner ear morphology is notably similar to that

18

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

of Falcarius (Lautenschlager et al., 2012). Additionally, theanterior canal extends slightly caudal to the crus commune, afeature seen in maniraptoriforms such as Struthiomimus altus(Witmer & Ridgely, 2009; Hanson et al., 2021; Bronzati et al.,2021).
Dentary
The posterior part of the right dentary bearing the last sevenalveoli is preserved, and at least the posteroventral end is
missing (Fig. 14). The preserved part appears subtriangular inshape tapering anteriorly with straight dorsal and ventral
margins in lateral view (Fig. 14E), which indicates that themissing anterior part might be shallower dorsoventrally than thepreserved anterior end (about 9 mm) when present. In dorsal
view, the body is straight and mediolaterally narrow, on which
the tooth row is approximately in line with the flat lateralsurface (Fig. 14A).
The lateral surface is well preserved in the posterior halfwhile it is somewhat damaged in the anterior half (Fig. 14E). A
shallow dentary groove emerges near the posterodorsal end of
the lateral surface and extends anteriorly toward a small
neurovascular foramen just ventral to the third alveolus from thelast. This foramen is anteroposteriorly elongated anddiscontinuous with anterior foramina unlike troodontids where
several foramina form a deep groove (Currie, 1987; Brusatte etal., 2010; Turner et al., 2012). All alveoli are separated to eachother by the interdental plates (Fig. 14C). Each interdental plateis smooth, subtriangular in medial view, and bordered fromneighboring plates by small foramen for the apex of thereplacement teeth situated mediodorsal to each alveolus. The
meckelian fossa is dorsoventrally broad posteriorly and abruptly
narrows anteriorly to be closed at the level of the sixth alveolus
from the last. The four last alveoli partially perforate the dorsal
wall of the meckelian fossa (Fig. 14F). In medial view, thepointed anterior end of the meckelian fossa is continuous with ashallow meckelian groove sitting approximately at midheight ofthe dentary in the preserved anterior end (Fig. 14C).
Dentition
Although the premaxilla and the anterior part of the dentary
are not preserved, two isolated mesial teeth were foundassociated with the rest of the skeleton (Fig. 15). These teeth aresubstantially different in size from each other. The smaller one
is potentially the mesialmost tooth exhibiting a characteristic
spatulate shape with convex labial surface and slightly concave
lingual surface (Fig. 15A–D). The apical margin of the crown isrounded in lingual view and is continuous with both the mesial
and distal carinae, both of which are on the lingual surface toenclose the slight concavity (Fig. 15C). Around the cervix, afaint constriction is present (Fig. 15D). The root narrows
apically by a slight inclination of the left lateral margin in
lingual view and ends bluntly (Fig. 15C).
Another, much larger mesial tooth representing a left
premaxillary or right dentary tooth exhibits a conical shape withsubtle carinae along the convex mesial and straight distal
margins (Fig. 15E–H). The mesial carina is present only in theapical third of the crown at most (Fig. 15E), whereas the distal

carina runs along the labial margin of the distal aspect and ends
slightly before reaching the cervix with curving slightly
basolingually (Fig. 15G). In mesial view, the labial margin isconvex whereas the lingual is almost straight with a slight
convexity near the cervix (Fig. 15E). The cervix exhibits a weak
constriction (Fig. 15H), which is more apparent than that of theanteriormost tooth (Fig. 15D). Although the crown shape of thelarger mesial tooth is quite similar to those of lateral teeth, theformer is smaller than the latter (Fig. 3). Additionally, thedorsoventral depths of the posterior parts of the maxilla anddentary, as well as the estimated depth in the anterior part of thelatter, are unlikely to house such a long root. In both mesial
teeth, there is no trace of any replacement teeth in the roots.
Among four erupted teeth in the left maxilla, the apicobasal
heights are somewhat variable with the greatest in the 5th andthe lowest in the 8th and 10th (Fig. 4B–F). As in the larger
mesial and dentary teeth, all preserved maxillary teeth havesuboval cross sections resulting in mediolaterally thicker
maxillary teeth than the ziphodont teeth seen in most basal
theropods, and do not bear denticles (Fig. 15I–N). In labial
view, the second and fifth maxillary teeth are recurved
apicodistally (Fig. 4C). In contrast, the 8th and 10th teeth are notrecurved, exhibiting the convex mesial and straight distal
margins in labial view and a faint constriction at each cervix. In
mesial view, the second maxillary tooth bears an apex ending
bluntly due to the wearing and/or postmortem damage mainlyon the mesial and slightly on the distal margins (Fig. 15I, J),
whereas apices of other maxillary teeth are well pointed (Fig.
15K–N). In addition, the crown in the second tooth lacks both
mesial and distal carinae (Fig. 15I, J), whereas those of the fifth,
eighth and 10th exhibit a short mesial carinae in the apical half
and a longer distal carina that nearly extends to the base (Fig.
15K–N).
On the dentary, two intact teeth are preserved (Fig. 14A–E).
The mesial tooth entirely lacks the crown whereas the lateral
tooth preserves nearly whole crown except for the apex. The
latter is quite similar to the 8th and 10th preserved teeth ofmaxilla by exhibiting unrecurved crown with a suboval crosssection and short mesial and longer distal carinae while lacking
any serrations. As in the maxilla, the size of the dentary teeth isrelatively large for the dorsoventral depth of the dentary housing
them.
Axial skeleton
Cervical vertebrae
Fukuivenator possessed at least 11 cervical vertebrae,including preserved 9 cervicals (1st to 3rd, 5th, 6th and 8th to11th) and missing 4th and 7th (Figs. 16–20; Table 2). This
number of cervical vertebrae is greater than those of most othertheropods (10 or less), and such numerical increase indicates theelongation of the neck known in therizinosaurs andoviraptorosaurs (Zanno and Makovicky, 2011). Atlantal
neurapophyses are added to the series originally described in
Azuma et al. (2016) in which they are misidentified as axial ribs.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

19

FIGURE 16. Cervical vertebrae of FPDM-V-8461. Left lateral views of left (A) and right (B) atlantal neurapophyses, axis (C) and 3rd (D), 5th (E), 6th(F), 8th (G), 9th (H), 10th (I) and 11th (J) cervical vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; ai, axis intercentrum; cdf,
centrodiapophyseal fossa; dp, diapophysis; ep, epipophysis; f, bone fragment; hp, hypapophysis; ic, intercentrum contact; oc, occipital condyle contact;pcdl, posterior centrodiapophyseal lamina; pf, pneumatic foramen; po, postzygapophysis; pocdf, postzygocentrodiapophyseal fossa; podl,
postzygodiapophyseal lamina; podl-f, intralaminar fossa of postzygodiapophyseal lamina; pp, parapophysis; pr, prezygapophysis; prdl,
prezygodiapophyseal lamina.

The left postzygapophysis of the third cervical is missing.
Whereas all of eight well-preserved centra are anteroposteriorly
long and dorsoventrally shallow in lateral view (Fig. 16), they
are not in the “hyperelongate” condition, unlike those ofornithomimids and therizinosaurs (Zanno et al., 2009); forexample, the third cervical centra is 10.1 mm in width at thenarrowest (in the posterior end) and 27.7 mm in length. At theposterior articular surface in each centrum, the lateral width issubequal to the dorsoventral height (Fig. 18C–J). In the third toeighth cervical centra, the anterior articular surfaces are strongly
inclined ventrally, and distinctly wider than high in anteriorview (Fig. 17D–G) as in other maniraptoriforms (Xu et al.,
2002b). In addition, the anterior surface of the ninth is inclined

somewhat ventrally (Fig. 17H) and posterior surfaces of thethird to the ninth are inclined somewhat dorsally (Fig. 16D–H).
The cervical centra exhibit intermediate condition between
amphiplatyan and platycoelous with flat-to-concave anterior
surfaces and slightly more concave posterior surfaces (Figs. 17,18) as in Deinonychus (Ostrom, 1969). The posterior extent of
the centrum in anterior to middle cervical vertebrae terminates
approximately at the level of the neural arch (Fig. 16C–F).
All the cervical centra possess at least one pneumatic foramen
on each side (Fig. 16). In addition, the 8th and 11th exhibit twopneumatic foramina at least on each side, in which pneumaticforamina are dorsoventrally separated by horizontally oriented
septa (Figs. 16J, 18K) as in Sinovenator, therizinosaurs and20

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 17. Cervical vertebrae of FPDM-V-8461. Anterior views of right (A) and left (B) atlantal neurapophyses, axis (C) and 3rd (D), 5th (E), 6th (F),
8th (G), 9th (H), 10th (I) and 11th (J) cervical vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; ai, axis intercentrum; cprf;
centroprezygapophyseal fossa; cprl, centroprezygapophyseal lamina; dp, diapophysis; f, bone fragment; hlp, hypantrum-like projection; iprl,
intraprezygapophyseal lamina; iprl-f, intralaminar fossa of intraprezygapophyseal lamina; oc, occipital condyle contact; pp, parapophysis; pr,prezygapophysis; prcdf, prezygocentrodiapophyseal fossa; prdl, prezygodiapophyseal lamina; sprf, spinoprezygapophyseal fossa; sprl,
spinoprezygapophyseal lamina.

some oviraptorosaurs (Gauthier, 1986; Turner et al., 2012).
Although the number of pneumatic foramina on the centra varies
among cervicals in Fukuivenator, similar condition is known atleast in Falcarius (Zanno, 2010) and Jianchangosaurus (Pu etal., 2013). The pneumatic foramina connect to the internal
pneumatopore except in the axis and the 11th. Most of thelateral surface of the axial centrum is occupied by ananteroposteriorly-elongated fossa below the neurocentral suture,and the pneumatic foramen is present as the deepest part in its
anterior half, just posterior to the parapophysis (Fig. 16C). The
pneumatic foramen of the 11th is present as a fossa in the rightside whereas it is separated into a dorsal deep pit and ventral
shallow pit in the left side (Fig. 16J). In the axis, theparapophysis is indistinctly present as a small bulge, situateddorsally, and separated from the ventral margin of the centrum
in lateral view (Fig. 16C). The parapophysis is present as a
distinct lateral projection from the anteroventral margin of thecentrum in the postaxial cervicals (Fig. 16D–J). In the sixth
cervical vertebra, the parapophysis is best preserved among
postaxial cervicals and has a distinct concavity on the middle
part of the facet in lateral view (Fig. 16F).
Except for the 10th and 11th, ventral surfaces of the cervicalcentra are essentially flat even between both parapophyses,
resulting in the lack of the carotid processes (Fig. 20C–G). The
ventral surface of the ninth is somewhat concave anteriorly due
to the enlargement of parapophyses (Fig. 20H). The ventral

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

21

FIGURE 18. Cervical vertebrae of FPDM-V-8461. Posterior (A–J) and close-up right posterolateral (K) views of left (A) and right (B) atlantal
neurapophyses, axis (C) and 3rd (D), 5th (E), 6th (F), 8th (G, K), 9th (H), 10th (I) and 11th (J) cervical vertebrae. Abbreviations: cpof,
centropostzygapophyseal fossa; cpol, centropostzygapophyseal lamina; dp, diapophysis; ep, epipophysis; f, bone fragment; fo, foramen; ic, intercentrum
contact; ipol, intrapostzygapophyseal lamina; pcdl, posterior centrodiapophyseal lamina; pf, pneumatic foramen; po, postzygapophysis; pocdf,
postzygocentrodiapophyseal fossa; podl, postzygodiapophyseal lamina; pp, parapophysis; spof, spinopostzygapophyseal fossa.

surface of the 10th is flat anteriorly, while it becomes convex
posteriorly (Fig. 20I). The 11th cervical centrum is ventrally
pinched to form a distinct median ridge or hypapophysis (Fig.
20J), whereas it is not as projected ventrally as those of
Sinraptor (Currie and Zhao, 1993a), Avimimus (Kurzanov,1981), Nothronychus (Kirkland and Wolfe, 2001) andDeinonychus (Ostrom, 1969). In the anterior-middle cervicals,
the lateroventral margins become angular posteriorly (Fig.
20C–F); however, they do not form “finlike” crests seen in
therizinosauroids and some basal oviraptorosaurs (Zanno et al.,2009).
On the neural arch of the atlas, paired atlantal neurapophyses
are well preserved. Each neurapophysis exhibits triradiate shapein lateral view represented by anterodorsal, anteroventral andposterior processes (Fig. 16A). In medial view, the main bodybears a tabular zygapophyseal facet facing medioventrally (Fig.
16B). The anteroventral process projects somewhat medially toform a large contact surface composed of a convex anterodorsal
part for the occipital condyle and a flatter posteroventral part for
the intercentrum (Fig. 17A, B). In posterior view, a small
foramen is present at the base of the anteroventral process,between the large contact surface and the postzygapophyseal
facet (Fig. 18A, B). The anterodorsal process is formed by athin, flange-like structure facing its broad surface laterodorsally,
which medially forms the roof of the neural canal and laterally
covers a part of the anteroventral process in dorsal view (Fig.

22

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 19. Cervical vertebrae of FPDM-V-8461. Dorsal views of right (A) and left (B) atlantal neurapophyses, axis (C) and 3rd (D), 5th (E), 6th (F),
8th (G), 9th (H), 10th (I) and 11th (J) cervical vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; ai, axis intercentrum; cdf,
centrodiapophyseal fossa; cpof, centropostzygapophyseal fossa; cprl, centroprezygapophyseal lamina; dp, diapophysis; ep, epipophysis; f, bone fragment;
hlp, hipantrum-like projection; ipol, intrapostzygapophyseal lamina; ipol, intrapostzygapophyseal lamina; iprl, intraprezygapophyseal lamina; pcdl,
posterior centrodiapophyseal lamina; po, postzygapophysis; podl, postzygodiapophyseal lamina; pp, parapophysis; pr, prezygapophysis; prcdf,
prezygocentrodiapophyseal fossa; prdl, prezygodiapophyseal lamina; spof, spinopostzygapophyseal fossa; sprf, spinoprezygapophyseal fossa; sprl,
spinoprezygapophyseal lamina.

19A, B). The posterior process is dorsoventrally thin, while
broad and triangular in dorsal view. The posterior end of thedorsal surface bears a short longitudinal ridge representing an
epipophysis.
On the neural arch of the axis, the prezygapophyseal facet ofthe axis is situated dorsal to the anterior end of the centrum as in
the posterior cervicals in lateral view (Fig. 16C) and dorsal to
the lateral margin of the centrum in anterior view (Fig. 17C).

The prezygapophyseal facets face laterally in the axis in contrast
to those of postaxial cervicals facing dorsally or mediodorsally(Fig. 17C–J). The diapophysis of the axis is present as a low
mound posteroventral to the fossa below the prezygapophysis,
and posteriorly accompanied by a shallow depression (Fig.
16C). The axial epipophysis is well developed to form a large,
posterolaterally-directed projection extending beyond the lateral
margin of the postzygapophysis (Figs. 19C, 20C). The neural

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

23

FIGURE 20. Cervical vertebrae of FPDM-V-8461. Ventral (A–J) and close-up left lateroventral (K) views of left (A) and right (B) atlantal
neurapophyses, axis (C) and 3rd (D), 5th (E), 6th (F), 8th (G), 9th (H, K), 10th (I) and 11th (J) cervical vertebrae. Note that the bone fragment in the 9th isdigitally removed in K. Abbreviations: acdl, anterior centrodiapophyseal lamina; ai, axis intercentrum; cdf, centrodiapophyseal fossa; cpof,
centropostzygapophyseal fossa; cprl, centroprezygapophyseal lamina; dp, diapophysis; ep, epipophysis; f, bone fragment; hp, hypapophysis; pcdl,
posterior centrodiapophyseal lamina; pf, pneumatic foramen; po, postzygapophysis; podl-f, intralaminar fossa of postzygodiapophyseal lamina; pp,
parapophysis; pr, prezygapophysis; prcdf, prezygocentrodiapophyseal fossa; prdl, prezygodiapophyseal lamina.

spine of the axis is anteroposteriorly wide through its depth andexpanded further posteriorly at its dorsal margin, so that its
anteroposterior length reaches that of the centrum (Fig. 16C).
The posterodorsal portion of the axial neural spine is flared
laterally as in the third while being dissimilar to that in the
middle to posterior cervicals (Fig. 19C).
Neural arches of the postaxial cervicals essentially resemble
each other. In lateral view, the prezygapophysis is projected

anteriorly and flexed slightly ventrally (Fig. 16D–J). The
prezygapophyses project well anteriorly in the third 3rd to 8th
cervicals to place the facets entirely anterior to the centrum (Fig.
16D–F), in contrast to the facets of the axis and 10th and 11th
cervicals that are situated dorsal to the anterior part of thecentrum (Fig. 16C, I, J), and the 9th cervical exhibit anintermediate condition (Fig. 16H). In anterior view, theprezygapophyses overhang the centrum to place

24

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

prezygapophyseal facets entirely lateral to the centrum in most
cervicals (Fig. 17D–J). The prezygapophyseal facets facedorsally in the fifth cervical in contrast to those of otherpostaxial cervicals facing mediodorsally. In the third to sixth
cervicals, the bases of both prezygapophyses connect to eachother by a horizontal interprezygapophyseal lamina (Fig.
17D–F). In the eighth and ninth cervicals, the lateral parts of theintraprezygapophyseal lamina are divided dorsoventrally by apair of intralaminar fossae (Fig. 17G, H). In the eighth and moreposterior cervicals, the spinoprezygapophyseal laminae
are well-developed accompanied with the presence ofa n t e r i o r projection of the lateroventral margin of the
spinoprezygapophyseal fossa (Fig. 17G–J). Although thisprojection is originally described as the hypantrum (Azuma etal., 2016), it does not reach hyposphene-like projection of thepreceding cervical. Ventrally, each prezygapophysis of the 3rd
to 11th cervicals is supported by the centroprezygapophyseal
and prezygodiapophyseal laminae (Fig. 20D–J). The
centroprezygapophyseal fossae are present between theintraprezygapophyseal and centroprezygapophyseal laminae in
the fifth to eighth cervicals and is reduced in the ninth to aremnant only present in the left side (Fig. 17E–H). In the axis
and third to sixth cervicals, the intrapostzygapophyseal lamina is
expanded posteriorly from the base of the neural spine to form abroad platform covering the neural canal in dorsal view (Fig.
19C–F). In the eighth and more posterior cervicals, the medial
part of the intrapostzygapophyseal lamina bends ventrally toform a hyposphene-like projection just below the
spinopostzygapophyseal lamina (Fig. 19G–J). A pair of deep
centropostzygapophyseal fossae are situated below the
intrapostzygapophyseal lamina in the sixth and more posteriorcervicals (Fig. 18F–J). Such a complex lamina systemsurrounding the neural canal, resulting in deep and wide grooves
for interspinous ligaments and additional deep sockets,
represents a unique feature among theropods (Azuma et al.,2016).
In postaxial cervicals, the posterior centrodiapophyseal
lamina is strongly developed as a thick, laterally-offset lamina
that demarcates the centrodiapophyseal fossa dorsally (Fig.
20D–J). The anterior centrodiapophyseal lamina recedes
posteriorly toward the posterior centrodiapophyseal lamina by
the expansion of the prezygocentrodiapophyseal fossa in theeighth and ninth cervicals. In the eighth cervical, thecentrodiapophyseal fossa is present as a small pneumaticforamen in the ventral aspect of the diapophysis and divided
anteroposteriorly by a transverse lamina in the right side (Fig.
20G). In the ninth cervical, the centrodiapophyseal fossa isabsent at least in the left side (Fig. 20K). In contrast, thecentrodiapophyseal fossa is developed as a large pneumaticforamen invading the conjoined base of the prezygapophysis
and diapophysis in the 10th and 11th cervicals (Fig. 16I, J).
Dorsal to the posterior centrodiapophyseal lamina, thepostzygocentrodiapophyseal fossa is well developed as aforamen in postaxial cervicals (Fig. 16D–J) and dorsallydemarcated by a well-developed horizontal

postzygodiapophyseal lamina in the third to eighth cervicals(Fig. 16D–G). In combination with the intraprezygapophyseal,
prezygodiapophyseal and intrapostzygapophyseal laminae, thepostzygodiapophyseal lamina contributes to form an extensive
horizontal platform in the anterior cervicals (Fig. 19D–G) that is
unique among theropods (Azuma et al., 2016). In the ninth andmore posterior cervicals, the medial part of thepostzygodiapophyseal lamina is divided dorsoventrally by apresence of large fossa (intralaminar fossa of
postzygodiapophyseal lamina) so that the lamina does notextend to the postzygapophysis (Fig. 16H–J). The dorsal
division of the postzygodiapophyseal lamina is posteriorly
continuous with the epipophysis in the 9th and 10th cervicals,
whereas with the postzygapophysis in the 11th cervicals. In
addition, in the ninth and more posterior cervicals, thepostzygocentrodiapophyseal fossa is divided into two foramina
by a posteroventrallydirected lamina, and another fossa develops
between the postzygodiapophyseal and spinoprezygapophyseal
laminae. In postaxial cervicals, epipophyses are placed above
postzygapophyseal facets.
In postaxial cervicals, the neural spines are generally thin andplate-like, with no pneumatic foramen nor fossa on each side
(Fig. 19C–J). Among preserved middle-posterior cervicals, only
the eighth bears a nearly complete spine, which is shallower
than the posterior centrum height (Fig. 16H) as in othercoelurosaurs except for tyrannosaurids (Holtz, 2001; Brusatte etal., 2010). The anteroposterior lengths of the neural spines areshorter than those of the centra unlike that of the axis (Fig.
16D–J), especially in the eighth and more posterior cervicals togive an X-shaped appearance in dorsal view (Fig. 19G–J) as inmost other coelurosaurs (Makovicky and Sues, 1998; Brusatte etal., 2014). The dorsal end of the neural spine is bifurcatedlaterally at least in the eighth and ninth cervical to bear a narrow
longitudinal sulcus dorsally (Fig. 17G, H) unlike othertheropods and rather like some sauropods (Azuma et al., 2016).
The pre- and postspinal fossae are present throughout thecervical series except for the prespinal fossa of the axis (Figs.
17C–J, 18C–J). The rugosity of the ligament attachment scars isabsent or only weakly developed within each fossa as in othercoelurosaurs except for tyrannosaurids (Brusatte et al., 2010).
Dorsal vertebrae
The separately preserved 11 dorsals probably constitute acomplete series (Figs. 21–25). Such number of dorsal vertebrae
represents an intermediate condition between the plesiomorphic
state (12 or more) seen in early-diverging maniraptorans and thederived state (10 or fewer) seen in some therizinosaurids such as
Suzhousaurus (Dong, 1979) and Nanshiungosaurus (Daqing etal., 2007), and some oviraptorosaurs (Turner et al., 2012). The
first centrum is severely damaged and the second dorsal lacks its
neural arch.
The first dorsal is a cervicodorsal vertebra that appears very
similar in morphology to the posteriormost cervical vertebrae(Figs. 16J, 21A). Although the parapophysis is not preserved,
the preserved anterodorsal margin of the centrum indicates thatthe parapophysis does not reach the dorsal margin of the

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

25

FIGURE 21. Dorsal vertebrae of FPDM-V-8461. Left lateral views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 8th (H), 9th (I), 10th (J)and 11th (K) dorsal vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; cdf, centrodiapophyseal fossa; cpof, centropostzygapophyseal
fossa; cprl, centroprezygapophyseal lamina; dp, diapophysis; ep, epipophysis; hp, hypapophysis; hs, hyposphene; il, interspinous ligament scar; pcdl,
posterior centrodiapophyseal lamina; pd, pleurocentral depression; po, postzygapophysis; pocdf, postzygocentrodiapophyseal fossa; podl-f, intralaminarfossa of postzygodiapophyseal lamina; pp, parapophysis; pr, prezygapophysis; prcdf, prezygocentrodiapophyseal fossa; prdl, prezygodiapophyseal lamina.

26

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 22. Dorsal vertebrae of FPDM-V-8461. Anterior views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 8th (H), 9th (I), 10th (J)and 11th (K) dorsal vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; cprf, centroprezygapophyseal fossa; cprl, centroprezygapophyseal
lamina; dp, diapophysis; ha, hypantrum; pp, parapophysis; pr, prezygapophysis; prcdf, prezygocentrodiapophyseal fossa; prdl, prezygodiapophyseal
lamina; sprf, spinoprezygapophyseal fossa.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

27

FIGURE 23. Dorsal vertebrae of FPDM-V-8461. Posterior views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6h (F), 7th (G), 8th (H), 9th (I), 10th (J)and 11th (K) dorsal vertebrae. Abbreviations: cpof, centropostzygapophyseal fossa; cpol, centropostzygapophyseal lamina; dp, diapophysis; ep,
epipophysis; hp, hypapophysis; hs, hyposphene; pcdl, posterior centrodiapophyseal lamina; po, postzygapophysis; pocdf, postzygocentrodiapophyseal
fossa; podl, postzygodiapophyseal lamina; pp, parapophysis; spof, spinopostzygapophyseal fossa.

28

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 24. Dorsal vertebrae of FPDM-V-8461. Dorsal views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 8th (H), 9th (I), 10th (J) and11th (K) dorsal vertebrae. Abbreviations: dp, diapophysis; ep, epipophysis; pcdl, posterior centrodiapophyseal lamina; po, postzygapophysis; podl,
postzygodiapophyseal lamina; pp, parapophysis; pr, prezygapophysis; prdl, prezygodiapophyseal lamina; sprf, spinoprezygapophyseal fossa.

centrum in lateral view as in cervical vertebrae. The intralaminarfossa of postzygodiapophyseal lamina divides thepostzygodiapophyseal lamina dorsoventrally (Fig. 21A) as inposterior cervicals (Fig. 16H–J). The transverse processes
bearing the diapophyses are displaced anteriorly to conjoin withthe prezygapophyses and widely separated from thepostzygapophyses in dorsal view (Fig. 24A) as in posteriorcervical vertebrae (Fig. 19H–J). The epipophysis is present

above the postzygapophyseal facet (Fig. 21A) and much smaller
than those of postaxial cervicals (Fig. 16C–J). The neural spine
is anteroposteriorly narrow in lateral view (Fig. 21A) as inposterior cervicals (Fig. 16H–J). The transverse process emerges
from the lateral margin of the prezygapophysis in dorsal view(Fig. 24A) as in the 10th and 11th cervicals (Fig. 19I, J).
However, in anterior view, the transverse process is long anddirected laterodorsally as in other dorsals (Fig. 22) while the one

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

29

FIGURE 25. Dorsal vertebrae of FPDM-V-8461. Ventral views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 8th (H), 9th (I), 10th (J)and 11th (K) dorsal vertebrae. Abbreviations: acdl, anterior centrodiapophyseal lamina; cdf, centrodiapophyseal fossa; dp, diapophysis; ep, epipophysis;
hp, hypapophysis; hs, hyposphene; pcdl, posterior centrodiapophyseal lamina; po, postzygapophysis; podl, postzygodiapophyseal lamina; pp,
parapophysis; pr, prezygapophysis; prdl, prezygodiapophyseal lamina.

in posterior cervicals is short and directed lateroventrally (Fig.
17I, J). This condition makes the diapophysis more distant fromthe parapophysis to bear a dorsal rib with the capitulum further
away from the tuberculum, as in other dorsal vertebrae.
All dorsal centra are amphicoelous, while the anteroposterior
length is greater than the lateral width and the dorsoventral
height in each centrum (Figs. 21–23). The first to third dorsal
centra are ventrally pinched to form small hypapophyses (Fig.

25A–C) as in the posteriormost cervical (Fig. 20J), whereas theventral margin of the first centrum is incompletely preserved.
The depth of each preserved hypapophysis is greater than thoseof Harpymimus (ventral keel sensu Kobayashi and Barsbold,
2005b), while slightly less than those of Nqwebasaurus(Choiniere et al., 2012), Mononykus (ventral process sensu Perle
et al., 1994) and Falcarius (Zanno, 2010). In addition, theventrally pinched centrum is present in more posterior dorsals to30

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 26. Fused sacral vertebrae with ribs of FPDM-V-8461. First through fourth sacral vertebrae with ribs in dorsal (A), anterior (B), left lateral (C),posterior (D), right lateral (E) and ventral (F) views. Abbreviations: acdl-f, intralaminar fossa of anterior centrodiapophyseal lamina; cdf,
centrodiapophyseal fossa; hs, hyposphene; ic, ilium contact; ns, neural spine; pcdl, posterior centrodiapophyseal lamina; pd, pleurocentral depression; po,postzygapophysis; pocdf, postzygocentrodiapophyseal fossa; prcdf, prezygocentrodiapophyseal fossa; spof, spinopostzygapophyseal fossa; sprf,
spinoprezygapophyseal fossa; sr, sacral rib; src, sacral rib contact; tp, transverse process.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

31

FIGURE 27. Isolated sacral vertebra with fused ribs of FPDM-V-8461. Fifth sacral vertebra with fused ribs in dorsal (A), anterior (B), left lateral (C),posterior (D), right lateral (E) and ventral (F) views, and its dorsal part in left posterolateral view (G). Abbreviations: hs, hyposphene; po,postzygapophysis; pocdf, postzygocentrodiapophyseal fossa; prcdf, prezygocentrodiapophyseal fossa; spof, spinopostzygapophyseal fossa; sprf,
spinoprezygapophyseal fossa; sr, sacral rib.

the sixth, gradually becoming less prominent (Fig. 25D–F). In
lateral view the parapophyses are situated entirely ventral to thetransverse processes (Fig. 21C–J) unlike those of alvarezsaurs(Choiniere et al., 2010a), while it approaches to the level withthe ventral margin of the transverse processes in the 11th
cervical (Fig. 21K). In anterior view, the parapophysis are
distinctly projected laterally below the base of the
prezygapophyses (Fig. 22). In dorsal view, the parapophyses are
partially obscured by the transverse process in the third andfourth dorsals (Fig. 24C, D), whereas they are situated entirely
anteriorly in more posterior dorsals (Fig. 24E–K). At least in thethird dorsal, the parapophysis is hypertrophied, with thedorsoventral depth of the parapophyseal facet greater than thehalf of that of the anterior articular surface (Fig. 21C) as in theanterior dorsals of Avimimus, therizinosaurids and sometyrannosauroids (Zanno et al., 2009; Brusatte et al., 2014). The
parapophysis is the largest in the third among dorsal vertebrae
and become gradually smaller in more posterior dorsal vertebrae(Fig. 21C–K). In the 9th and more posterior dorsals, theparapophysis is projected well laterally (Fig. 22I–K) like thestalked condition seen in alvarezsaurs and dromaeosaurids(Norell and Makovicky, 1999; Hwang et al., 2004). Instead of
the pneumatic foramen, an indistinct pleurocentral depression ispresent on each lateral side of dorsal vertebra, which is lessdistinct in anterior five centra due to their ventrally pinched
condition (Fig. 21).
The neural arch is dorsoventrally deep, with the depth
between its base and the prezygodiapophyseal lamina greater

32

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 28. Anterior to middle caudal vertebrae of FPDM-V-8461. Left lateral views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 9th(H), 10th (I), 11th (J), 12th (K), 13th (L), 14th (M) and 15th (N) caudal vertebrae. Abbreviations: aa, anterior ala of neural spine; acdl, anterior
centrodiapophyseal lamina; fm, bone fragment; ha, hypantrum; mp, mound-like dorsal projection; pa, posterior ala of neural spine; po, postzygapophysis;
pocdf, postzygocentrodiapophyseal fossa; pr, prezygapophysis; prdl, prezygodiapophyseal lamina; tp, transverse process.

than that of the centrum at least in the third dorsal (Fig. 22) as in
the anterior dorsals of therizinosaurs (Zanno et al., 2009). In
lateral view, the prezygapophyses are at the same level with thepostzygapophyses (Fig. 21) unlike those of ornithomimids andlate-diverging tyrannosauroids in which the former locatedbelow the level of the latter (Brusatte et al., 2010). Just below
the prezygapophysis, the centroprezygapophyseal fossa ispresent in the anterior dorsals, whereas it is absent in the fifth
and more posterior dorsals (Fig. 22). The hypantrum is absent in
the third, while present in more posterior dorsals, and its facet isrelatively broad, dorsoventrally narrow and anteroposteriorly

elongated.
The transverse processes of the anterior dorsal vertebrae are
mediolaterally long and dorsoventrally thin (Fig. 22). The
prezygodiapophyseal lamina forms the anterior margin of thetransverse process in the first to eighth dorsals (Figs. 21C–H,
22A, C–H). Similarly, the prezygocentrodiapophyseal fossa ispresent in the first to eighth dorsals, with the remnant in the
ninth (Fig. 21I). The anterior and posterior centrodiapophyseal
laminae exist throughout the dorsal series and contact with eachother on the ventral surface of transverse process, demarcating a
triangular centrodiapophyseal fossa (Fig. 21). The anterior

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

33

FIGURE 29. Middle to posterior caudal vertebrae of FPDM-V-8461. Left lateral views of 16th (A), 17th (B),18th (C), 19th (D), 20th (E), 21st (F), 22nd(G), 23rd (H), 24th (I), 25h (J), 26th (K), 27th (L), 28th (M), 29th (N), 30th (O) and 31st–32nd (P) caudal vertebrae. Abbreviations: f, bone fragment; g,
gap between fused centra and fused zygapophyses; po, postzygapophysis; pr, prezygapophysis.

centrodiapophyseal lamina forms the anterior margin of thetransverse process in the 9th to 11th dorsals instead of thedisappearing prezygodiapophyseal lamina (Fig. 21I–K).
The hyposphene lamina is flared lateroventrally in the thirdand more posterior dorsal vertebrae (Fig. 23C–K). In addition,
the hyposphene lamina is strongly curved lateroventrally in the
middle and posterior dorsals to form a semi-canal (i.e.,
open-half-cylinder in shape) in posterior view with thepostzygapophyseal facet (Azuma et al., 2016). Such feature isalso present in the sacral and anteriormost caudal vertebrae(Figs. 26D, 27D, 31A). The opposite hyposphenes are separated
as two laminae and ventrally conjoined by an inset web of bonecomprising dorsal margin of the neural canal (Figs. 24C–K,
25C–K) as in ornithomimosaurs and some tyrannosauroids(Turner et al., 2012; Brusatte et al., 2014).
The dorsal neural spines are essentially rectangular in thethird and more posterior dorsal vertebrae, and its dorsal ends areflared only slightly anteroposteriorly in lateral view (Fig.
21C–K) to form the spine table in anterior view (Fig. 22) unlike
some theropods (Makovicky and Sues, 1998). The neural spine

ends at nearly the same level as the posterior centrum face in thethird and more posterior dorsals (Fig. 21C–K). In well-preserved
dorsal neural spines of the sixth, seventh and eighth dorsals, theinterspinous ligament scars are visible in the anterior andposterior aspects and ends slightly below the apex of the neuralspine (Fig. 21F–H) as in deinonychosaurs (Turner et al., 2012).
In the posterior dorsals, the neural spines are slightly greater inheight than the anteroposterior width (Fig. 21I–K).
Sacral vertebrae
The sacral series consists of four fused and one isolated sacralvertebrae (Figs. 26, 27; Table 2), with most of neural spines
missing in the first four. In contrast to Azuma et al. (2016)
which regards the isolated one as the sixth or more posterior
sacral, CT analyses performed in the present study identify it as
the fifth with its anterior surface of the centrum matching theposterior surface of the fused sacrals. In addition, the posterior
surface of the fifth is mostly flat and does not exhibit any traces
for fusion (Fig. 27D), suggesting that it articulates with theunfused first caudal vertebra. The ventral margin of the fifthsacral is inclined posteroventrally in lateral view, forming the34

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 30. Caudal vertebrae of FPDM-V-8461. Anterior views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 9th (H), 10th (I), 11th (J),12th (K), 13th (L), 14th (ML), 15th (N), 16th (O), 17th (P),18th (Q), 19th (R), 20th (S), 21st (T), 22nd (U), 23rd (V), 24th (W), 25th (X), 26th (Y), 27th(Z), 28th (AA), 29th (AB), 30th (AC) and 31st–32nd (AD) caudal vertebrae. Abbreviations: f, bone fragment; ha, hypantrum; pr, prezygapophysis; sprf,
spinoprezygapophyseal fossa; tp, transverse process.

arched ventral margin of the sacrum (Figs. 26C, 27C). In ventral
view, the second to fifth sacrals exhibit ventrally-flattened
centra with extremely shallow sulcus at least in a portion of each(Figs. 26F, 27F). Only the posterior half of the isolated sacralvertebra shows a keeled ventral surface (Fig. 27F), lessprominent than those in alvarezsaurids (Perle et al., 1994;
Novas, 1997). Pneumatic foramina are absent on lateral surfacesof sacral vertebrae, whereas a pleurocentral depression is present
on each lateral side of the first centrum (Fig. 26C, E).
Parapophyses are clearly absent in the first sacral vertebra(Fig. 26E). Instead, the anterior centrodiapophyseal lamina is

divided into two to bear an intralaminar fossa as a narrow sulcus
between them. In the second to fourth sacrals, sutures against
the sacral ribs are expanded anteriorly from the diapophyseal
region to invade the anterodorsal margin including theparapophyseal region of each vertebra to reach the border
against the preceding sacral (Fig. 26C, E). The anterior marginof this suture never or only slightly invades the posterior marginof each preceding sacral centrum. The right rib of the third
sacral is nearly complete and is bifurcated dorsoventrally (Fig.
26E). The dorsal part of the rib is pointed laterodorsally to form
a narrow facet for the ilium, whereas the ventral part projects

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

35

FIGURE 31. Caudal vertebrae of FPDM-V-8461. Posterior views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 9th (H), 10th (I), 11th (J),12th (K), 13th (L), 14th (M), 15th (N), 16th (O), 17th (P),18th (Q), 19th (R), 20th (S), 21st (T), 22nd (U), 23rd (V), 24th (W), 25th (X), 26th (Y), 27th(Z), 28th (AA), 29th (AB), 30th (AC) and 31st–32nd (AD) caudal vertebrae. Abbreviations: f, bone fragment; hs, hyposphene; po, postzygapophysis; pr,prezygapophysis; spof, spinopostzygapophyseal fossa; tp, transverse process.

horizontally and expands anteroposteriorly to form a long iliac
facet. The fifth sacral ribs are completely fused to the fifth sacralvertebra, where the suture between them appear indistinct anddo not invade the posterior margin of the preceding sacral (Fig.
27C, E). Additionally, the fifth sacral rib is bifurcateddorsoventrally as in the third, although the dorsal part of theformer is well expanded to form a long facet for the ilium as in
the ventral part. The zygapophyses are fused to each other at
least in the first to fourth sacrals to form a platform lateral to theneural spines (Fig. 26A) as in dromaeosaurids (Norell and
Makovicky, 2004). The hyposphene preserved in the fifth sacralis small and not sheet-like (Fig. 27D) as in most coelurosaurs(Brusatte et al., 2010). The pre- and postzygocentrodiapophyseal
fossae are well excavated besides the lacking
prezygocentrodiapophyseal fossa in the first (Fig. 26A, C). The
postzygocentrodiapophyseal fossa in the fifth sacral exhibits asmall pneumatic foramen within it on the left side (Fig. 27G) as
in the first caudal vertebra (Fig. 28A).
Caudal vertebrae
Preserved 31 caudal vertebrae represent most part of acomplete set of 32 caudals, lacking the 8th and the majority of
the 4th (Figs. 28–33). The anterior caudals include nine36

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 32. Caudal vertebrae of FPDM-V-8461. Dorsal views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 9th (H), 10th (I), 11th (J),12th (K), 13th (L), 14th (M), 15th (N), 16th (O), 17th (P),18th (Q), 19th (R), 20th (S), 21st (T), 22nd (U), 23rd (V), 24th (W), 25th (X), 26th (Y), 27th(Z), 28th (AA), 29th (AB), 30th (AC) and 31st-32nd (AD) caudal vertebrae. Abbreviations: f, bone fragment; mp, mound-like dorsal projection; po,postzygapophysis; pr, prezygapophysis; sprf, spinoprezygapophyseal fossa; tp, transverse process.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

37

FIGURE 33. Caudal vertebrae of FPDM-V-8461. Ventral views of 1st (A), 2nd (B), 3rd (C), 4th (D), 5th (E), 6th (F), 7th (G), 9th (H), 10th (I), 11th (J),12th (K), 13th (L), 14th (M), 15th (N), 16th (O), 17th (P),18th (Q), 19th (R), 20th (S), 21st (T), 22nd (U), 23rd (V), 24th (W), 25th (X), 26th (Y), 27th(Z), 28th (AA), 29th (AB), 30th (AC) and 31st-32nd (AD) caudal vertebrae. Abbreviations: ha, hypantrum; po, postzygapophysis; pr, prezygapophysis;
tp, transverse process; vs, ventral sulcus.

38

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMATABLE 2. Measurements on the vertebrae of FPDM-V-8461.

*Specimen incomplete in measured dimension.

TABLE 3. Measurements on the chevrons of FPDM-V-8461.

*Specimen incomplete in measured dimension.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

39

FIGURE 34. Chevrons of FPDM-V-8461. Dorsal (A–F), left lateral (G–L), ventral (M–R), anterior (S–X) and posterior (Y–AD) views of 9th (A, G, M,
S, Y), 11th, (B, H, N, T, Z), 14th (C, I, O, U, AA), 15th (D, J, P, V, AB), 16th (E, K, Q, W, AC) and 17th (F, L, R, X, AD) chevrons.

preserved caudals with distinct transverse processes (Fig.
30A–I) which are absent in the posterior ones represented by the
11th and more posterior caudals (Fig. 30J–AD). The loss of thetransverse processes represents a transitional point between theanterior and posterior caudals, which occurs more anteriorly inalvarezsaurs and paravians (Makovicky et al., 2005; Turner etal., 2012) or absent in oviraptorosaurs (Osmólska et al., 2004).
Notably, the posteriormost two caudals are fused with eachother (Fig. 29P), and only about 1.5 times longer than thepreceding caudal vertebrae. The fragments between
prezygapophyses in the 28th and fused posteriormost caudals
are probably derived from the postzygapophyses of the40

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

preceding caudals.
The caudal centra lack any pneumatic foramina and are allamphiplatyan except for the posterior caudals (Figs. 28, 29). The
anteroposterior length of each caudal vertebra including pre- andpostzygapophyses becomes greater posteriorly at least to the18th caudal, which is nearly 50% longer anteroventrally than thelongest dorsal vertebra (11th; Table 2). Similarly, the centrum
length of each caudal becomes greater to the 16th, whereas its
mediolateral width and dorsoventral height are reduced
sequentially (Figs. 30, 31). In the first three caudals, eachcentrum is box-like in lateral view and its transverse section isnearly rectangular (Fig. 28A–C). The ventral surface of each 1st
to 14th caudal centra exhibits a longitudinal sulcus, whichbecomes shallower posteriorly into nearly completely flat in the
16th and more posterior caudals (Fig. 33). In the fifth and moreposterior caudal centra, the anteroposterior length is greater than
twice the mediolateral width and dorsoventral height (Fig. 31).
In lateral view, each neural spine of the first two caudals
exhibits beam-like shape and its posteroventral margin issituated slightly anterior to the posterior surface of the centrum(Fig. 28A, B). In the third and subsequent caudals, thedorsoventral height of each neural spine is low anteriorly andtall posteriorly, expressing L-shaped condition in lateral view(Fig. 28C–I). In the 5th to 10th caudals, the neural spine
becomes lower and separated into the anterior and posterior alae
(Fig. 28E–I), with the former absent in the 11th to 18th caudals(Figs. 28J–N, 29A–C). In more posterior caudals, the remnant ofneural spine is present as a low ridge (Fig. 29D–P) as in sometyrannosauroids, ornithomimosaurs, therizinosaurs,
oviraptorosaurs and avialans (Russell and Dong, 1993; Turner etal., 2012).
The transverse processes of the anterior caudals appear
sub-rectangular and slightly narrow distally in dorsal view (Fig.
32). In the anterior caudals, the mediolateral length of eachtransverse process is subequal to the mediolateral width of thecentrum (Fig. 30A–G), whereas such feature is obscure in the
9th and apparently reduced in the 10th (Fig. 30H, I). Although
the diapophysis is absent in all caudals, the remnant of anterior
centrodiapophyseal lamina is visible in the left lateral view ofthe first caudal (Fig. 28A). In the first caudal, a slightpneumaticity is present within the postzygodiapophyseal fossa
in lateral view, as in the fifth sacral (Fig. 27G). In the second
and third caudals, a low, mound-like dorsal projection isobservable just above the proximoanterior margin of thetransverse process (Figs. 28B, 32B).
In the posterior caudals, the prezygapophyses are elongatedwell anteriorly (Fig. 29). In the well-preserved posterior caudals,
the prezygapophyses overlap about the posterior third or more of
the preceding caudal centra, whereas the postzygapophyses
overlap only the anterior fifth or less of the following caudal
centra (Figs. 28K–M, 29B, C, J). The distal end of the
prezygapophysis is preserved in both sides of the 14th caudal
and bifurcated (Figs. 28M, 32M, 33M) unlike other known
dinosaurs (Azuma et al., 2016). In 14–16th, 19th and 22nd
caudals, a narrow spine-like process is preserved posterior to thepostzygapophyseal facet (Figs. 28M, N, 29A, D, G). The
postzygapophyseal facet is indistinct in the 22nd and moreposterior caudals (Fig. 29H–N).
As mentioned, the posteriormost two caudals (31st and 32nd)
are fused with each other. The suture between these two centra
is marked by a slight eminence (Fig. 32AD) unlike theprominent line or tubercle seen in Beipiaosaurus (Liao et al.,2021). The suture is also visible between the neural arches dueto the remnants of zygapophyses (Figs. 29P, 32AD). In addition,a narrow gap between the fused centra and fused zygapophyses
is visible in lateral view (Fig. 29P), which are unreported inother theropods with fused posteriormost caudals. The dorsal
margin of the posteriormost caudals bears a narrow, nearly flat
dorsal surface (Fig. 32AD), whereas the ventral margin forms abroader, flat or slightly concave ventral surface (Fig. 33AD) as
in the preceding caudals (Fig. 33O–AC) and the pygostyle of
Fukuipteryx (Imai et al., 2019), in contrast to other fused
posteriormost caudals in non-pygostylian theropods (Lee et al.,2014; Barsbold et al., 2000a:fig. 1). The anterior and posterior
ends of the ventral margin projects ventrally to form an arch inlateral view (Fig. 29P). The anterior surface of the fusedcentra is concave to fit with the bulbous posterior surface of the
30th caudal (Fig. 29O, P).
Chevrons
Preserved six chevrons comprise two anterior and fourposterior ones (Fig. 34). Each of them engages with theposteroventral margin of the 9th, 11th, 14–17th caudals (Fig. 2).
Because it is unknown whether every caudal vertebra isequipped with a chevron, these chevrons are numbered hereaccording with the corresponding caudal vertebra. As preserved,
both the anterior and posterior chevrons are relatively long,
slender, and recurved posteriorly in distal portions (Fig. 34G–K;
Table 3). In dorsal view, the middle part of the haemal canal iscovered by a narrow bridge bearing an articular facet for thecaudal vertebrae (Fig. 34A–F). The articular facet is laterally
concave and anteroposteriorly convex.
The proximal margins of the lateral walls of the haemal canal
are anteroposteriorly expanded to form paired processes directedboth anteriorly and posteriorly (Fig. 34B–F), which are missingin the 9th chevron (Fig. 34A). Posterior three chevrons bear
horizontal ridges on lateral sides at the level of theanteroposterior expansion (Fig. 34J–L). The lateral walls distally
become restricted anteroposteriorly and meet at the sagittal
plane to form a distal blade (Fig. 34S–AD). The blade is more
prominently restricted and recurved posteriorly in the posteriorchevrons than the anterior ones (Fig. 34G–K). The distal ends ofthe posterior chevrons are not expanded anteroposteriorly as in
the anterior chevrons (Fig. 34I, K).
Cervical ribs
Preserved nine cervical ribs represent left 5th and 7th to 11th
and right 6th, 9th and 11th (Figs. 35, 36). Although possible
another cervical rib is adhered to the anterolateral part of theninth cervical centrum (Fig. 17H), its identification is
problematic due to the severe damage causing unmatched
articulation to any cervical diapophyses or parapophyses.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

41

FIGURE 35. Cervical ribs of FPDM-V-8461. Lateral (A–I), anterior (J–R) and posterior (S–AA) views of left 5th (A, J, AA), 7th (B, K, Z), 8th (C, L, Y),
9th (D, M, X), 10th (E, N, W) and 11th (F, O, V) and right 6th (I, R, S), 9th (H, Q, T) and 11th (G, P, U) cervical ribs. Abbreviations: ap, anterior process;cf, capitular facet; ctw, capitular-tubercular web; f, bone fragment; n, notch for probable contact with preceding rib; tf, tubercular facet; tl, tubercularlamina.

42

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 36. Cervical ribs of FPDM-V-8461. Medial (A–I), dorsal (J–R) and ventral (S–AA) views of left 5th (D, J, S), 7th (C, K, T), 8th (B, L, U), 9th(A, M, V), 10th (F, N, W) and 11th (E, O, X) and right 6th (G, R, AA), 9th (H, Q, Z) and 11th (I, P, Y) cervical ribs. Abbreviations: cf, capitular facet;ctw, capitular-tubercular web; n, notch for probable contact with preceding rib; pf, pneumatic foramen; tf, tubercular facet; tl, tubercular lamina.

OSTEOLOGY OF FUKUIVENATOR PARADOXUSTABLE 4. Measurements on the cervical ribs of FPDM-V-8461.

*Specimen incomplete in measured dimension.

TABLE 5. Measurements on the dorsal ribs of FPDM-V-8461.

*Specimen incomplete in measured dimension.

43

When in articulation with the corresponding cervical vertebra,
each cervical rib is triangular in lateral view by a tuberculum
directed dorsally and the shaft directed posteriorly or
posteroventrally, the latter being much shorter than those of
dorsal ribs (Figs. 35A–I; Tables 4, 5). The capitulum is directed
medially or mediodorsally and not visible in lateral view. In
addition, a short anterior process is present at the anteroventral
corner except in the posteriormost cervical rib. The anterior
process bears a longitudinal ridge projected ventrally that
continues toward the distal end of the shaft (Fig. 36S–AA). Thisridge forms the lateral margin of the concave ventral surface ofthe capitulum. In the eighth and ninth ribs, a sharp longitudinalnotch is present on the lateroventral side of the anterior end of
the longitudinal ridge for a possible contact with the distal endof the preceding ribs and makes the anterior process bifurcated(Figs. 35C, D, H, 36U, V, Z). The size of tubercular facet isabout one-half of that of capitular facet, which is elongatedanteroposteriorly (Fig. 36J–R). The incomplete shafts of thefifth and sixth cervical ribs are distally expanded mediolaterally,
whereas those of the eighth and more posterior ribs are tapered(Fig. 36S–AA).
In dorsal view, a deep pneumatic cavity excavates the main
body between the tubercular and capitular facets and continues
posteriorly to form a concave dorsal surface of the shaft (Fig.
36J–R). The cavity is surrounded and divided by thin laminae
best developed in the anterior margin corresponding to thecapitular-tubercular web (sensu Currie and Zhao, 1993a) seen in
the dorsal ribs. In the 9th and 10th ribs, another lamina extends
posteroventrally from the medial margin of the tubercular facet(Figs. 35T, U, Z, 36A, F, I), and the remnant of this lamina ispresent in the 11th rib (Figs. 35S, AA, 36G, H). The cavity
posterior to the capitular-tubercular web is further excavated byseveral foramina in the right sixth, in which the capitulum andtuberculum bear two and one foramina, respectively. In dorsal
view, another small cavity excavates the anterior process anddivided by thin, laterally-oriented laminae (Fig. 36L–N, Q).
Dorsal ribs
While most dorsal ribs are fragmented, right ones are better
preserved than the left ones, including some nearly complete
series lacking the second and fifth ribs (Figs. 37–41; Table 5).
The distal end is not completely preserved in any dorsal ribs. No
trace of ossified uncinate process is recognizable. When inarticulation with a corresponding dorsal vertebra, thelongitudinal axis of the capitulum is oriented anteromedially(Fig. 41A–H) and the shaft and the tuberculum exhibit acontinuous flat surface facing laterally (Fig. 39A–H).
In the first dorsal rib, the capitulum is widely separated fromthe tuberculum by a notch in anterior view (Fig. 37D). In thethird and more posterior dorsal ribs with the proximal partpreserved, the notch is occupied by the capitular-tubercular web,which is much less developed in the first rib. The capitular facet
is preserved in the 1st, 3rd, 6th, 7th, 8th and 11th. Among them,
the area of the facet is largest in the third rib and successively
reduced in more posterior ribs (Fig. 40).
Proximally, the lateral margin of the shaft is expanded44

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 37. Dorsal ribs of FPDM-V-8461. Anterior views of right 1st (D), 3rd (C), 4th (B), 6th (A), 7th (H), 8th (G), 10th (F) and 11th (E) dorsal ribs.
Abbreviations: air, anterior intercostal ridge; cf, capitular facet; cg, costal groove; ctw, capitular-tubercular web; tf, tubercular facet.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

45

FIGURE 38. Dorsal ribs of FPDM-V-8461. Posterior views of right 1st (A), 2nd (B), 4th (C), 6th (D), 7th (E), 8th (F), 10th (G) and 11th (H) dorsal ribs.
Abbreviations: ctw, capitular-tubercular web; pir, posterior intercostal ridge; tf, tubercular facet.

46

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 39. Dorsal ribs of FPDM-V-8461. Lateral views of right 1st (E), 2nd (D), 4th (C), 6th (B), 7th (A), 8th (H), 10th (G) and 11th (F) dorsal ribs.
Abbreviations: cg, costal groove; ctw, capitular-tubercular web.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

47

FIGURE 40. Dorsal ribs of FPDM-V-8461. Medial views of right 1st (A), 2nd (B), 4th (C), 6th (D), 7th (E), 8th (F), 10th (G) and 11th (H) dorsal ribs.
Abbreviations: air, anterior intercostal ridge; cf, capitular facet; cg, costal groove; pir, posterior intercostal ridge; tf, tubercular facet.

48

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 41. Dorsal ribs of FPDM-V-8461. Dorsal (A–H) and ventral (I–P) views of right 1st (A, L), 2nd (B, K), 4th (C, J), 6th (D, I), 7th (E, P), 8th (F,O), 10th (G, N) and 11th (H, M) dorsal ribs. Abbreviations: air, anterior intercostal ridge; cf, capitular facet; ctw, capitular-tubercular web; pir, posterior
intercostal ridge; tf, tubercular facet.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

49

FIGURE 42. Gastralia of FPDM-V-8461. Right medial gastralia in dorsal (A, G, M), anterior (B, H, N), medial (C, I, O), posterior (D, J, P), lateral (E,K, Q) and ventral (F, L, R) views. Abbreviations: cag, caudal lateral groove; crg, cranial lateral groove; vb, ventral boss; vg, ventral groove; wle,
wing-like expansion.

anteroposteriorly to form a flat lateral surface. Such lateralexpansion is extensive in the first four among the thoracic series,
probably for bearing the pectoral girdle (Fig. 39C–E). In
contrast, the medial margin is narrow anteroposteriorly (Fig.
40), which makes a T-shaped cross section of the proximal
shaft. The anterior and posterior margins of the lateral expansion
represent intercostal ridges that define lateral margins of theconcave anterior and posterior surfaces. The anterior ridge isdeeper than the posterior ridge in the first rib, whereas, in thethird and more posterior ribs, the anterior ridge is shallower anddiminishes more proximally than the posterior ridge. In addition,
the anterior ridge is medially invaded by the costal groove,
which is particularly elongated in the 8th and 10th ribs (Fig. 37).
In anterior ribs, the intercostal ridges diminish before reachinglateral vertex of the shaft, so that only the flange-likemedial margin remains in the distal part to form a
mediolaterally-elongated elliptical cross section of the rib (Fig.
41K). However, the medial margin is twisted posteriorly in theposterior ribs to form an anteroposteriorly-elongated ellipticalcross section of the distal part (Fig. 41M–P). The tuberculum ispresent as a proximal protrusion of the shaft so that it alsobears the intercostal ridge (Figs. 37, 38). The anterior concave
surface of the capitular-tubercular web is distinguished fromthe one in the shaft by a stout ridge at the base of the
tuberculum in the first rib (Fig. 37D). The tubercular facet
faces proximomedially with an elliptical outline (Figs. 40,
41A–H), somewhat anteriorly in the third rib (Fig. 37C), andposteriorly in the 1st, 7th and 10th ribs (Fig. 38A, E, G).
Gastralia
Although several possible fragments of gastralia arecollected, three of them are identified and described here (Fig.
42). Each of the three is the right medial gastralium preserving
its medial ends. In anterior view, the shaft is curved
laterodorsally and becomes thinner toward the medial end (Fig.
42B, H, N). The lateral part of the shaft bears longitudinalgrooves on both the anterior (craniolateral groove sensu

50

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 43. Scapula of FPDM-V-8461. Left scapula in dorsal (A), anterior (B), lateral (C), posterior (D), medial (E) and ventral (F) views.
Abbreviations: ap, acromion process; cc, coracoid contact; gf, glenoid fossa; nf, nutrient foramen; pgf, paraglenoid fossa.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

51

FIGURE 44. Pectoral girdle of FPDM-V-8461. Left pectoral girdle (A) and coracoid (B–G) in lateral (A, D), dorsal (B), anterior (C), posterior (E), medial(F) and ventral (G) views. Abbreviations: ap, acromion process; cf, coracoid foramen; ct, coracoid tuber; gf, glenoid fossa; pvp, posteroventral process; sc,scapula contact; sf, subglenoid fossa; vb, ventral blade.

Claessens, 2004) and posterior aspects, with the latter
continuing more medially than the former (Fig. 42D, J, P).
Medially, the shaft is expanded posteriorly in dorsal view
(wing-like expansion sensu Claessens, 2004) to form relatively
broad dorsal surface, while its width is constricted near the
medial end by the depression of the posterior margin (Fig.

42A, G, M). The restricted part bears a short longitudinalgroove which faces ventrally and continues to the concave
ventral surface of the neighboring expanded part (Fig. 42F, L,
R). Slightly lateral to the expanded part, the ventral surface
bears a blunt to prominent boss near the anterior margin.

52

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMATABLE 6. Measurements on the pectoral girdles of FPDM-V-8461.

*Specimen incomplete in measured dimension.

Appendicular skeleton
ScapulaBoth scapulae are preserved well. The left is nearly complete(Fig. 43) whereas the distal end and the acromion margin of theright scapula are damaged. The scapula is dorsoventrally
elongated and approximately straight in lateral view (Fig. 43C).
The dorsoventral height of the scapula is about nine times
greater than the minimum anteroposterior width (14.5 mm),being approximately equal to the proximodistal length of thehumerus (Table 7). Both the anterior and posterior margins of
the scapular blade are slightly arched to constrict theanteroposterior width at its midlength. The ventrally facing
contact surface for the coracoid is rugose and bears alongitudinal depression at its midline (Fig. 43F). In lateral view,the coracoid contact is almost straight and slightly swelled
ventrally in its posterior half (Figs. 43C, 44A).
In the ventral fourth of the scapula, the acromion process
projects anteriorly from the anterior margin of the blade (Fig.
43C). The ventral half of the lateral surface of the acromion
region is concave to compose a broad fossa with the dorsal halfof the coracoid (Fig. 44A). At the level of the acromion process,the posterior margin of the scapula curves posteroventrally tooffset the glenoid well from the blade. The glenoid fossa of thescapula faces posteroventrally and its contribution to the glenoid
is greater than the one of coracoid as in other maniraptorans
except for alvarezsaurs and late-diverging dromaeosaurids(Brusatte et al., 2014). In lateral view, the angle between thescapula and coracoid within the glenoid is approximately 110
degrees, much greater than the approximate right angle seen intyrannosauroids and alvarezsaurs (Brusatte et al., 2010), while
the glenoid is rather like a continuous arc. On the lateral surface
immediately anterior to the glenoid, a deep, circular fossa ispresent as in Coelurus and Tanycolagreus (Brusatte et al.,2014), termed here as the paraglenoid fossa (Fig. 43C).
Coracoid
The left coracoid is complete (Fig. 44), whereas the rightlacks the anteroventral part. The coracoid is anteroposteriorly
longer than dorsoventrally high (Table 6). The ventral blade isshallow and the posteroventral process is elongated (Fig. 44D,
F) as in alvarezsaurs (Choiniere et al., 2010b) andornithomimosaurs (Makovicky et al., 2004). The anterodorsal

part of the lateral surface is concave to form a broad fossa withthe anteroventral part of the scapula (Fig. 44A). The coracoid
foramen penetrates mediodorsally from the posterior part of theconcavity (Fig. 44D). The ventral half of the coracoid forms theventral blade by a thin bony plate with gently curved ventraledge. The ventral blade and the anterodorsal concavity areseparated by a blunt, anteroposteriorly oriented ridge thatposteriorly meets a mound-like coracoid tuber (or biceps
tubercle). The posterior third of the ventral blade forms apointed posteroventral process (or postglenoid process), which
is well separated from the glenoid by a deep notch in lateralview. The glenoid occupies the posterodorsal corner of thecoracoid and faces mainly posterodorsally. A short ridgeconnects the coracoid tuber and the glenoid. Another ridge
emerges from the coracoid tuber toward the posteroventral
process of the coracoid. These two ridges are relatively bluntand only slightly project laterally to form the anteroventral rim
of a shallow subglenoid fossa, which contrasts with the deep
subglenoid fossa in paravians (Norell and Makovicky, 1999;
Hwang et al., 2002; Turner et al., 2012) and mostornithomimosaurs (Sereno, 2017).
The medial surface is flat ventrally, concave at the middle andconvex dorsally, so that the coracoid foramen exits on theconvex surface (Fig. 44F) as in some alvarezsaurids andMicrovenator (Brusatte et al., 2014). The dorsal surface isrugose and mediolaterally thin and thickens posteriorly to meet
the thickness of the glenoid (Fig. 44B). In lateral view, thedorsal margin is nearly straight with a shallow concavity in theposterior part to fit with the ventral margin of the scapula (Fig.
44A, D).
Humerus
The left humerus is well preserved with only missing
proximalmost part (Fig. 45). The right humerus preserves only
the distal end. The length of the humerus is approximately thesame with the scapula and about 70% of the femur (Table 7).
The overall shape is slightly sigmoidal in lateral view withcurving posteriorly in proximal whilst anteriorly in distal parts.
A large, distinct deltopectoral crest projects anteriorly fromthe proximal third of the humerus to make it quadrangular inlateral view (Fig.43C). The crest has approximately the same
anteroposterior thickness with the shaft in medial view (Fig.
45E). The anterior extremity of the crest has a slight medial
expansion to bear a shallowly concave surface facing
anterolaterally (Fig. 45A, B), possibly for the insertion of the m.
deltoideus clavicularis (Burch, 2014). This surface isdemarcated laterally by a ridge, which continues further distallywith keeping a constant distance from the anterior margin of thebone toward the presence of a distinct pit in at the middle of thela te ra l s urfa c e (Fig. 45C ). The la te ra l s urfa c e of t hedeltopectoral crest is flat to slightly concave. Distally, a verticalflute emerges at the level of the distal extremity of thedeltopectoral crest as in Falcarius and Microvenator (Zanno,2006). The flute runs distally along the lateral ridge toward thepit, whereas it curves posteriorly to avoid a slight tabular
protrusion emerging just proximal to the pit.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

53

FIGURE 45. Humerus of FPDM-V-8461. Left humerus in proximal (A), anterior (B) lateral (C), posterior (D), medial (E) and distal (F) views.
Abbreviations: dc, deltopectoral crest; dci, m. deltoideus clavicularis insertion; dd, m. distolateral depression; ecc, ectepicondyle; enc, entepicondyle; nf,
nutrient foramen; rc, radial condyle; tp, tabular process; uc, ulnar condyle; vf, vertical flute.

Distally, the humerus is expanded unusually medially inanterior view to form broad, slightly concave anterior andposterior surfaces in contrast to a narrow shaft (Fig. 45B). In
anterior view, the distal margin is perpendicular to the long axisof the bone and its width (35 mm) is about three times greaterthan the minimum width of the shaft (14 mm). The distal articularcondyles are bulbous and project more anteriorly than distally asin therizinosaurs and late-diverging alvarezsaurs (Turner et al.,2012). The radial condyle is expanded well proximally andanteroposteriorly so that it is apparently larger than the ulnar
condyle in both anterior and distal views (Fig. 45B, F). However,the ulnar condyle is expanded further medially than the radialcondyle laterally in distal view. The lateral surface of the distal
end bears a distinct depression resembling the origin scar of M.
extensor metacarpi radialis in avialans (Vanden Berge, 1975).
The entepicondyle is spherical in anterior view and separated
from the ulnar condyle by a distinct fossa on the anterodistal
margin between them. The entepicondyle is located well medial
to the ulnar condyle as in therizinosaurs (Zanno et al., 2009).
Ulna
The left ulna is nearly complete with missing the distal
extremity (Fig. 46B, C, F, H, I, K). The right ulna is represented
by the proximal half of the shaft lacking its posterior part of theproximal end. The ulna is relatively long as the preserved length

54

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 46. Radii and ulna of FPDM-V-8461. Proximal (A, B), anterior (C, D, P), lateral (E, F, O), posterior (G, H, N), medial (I, J, M) and distal (K,L, Q) views of left radius (A, D, E, G, J, L), left ulna (B, C, F, H, I, K) and distal part of right radius (M–Q). Abbreviations: alc, anterolateral crest; alp,
anterolateral process; amc, anteromedial crest; cp, coronoid process; lc, lateral cotyla; mc, medial cotyla; nf, nutrient foramen; ol, olecranon; plr,posterolateral ridge; pmr, posteromedial ridge; rc, radius contact.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

exceeds 90% of the humerus (Table 7) like those of alvarezsaurs
and some paravians (Clark et al., 2006; Turner et al., 2012),
while it is less than 70% in length of the femur.
In lateral view, the shaft is bowed posteriorly (Fig. 46F) as inmaniraptorans except for alvarezsaurs, therizinosauroids andsome oviraptorosaurs (Gauthier, 1986; Maryańska et al., 2002,
Brusatte et al., 2014) so that the ulna is well separated from theradius along their length except both ends when in articulation.
The shaft is proximally expanded anteriorly to bear smooth
triangular surfaces on the lateral and medial aspects (Fig. 46F,
I). The proximal end is anteroposteriorly higher than the
mediolaterally wide, whereas the olecranon is slightly wider
than high (Fig. 46B). The proximal end bears a prominentcoronoid process anterolateral to the large olecranon, which areseparated by a faint depression on the posterolateral aspect (Fig.
46F, H) and not hypertrophied as those of Compsognathus(Ostrom, 1978), Sinosauropteryx (Hwang et al., 2004) andalvarezsaurs (Choiniere et al., 2010b). The lateral surface justdorsal to the coronoid process bears a faint depression to contact
with the radius (Fig. 46C, F) and lacks the bicipital scar that isseen in most paravians (Turner et al., 2012). The lateral cotyla
situated proximally on the coronoid process is small and convex,
whereas the medial cotyla on the main shaft is large andconcave; these cotylae are not clearly separated and rather form
a continuous surface (Fig. 46B, C). The distal end is expanded
mediolaterally to reach the width (12.2 mm) twice as large as
the one of the midshaft (6.1 mm). The anterior margin of the
shaft is formed by a distinct longitudinal ridge through its
length, and it distally forms the lateral margin of the triangulardepression facing anteromedially. The lateral margin of this flat
surface contacts the posteromedial margin of the radius when inarticulation.
RadiusAlthough both radii are well preserved, the left lacks its distalend (Fig. 46A, D, E, G, J) and the right has somewhat damaged
proximal and distal surfaces (Fig. 46M, N). The nearly complete
length of the right radius reaches about 80% of the humerus(Table 7).
The shaft of the radius is approximately straight in bothanterior and lateral views (Fig. 46D, E, M, N). The proximal end
is unusually flared posterolaterally to separate the shaft from theulna. The proximal surface is deeply concave in both left andright radii so that the proximal end is notably funnel-shaped.
The shaft has a smooth medial surface which isanteroposteriorly broader in the middle part (Fig. 46J). Middle
parts of the antero- and posteromedial margins are formed byblunt longitudinal ridges, except for a short longitudinal crest in
the former at the proximal fourth of the shaft (Fig. 46D).
Proximal to this crest, another prominent crest emerges on thedorsal margin and extends toward the anterolateral margin of theproximal end. The lateral surface is flat with the proximal
portion slightly twisted anteriorly (Fig. 46E). The posterolateral
margin of the distal shaft is formed by a distinct ridge (Fig.
46G). The posterior surface between the posteromedial andposterolateral ridges are also flat, so that the cross section of the55

mid-shaft exhibits a sub-quadrangular profile. The distal end isflared anteroposteriorly to bear a broad articular surface for the
carpals (Fig. 46M–Q). In addition, a distinct process is present
in the anterolateral margin of the distal end.
Metacarpus
While carpal elements are missing, the complete left
metacarpus (Fig. 47A, G, O) and right metacarpal I arepreserved. Metacarpals exhibit no sign of fusion with any distal
carpal.
Metacarpal I is considerably shorter than other metacarpals
unlike those of ornithomimids (Pérez-Moreno et al., 1994), with
its length about the half of metacarpal II and 60% of metacarpal
III, while its length being greater than the width (Table 7). The
element is dorsoventrally thin and mediolaterally wide incontrast to metacarpal III (Fig. 48Y). Additionally, it is roughly
hourglass-shaped in dorsal view due to medially-expanded
proximal and distal ends (Fig. 47A). Even in the narrowest
point, the mediolateral width of metacarpal I is greater than thatof the midshaft of metacarpal II. The dorsal surface appears
slightly concave and faces somewhat medially. The proximal
end is triradiate in proximal view, with its sharpest apex directed
medially (Fig. 48V). The lateral surface of the proximal third isconcave (Fig. 47Y) to receive the convexity in theproximoventral part of the medial surface of metacarpal II (Fig.
48AF). The lateroventral margin of metacarpal I bears a small
process partially underlying the metacarpal II (Fig. 48Z) to alesser extent than the rectangular buttress of therizinosaurs(Russell and Dong, 1993). The distal end is marked by a highly
ginglymoid joint through well developed distal condyles, among
which the medial condyle is slightly larger than the lateral (Fig.
47D)
Metacarpal II is the longest among three metacarpals andlonger than the longest manual phalanx (I-1), while it issubstantially shorter than the combined length of phalanges II1
and 2 (Fig. 47G–I) unlike those of therizinosauroids (Zanno,2006). The nearly straight shaft bows slightly medially in dorsal
view (Fig. 47G). The proximal fifth of the dorsal surface and theproximodorsal end of the medial surface are shallowly concave(Figs. 47G, 48AF). The laterodorsal margin of the shaft isformed by a blunt longitudinal ridge, which has a tiny
protuberance laterodistal to the concavity on the dorsal surface(Fig. 47G). While the reminder of the dorsal, medial and lateralsurfaces of the shaft are convex, the ventral surface is notably
flat and is laterally demarcated by a longitudinal ridge (Fig.
48K). In proximal view, the well-preserved dorsal part exhibits aconvex articular surface (Fig. 48O). The distal end exhibits aginglymoid joint (Fig. 47K), which is less developed than the
one in metacarpal I whereas unlike the rounded ones ofornithomimids (Turner et al., 2012). The lateral collateral
ligament fossa is much shallower than the medial one (Figs.
47AB, 48AF).
Metacarpal III is the slenderest among the three metacarpals,
being dorsoventrally thick and mediolaterally narrow (Fig. 47O,
AF) in contrast to metacarpal I. Both the proximodistal lengthand the dorsoventral thickness are slightly less than those of

56

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 47. Metacarpus and manual phalanges of FPDM-V-8461. Dorsal (A–C, G–J, O–S), distal (D–F, K–N, T–X) and lateral (Y–AJ) views of left
metacarpals I (A, D, Y), II (G, K, AB) and III (O, T, AF), left phalanges I-1 (B, E, Z), I-2 (C, F, AA), II-1 (H, L, AC), II-2 (I, M, AD), II-3 (J, N, AE),
III-1 (P, U, AG), III-2 (Q, V, AH) and III-3 (R, W, AI), and mirrored right phalanx III-4 (S, X, AJ). Abbreviations: ft, flexor tubercle; ldp, laterodorsal
protuberance; pl, proximodorsal lip.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

57

FIGURE 48. Metacarpus and manual phalanges of FPDM-V-8461. Ventral (A–E, K–N, S–U, Z), proximal (F–J, O–R, V–Y) and medial (AA–AL) viewsof left metacarpals I (S, V, AJ), II (K, O, AF) and III (A, F, AA), proximal part of left metacarpus in articulation (Y, Z), left phalanges I-1 (T, W, AK), I-2(U, X, AL), II-1 (L, P, AG), II-2 (M, Q, AH), II-3 (N, R, AI), III-1 (B, G, AB), III-2 (C, H, AC) and III-3 (D, I, AD), and mirrored right phalanx III-4 (E,J, AE). Metacarpals II and III in Z are shaded to highlight the lateroventral process of the metacarpal I. Abbreviations: ft, flexor tubercle; lp, lateroventral
process; pc, proximal concavity; plr, proximolaterally-directed ridge; pvc, proximoventral concavity.

58

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMATABLE 7. Measurements on the forelimbs of FPDM-V-8461.

*Specimen incomplete in measured dimension.

metacarpal II (Table 7). The proximal end is triangular inproximal view with its apex directed dorsally, and the proximal
surface forms a deep concavity (Fig. 47F) as in Mononykus andShuvuuia (Brusatte et al., 2014). Distally, the ventral surface
becomes narrow (Fig. 48A) while the dorsal surface widens toform a flat surface in distal two-thirds of the shaft (Fig. 47O).
The distal end is dorsoventrally taller than mediolaterally wide
and is less ginglymoid than other metacarpals (Fig. 47T). The
condyles are discrete ventrally and merged dorsally. The lateralcollateral ligament fossa is substantially broader than the medial
one (Figs. 47AF, 48AA).
Manual phalanges
As in the metacarpus, the left manual phalanges are nearlycomplete except for the missing third ungual (III-4), which ispreserved in the right (Figs. 47–48). Additional right elements
include only II-2 and III-1. All non-ungual phalanges bear deep
collateral ligament fossae in medial and lateral sides of the distaltrochleae. In the penultimate phalanx, the collateral ligament
fossae are located very close to the proximodorsal extremity ofthe distal articular surface, which is more dorsally than those ofother non-ungual phalanges. The distal articular surface of thepenultimate phalanx extends more proximally in the ventral sidethan the dorsal side so that the surface faces somewhat
distoventrally.
I-1 is the largest among non-ungual phalanges, in which theshaft is subcircular in cross section due to the flat ventral surface(Fig. 48T) and its diameter is approximately equal to that of theradius. Proximally, the ventral surface becomes concave so thatthe medio- and lateroventral margins form longitudinal ridges,
which are less prominent than the flexor processes seen inornithomimosaurs and alvarezsaurids (Sereno, 2001). The shaft
is twisted to turn the distal trochlea about 20 degrees clockwise
in distal view of the left I-1 when the ventral surface of theproximal end is oriented horizontally (Fig. 47E). II-1 isessentially similar to I-1, except that it is shorter proximodistallyand narrower lateromedially so that the shaft of II-1 issubcylindrical in cross section. The ventral surface of the shaft isflat except for proximolaterally-directed ridge emerging at about
the center of the midshaft to join with the lateroventral margin at
the proximal fourth of the bone (Fig. 48L). II-2 is about 10%
longer than II-1 (Table 7). Phalanges of digit III are
mediolaterally narrower than those of digits I and II. In addition,
III-1 and 2 are notably short in contrast to III-3, which is longer
than the combined length of III-1 and 2. In dorsal view, themedial margin of III-1 is strongly concave (Fig. 47P), apparently
to avoid the contact with the metacarpophalangeal joint of thedigit II when in articulation.
All unguals are strongly recurved (Fig. 47AA, AE, AJ) unlike
those of ornithomimosaurs and alvarezsaurids (Brusatte et al.,2014) and lateromedially narrow (Fig. 47C, J, S). The
proximodistal length of each ungual is approximately the samewith its penultimate phalanx (Table 7). All medial and lateralgrooves are shallow (Figs. 47AA, AR, AJ, 48AE, AI, AL). Eachungual has a large flexor tubercle (Fig. 47AA, AE, AJ) unlike
small ones of tyrannosauroids and ornithomimosaurs (Brusatte
et al., 2010), and is located substantially distal to the proximal
end unlike proximally located ones of pennaraptorans andlate-diverging therizinosauroids (Nicholls and Russell, 1985; Xu
et al., 2011; Brusatte et al., 2014). The first ungual isdorsoventrally deeper and mediolaterally thicker than others andthe most strongly recurved in lateral view (Fig. 47AA).
However, the proximodistal length of the first ungual is slightly
shorter than the second. The second ungual is relatively
elongated proximodistally and bears a prominent proximodorsal
lip (Fig. 47AE) as in alvarezsaurs and maniraptorans (Currie andRussell, 1988; Zanno et al., 2009). The third ungual is thethinnest and shortest among the unguals, while it is morestrongly recurved than the second (Fig. 47S, AJ).
Pubis
Although both ilia and ischia are missing, nearly complete
shaft of the right pubis is preserved (Fig. 49). The shaft isstraight excepting that the proximal end is slightly bowedlaterally (Fig. 49B, C). Proximally, the cross section of the shaft
is anteroposteriorly elongated and thin lateromedially. The shaft

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

59

FIGURE 49. Pubis of FPDM-V-8461. Right pubis in proximal (A), lateral (B), anterior (C), medial (D), posterior (E) and distal (F) views. Abbreviation:pa, pubic apron.

60

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 50. Femur of FPDM-V-8461. Left femur in proximal (A), medial (B), anterior (C), lateral (D), posterior (E) and distal (F) views. Abbreviations:at, accessory trochanter; fc, fovea capitis; fh, femoral head; fo, fossa; ft, fourth trochanter; gt, greater trochanter; hlp, hook-like process; hr, horizontal
ridge; lr, lateral ridge; lt, lesser trochanter; nf, nutrient foramen; pt, posterior trochanter; tf, trochanteric fossa; ts, trochanteric shelf.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

61

FIGURE 51. Femur of FPDM-V-8461. Right femur in proximal (A), lateral (B), anterior (C), medial (D), posterior (C) and distal (F) views.
Abbreviations: et, ectocondylar tubercle; fg, flexor groove; fo, fossa; ft, fourth trochanter; hr, horizontal ridge; lc, lateral condyle; lr, lateral ridge; lt, lesser
trochanter; pt, posterior trochanter; ts, trochanteric shelf.

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

distally becomes thicker so that the cross section is subcircular,
except for the pubic apron. The pubic apron emerges as a low
ridge from the middle of the medial surface of the proximal end(Fig. 49D). The ridge extends anterodistally and becomes more
prominent to form a thin flange-like apron projected from theanteromedial margin of the shaft. The mediolateral width of theapron reaches its apex at the midlength of the preserved shaft
and then diminishes distally (Fig. 49E). Distal to the apex, themedial margin of the apron is rough possibly due to the contact
with the opposing pubis. However, the distalmost part of theapron turns to a low ridge with a smooth surface, indicating
absence of a contact with the opposing pubis at least in the distal
fourth of the shaft. The preserved distal end of the shaft exhibitsa slight expansion in anteroposterior orientation indicating thepresence of a pubic boot.
Femur
Both femora are preserved with both the proximal and distal
ends damaged. However, the proximal end is better preserved in
the left (Fig. 50) and the distal end better in the right (Fig. 51), sothat the original length of the femur can be estimated as 190 mm
by mirroring the one and superimposing on the other.
In anterior view, the proximal margin of the femoral head isstraight and perpendicular to the long axis of the shaft (Fig. 50C).
The head bears a blunt, small hook-like process protruding
distally at its mediodistal margin. In medial view, the fovea
capitis is present as a slight depression on the proximoventral
margin (Fig. 50B) and posteriorly continuous with thetrochanteric fossa on the posterior surface of the head (Fig. 50E).
Although the presence of this trochanteric fossa makes a partial
constriction on the region bridging the head and the greater
trochanter in proximal view (Fig. 50A), the constriction is not asprominent as those of some oviraptorosaurs (Balanoff and Norell,2012) and therizinosauroids (Zanno et al., 2009). In addition, theanterior margin of the proximal end is straight in proximal viewas in other maniraptorans (Brusatte et al., 2014).
The greater and lesser trochanters are separated from each otherby a deep cleft (Fig. 50D). The cleft is continuous with a fossa on
the anterior surface just below the greater trochanter and isventrally demarcated by a horizontal ridge (Fig. 50A, C). The
lesser trochanter is largely damaged so that the original dorsal
height is unknown, whereas it projects anteroproximally as abroad flange that is anteroposteriorly wider than the greater
trochanter; i.e. the alariform shape (Fig. 50D). There is a faintswelling on the anterodistal margin of the lesser trochanter
probably representing the accessory trochanter.
At the level with the base of the lesser trochanter, theposterior trochanter forms a blunt swelling on the lateral half ofthe ventral aspect (Fig. 50E). In lateral view, a longitudinal ridge
extends from the posteroventral termination of the lesser
trochanter to the anterior margin of the trochanteric shelf (Fig.
50D). Another longitudinal ridge extends distally from theventral end of the trochanteric shelf to demarcate thelateroventral margin of the shaft, possibly corresponding to thelateral ridge seen in many maniraptorans (Norell and
Makovicky, 1999; Hutchinson, 2001). Slightly distal to the levelof posterior trochanter, the fourth trochanter emerges as alongitudinal ridge on the posteromedial margin (Fig. 50E). The
anterior surface near the distal end of the femur exhibits a flat
surface (Fig. 51C). The medial part of the distal end isincompletely preserved and the presence of mediodistal crest is
unknown. In the distal epiphysis, the ectocondylar tubercle isprojected posterodistally and separated from the lateral condyleby a deep cleft in both lateral and distal views (Fig. 51B, F),
while they are not as developed to form a trochlea seen inpygostylians (Clarke et al., 2006). The ectocondylar tubercle
forms the lateral wall of the flexor groove that opens distally(Fig. 51E).
Tibia
Both tibiae are preserved (Fig. 52). The right tibia lacks theproximal end including most of the cnemial crests and the distal
end. Despite the incompleteness, the left tibia is longer than theestimated length of the femur, indicating that the original lengthof the tibia is substantially longer than the femur (Table 8).
In anterior view, the shaft curves medially and expands
mediolaterally toward the distal end (Fig. 52A). The fibular crest
emerges at the level slightly above the distal extremity of thecnemial crest as a bulge of the ridge that runs distally from theproximal end (Fig. 52B). The fibular crest is positioned further
away from the distal end indicating that the crest is proximallylocated and never extends to the midshaft of the tibia,
contrasting with those of therizinosauroids (Xu et al., 2002c;
Zanno et al., 2009; Zanno, 2010). Posteriorly, the preserved
proximal end bears a shallow concavity, which probably
corresponds to the remnant of the cleft between the medial andlateral proximal condyles (Fig. 52C). The distalmost part of theanterior surface is nearly entirely covered by the astragalus andcalcaneum when in articulation (Fig. 52G). In addition, thelaterodistal part of the anterior surface bears a blunt longitudinalridge demarcating the lateral margin of the area covered by theascending process of the astragalus (Fig. 52A). The narrowsurface lateral to this ridge is flat to contact with the fibula andthe calcaneum and faces anteriorly and slightly laterally as in
Falcarius (UMNH VP 12362). The lateral margin of the distal
shaft is kinked to form another distinct ridge. The distal
extremity of this ridge diverges from the lateral margin due tothe presence of the shoulder of the lateral malleolus. The
shoulder and the distal margin of the medial malleolus are
situated more proximally than those of lateral malleolus (Fig.
52A, G) as in Falcarius (Zanno, 2010). The posterior surface ofthe distal shaft bears a sharp longitudinal ridge demarcating thebroad posterolateral and narrow posteromedial surfaces. Thisridge is slightly curved distomedially, resulting in theposteromedial projection of the distal end located lateral to themedial edge by about one third of the mediolateral width (Fig.
52E).
FibulaThe proximal and distal parts of the right fibula are preserved(Fig. 52). The proximal part lacks the proximal end, while
exhibiting the base of the proximomedial fossa as aproximally-shallow concavity that is expanded anteroposteriorly

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

63

FIGURE 52. Tibiae of FPDM-V-8461. Left (A–F) and distal half of right (G) tibiae in anterior (A, G), lateral (B), posterior (C), medial (D), proximal (E)and distal (F) views. Abbreviations: ac, astragalus contact; acm, ascending process contact margin; apc, ascending process contact; cc, calcaneum contact;fco, fibula contact; fcr, fibular crest; ic, probable remnant of the intercondylar cleft between the medial and lateral proximal condyles; lm, lateralmalleolus; lms, shoulder of lateral malleolus; mm, medial malleolus; mms, shoulder of medial malleolus; pp, posteromedial projection.

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 53. Fibula, astragalus and calcaneum of FPDM-V-8461. Proximal part of right fibula (A–F), distal parts of right fibula and mirrored left tibia(shaded) in articulation with right astragalus and calcaneum (G), distal part of right fibula (H–M), right calcaneum (N, W, P, R, U, X) and astragalus (O,V, Q, S, T, Y) in proximal (A, H, N, O), anterior (B, G, I, R, S), medial (C, J, T, U), posterior (D, K, V, W) lateral (E, L, P, Q) and distal (F, M, X, Y)
views. Abbreviations: ac, astragalus contact; af, anteroproximal facet; ap, ascending process; cc, calcaneum contact; fc, fibula contact; it, iliofibularis
tubercle; pf, proximomedial fossa; ppe, proximally-pointed eminence; tc, tibia contact.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS(Fig. 53C). The anterior, ventral and posterior margins of theproximomedial fossa are indistinct unlike well-defined ones oftyrannosaurs and ornithomimosaurs (Brusatte et al., 2014).
Distal to the proximomedial fossa, a rough surface occurs as acontact with the fibular crest of the tibia, demarcated by theanterior margin of the fibula and a longitudinal ridge along the
midline of the medial surface. In lateral view, the proximal endof the iliofibularis tubercle is preserved as a stout ridge forming
the lateral margin of the fibula accompanying a rough, slightly
concave anterior surface (Fig. 53B, E). When in articulation
with the tibia, the tubercle is situated at the midheight of thefibular crest, which, in turn, is situated much proximal to themidshaft of the tibia. The distal part of the fibula has a distal
articular surface for the proximal tarsals (Fig. 53G) indicatingthat the fibula originally has the length subequal to that of thetibia. The articular surface extends well proximally along theanterior margin so that it contributes a concave surface with theanteroproximal facet of the calcaneum when in articulation (Fig.
53G).
TarsusBoth astragali, right calcaneum and right distal tarsal III arepresent (Figs. 53, 54). Unlike those of some maniraptorans,
there is no sign for the fusion of tarsal bones. When theastragalus and calcaneum are articulated, both the lateral andmedial condyles are similar in size and the former projects moredistally than the latter in anterior view (Fig. 53G).
The ascending process of the astragalus is tall and broad tocover most of the anterior surface of the distal end of the tibia(Fig. 53G). The anterior surface of the ascending process bears a
broad fossa that becomes deeper in its distal part and distinctly
demarcated by the dorsal margin of the distal condyles (Fig.
53S). Within the fossa, just proximal to the intercondylar sulcus,
two distinct circular pits are present. The anteroproximal end of
the lateral condyle forms a proximally-pointed eminence (Fig.
53Q). In contrast to the anterior side, there is no proximal
expansion on the posterior side (Fig. 53V). Distally, the medial
and lateral condyles are separated by a broad and distinct sulcus
in anterior view, while it becomes indefinite in distal view
unlike the distinct ones of alvarezsaurs, troodontids and avialans(Turner et al., 2012). The anterior surface of the intercondylar
sulcus bears a shallow horizontal groove. In distal view, themediolateral width of the medial condyle is slightly greater thanthat of the that of the lateral condyle and anteroposterior depths
of both condyles (Fig. 53Y). Both condyles mesially constricted
anteroposteriorly in distal view.
The proximal surface of distal tarsal III is convex (Fig. 55F)
whereas the distal one is concave with an anteroposteriorly
oriented ridge separating the facet for metatarsals II and III
mediolaterally (Fig. 54R). Distal tarsal III is proximodistallythin and thickens posteriorly in lateral view (Fig. 55N). The
posterior margin of the bone bears a large swelling protruding
posteriorly in the middle to lateral part, as well as a slight
depression in the medial part (Fig. 55H).
Metatarsus
The right metatarsus is mostly complete whereas the left lacks

65

metatarsal IV (Figs. 54, 55). No trace for the fusion isobservable between the metatarsal elements. When inarticulation, metatarsals II–IV are proximodistally elongated asa whole in anterior view. In addition, metatarsals II–IV areclosely appressed only at their proximal ends and distally
diverged from each other as in therizinosaurs (Zanno et al.,2009).
Metatarsal I bears a triangular shape in anterior view with asharp point directed proximally (Fig. 54F). The lateral side bears
a broad concave surface in the proximal half to contact withmetatarsal II (Fig. 55S). The distal half of the lateral surface ismostly occupied by the collateral ligament fossa, which is much
deeper than the medial one (Fig. 54L).
Metatarsal II is nearly straight in anterior view. In proximal
view, the bone has a semicircular profile with a straight medial
margin and the posterior two-thirds of the proximal surface isdepressed for the contact with distal tarsal III (Fig. 55B). The
proximal end is expanded anteroposteriorly in lateral view andexhibits a flat lateral surface facing slightly posteriorly for thecontact with metatarsal III. The proximal end bears a posterior
projection with a narrow rugose surface (Fig. 55I). The
proximalmost part of the anteromedial margin forms a sharp
angle, although it abruptly becomes blunt due to the presence ofa small boss (Fig. 55R) which probably corresponds to the
tuberositas m. tibialis cranialis of extant birds (Baumel andWitmer, 1993). As in most non-avian theropods, definite signsof articulation with metatarsal I are absent in both medial andposterior surfaces. However, at the level slightly distal to themidshaft, the medial surface bears a posterodistally directedgroove (Fig. 54H). The intercondylar sulcus is slightly visible
within the distal margin in anterior view (Fig. 54E), while it is
not as specialized as those of dromaeosaurids (Norell and
Makovicky, 1997). The mediolateral width of the distal trochlea(15.8 mm) is approximately the same with those of metatarsals
III (15.0 mm) and IV (14.7 mm).
Metatarsal III bears a stout shaft exposed both anteriorly andposteriorly when in articulation with other metatarsals throughits entire length, unlike the arctometatarsalian condition seen insome coelurosaurs (Holtz, 1995). The proximal end exhibits aflat medial surface for the contact with metatarsal II (Fig. 54I)
and a lateral projection followed posteriorly by a concavity forthe contact with metatarsal IV (Fig. 55Q). Other than these, no
obvious contact surface for neighboring metatarsals is observed.
In lateral view, the shaft is proximally expanded
anteroposteriorly in the proximal half, whereas it isanteroposteriorly thin in the distal half. The proximal half of the
shaft having a convex anterior surface with a small boss at the
middle of the proximal fifth (Fig. 54D), which probably
corresponds to the tuberositas m. tibialis cranialis of extant birds(Baumel and Witmer, 1993). In contrast, the distal half of the
shaft forms a flat anterior surface. The distal margin of
metatarsal III is approximately flat in anterior view and not
ginglymoid. In anterior view, the distal articular surface isexpanded more proximally above the medial condyle than thelateral condyle.

66

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 54. Distal tarsal III and metatarsus of FPDM-V-8461. Anterior (A–F), medial (G–L) and distal (M–S) views of right distal tarsal III (A, G, R),mirrored left metatarsal I (F, L, Q), right metatarsals II (E, H, P), III (D, I, O), and IV (C, J, N), mirrored left metatarsal V (B, K, M) and right distal tarsal
III in articulation with metatarsus incorporating mirrored left metatarsals I and V (S). Abbreviations: m2c, metatarsal II contact; m3c, metatarsal III
contact; m4c, metatarsal IV contact; pdg, posterodistally-directed groove; ttc, tuberositas m. tibialis cranialis.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

67

FIGURE 55. Distal tarsal III and metatarsus of FPDM-V-8461. Proximal (A–G), posterior (H–M) and lateral (N–S) views of right distal tarsal III (F, H,
N), mirrored left metatarsal I (A, M, S), right metatarsals II (B, I, R), III (C, J, Q) and IV (D, K, P), mirrored left metatarsal V (E, L, O), and right distal
tarsal III in articulation with metatarsus incorporating mirrored left metatarsals I and V (G). Abbreviations: d3c, distal tarsal III contact; m2c, metatarsal II
contact; m3c, metatarsal III contact; m4c, metatarsal IV contact; ttc, tuberositas m. tibialis cranialis.

68

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 56. Pedal phalanges of FPDM-V-8461. Dorsal (A, C–E, I–L, Q–U), distal (B, F–H, M–P, V–Z) and lateral (AA–AM) views of left phalanges
I-2 (A, B, AA), II-1 (C, F, AD), II-2 (D, G, AC), II-3 (E, H, AB), III-1 (I, M, AH), III-2 (J, N, AG), III-3 (K, O, AF), III-4 (L, P, AE), IV-1 (Q, V, AM),
IV-2 (R, W, AL), IV-3 (S, X, AK) and IV-4 (T, Y, AJ), IV-5 (U, Z, AI). Abbreviations: ft, flexor tubercle; hp, hyperextensor pit.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

69

FIGURE 57. Pedal phalanges of FPDM-V-8461. Ventral (A–E, K–N, S–U, Y), proximal (F–J, O–R, V–X, Z) and medial (AA–AM) views of leftphalanges I-2 (Y–AA), II-1 (S, V, AB), II-2 (T, W, AC), II-3 (U, X, AD), III1 (K, O, AE), III-2 (L, P, AF), III-3 (M, Q, AG), III-4 (N, R, AH), IV-1 (A, F,AI), IV-2 (B, G, AJ), IV-3 (C, H, AK) and IV-4 (D, I, AL), IV-5 (E, J, AM). Abbreviation: ft, flexor tubercle; pr, proximomedial ridge.

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SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMATABLE 8. Measurements on the pubis and hindlimbs of FPDM-V-8461.

*Specimen incomplete in measured dimension.

Metatarsal IV exhibits a stout shaft as in metatarsals II and
III. The proximal part is slightly expanded posteroproximallyunlike those of metatarsals II and III (Fig. 54J). The medialsurface of the proximal end is occupied by a broad longitudinalgroove, which roughly fits with the lateral projection in theproximal end of metatarsal III. The shaft of metatarsal IV
bears convex anterior and flat posterior surfaces to form asemicircular shape in cross-section. In addition, the shaft isslightly mediolaterally wider than dorsoventrally deep. In
distal view, however, the anteroposterior depth (15.0 mm) of
the distal end is subequal to the mediolateral width (14.7 mm)
in metatarsal IV (Fig. 54N).
Metatarsal V is crescent-shaped in lateral view with adistinctive proximal articular surface and a pointed distal end
without articular surface (Fig. 55O). The bone islateromedially thin through its entire length (Fig. 54B). The
medial surface constitutes a shallow concavity to contact withmetatarsal IV in the proximal third of the element. The
posterior margin of the distal half is thick and rugose incontrast to a thin, sharp margin of the proximal half (Fig. 55L).
Pedal phalanges
Pedal phalanges are almost completely known in the left pes
except for I-1 and the distal end of the second ungual (Figs.
56, 57). The right pedal phalanges are represented by I2, II-1,
III-1, III-3, IV-1, IV-3, IV-4 and IV-5.
All non-ungual phalanges share a proximodistally-elongatedslender shape in dorsal view unlike the square-shaped
non-ungual phalanges in most large-bodied theropods.
Particularly, those of digits II and III are three times longer ormore proximodistally than wide lateromedially (Fig. 56C–E,
I–L). In ventral view, phalanx II-1 bears a short ridge laterally
demarcating a narrow rugose surface in the proximomedial
margin (Fig. 57S). In contrast, phalanx III-1 and IV-1 exhibit
two ridges in medial and lateral margins near the proximal end(Fig. 57A, K). Proximodorsal to the distal articular surface,each non-ungual phalanx is excavated by the hyperextensor pit
except for the penultimate one (Fig. 56C, I, J, Q–S). In eachdistal trochlea, the lateral condyle is larger than the medial one
in the second digit (Fig. 56F, G), whereas the condition isreversed in the fourth digit (Fig. 56V–Y). The dorsoventral
depth of the distal articular surface of pedal phalanx II-2 isapproximately equal to that of the proximal articular surface(Fig. 55AD) unlike those of some troodontids (Turner et al.,2012).
Pedal unguals essentially resemble those of most othertheropods. Each pedal ungual is distally recurved, pointed
ventrally and lateromedially thin with a small lip overhanging
the proximal articular surface dorsally. Just below thedorsoventrally-elongated articular surface, a tuber for theattachment of the flexor tendon is present (Fig. 56AA, AB,
AE, AI). Among four pedal unguals, the first is the smallest
and the second is the largest in size, although it is not
significantly larger than the third unlike those ofdeinonychosaurs (Turner et al., 2012).

OSTEOLOGY OF FUKUIVENATOR PARADOXUSDISCUSSION
Phylogenetic position of Fukuivenator
A theropod genus Fukuivenator was erected in Azuma et al.
(2016) with 14 autapomorpies as a basal member ofManiraptora. Maniraptora originally established by Gauthier(1986) is defined as a stem-based clade containing Passer
domesticus and all coelurosaurs closer to it than to Ornithomimus
velox (Maryańska et al., 2002). The phylogenetic position of
Fukuivenator proposed by Azuma et al. (2016) within apolytomy of Ornithomimosauria and other clades leaves
maniraptoran affinity of this genus uncertain.
The phylogenetic analysis performed in this study recovered
62 MPTs with a length of 3453 steps (consistency index =0.313; retention index = 0.772). Additional TBR branchswapping on these MPTs resulted in >10000 total MPTs. In the
resulted strict consensus tree, Fukuivenator is recovered as an
unambiguous member of Maniraptora, and in total of 12
autapomorpies of the genus are proposed within the clade in thepresent study (see “Emendation of diagnosis” for detail). As aresult, Fukuivenator is placed at the base of the clade
Therizinosauria (Fig. 58), although the robustness of thisplacement should be accepted with caution because statisticalsupports for the clade are not strong. Nevertheless, the result
indicates a slightly more crownward position of Fukuivenator
than previously proposed in Azuma et al. (2016), in which no
positive relationship with Therizinosauria is suggested. In
addition, the polytomy unresolved in Nesbitt et al. (2019)
composed of Compsognathidae, Ornithomimosauria,
Therizinosauria, Alvarezsauria and Paraves is resolved in the
reduced consensus tree of the present study.
According to the analysis, the monophyly of Maniraptora was
supported by 19 unambiguous synapomorphies andFukuivenator presents two of them:71

8(1) subotic recess on braincase (also in ornithomimids andsome troodontids);90(1) 11 or more cervical vertebrae (also in Citipati,
Jinfengopteryx and some avialans);
98(1) two pneumatic foramina on each side of cervicalvertebrae (also in some oviraptorosaurs, Sinovenator and someavialans);
117(0) distal caudal neural spines present as low ridges (alsoin Zuolong, tyrannosauroids, ornithomimosaurs and someoviraptorosaurs);309(1) 11 dorsal vertebrae (also in Patagopteryx);366(1) distal articular condyles present on anterior surface of
humerus (also in alvarezsaurids and most avialans);
789(1) deep dorsal neural arches;
824(0) metatarsals II–IV divergent from each other.
The unusual morphological condition of Fukuivenator
resulted in 46 diagnostic features, which is the largest in number
among all taxa in the reduced consensus tree. Among them, nine
diagnostic features are autapomorphic within Maniraptora:

780(2) foramen magnum larger than occipital condyle;840(1) straight anterior margin of femur in proximal view(also in some tyrannosauroids).

25(0) short anteromedial process of maxilla;
103(1) zygapophyses of dorsal vertebrae situated lateral to theneural canal and hyposphenes connected by an inset web (alsoin Ornithomimosauria and some tyrannosauroids);
526(1) pneumatic posterior process of lacrimal (also inlate-diverging tyrannosauroids);
613(1) posteroventrally facing basisphenoid recess (also insome tyrannosauroids);
661(1) well-developed posterior centrodiapophyseal laminae
in anterior-middle cervicals (also in Tyrannosaurinae);
664(1) nearly horizontal posterior centrodiapophyseal lamina
in anterior-middle cervicals (also in basal tyrannosauroids);
680(0) relatively short scapula (also in Ornithomimosauria,
some compsognathids, some basal tyrannosauroids);
843(0) laterally-displaced medial ridge on posterior surface ofdistal tibia;
844(1) fossa on lateral surface of scapula immediately above
glenoid (also in Coelurus and Tanycolagreus).

Fukuivenator also presents four unambiguous synapomorphies
for maniraptorans diverging later than Alvarezsauria:

Remaining 37 diagnostic features of Fukuivenator are
homoplastic within Maniraptora:

150(1) proximodorsal lip of manual unguals (also in
Bicentenaria);
488(1) wide maxillary fenestra (also in some tyrannosauroids,some compsognathids and Garudimimus);688(0) bowed ulnar shaft (also in Zuolong, some early-diverging
tyrannosauroids, Ornitholestes and Archaeornithomimus);839(1) longer scapula contribution to the glenoid (also insome tyrannosauroids).

10(1) basisphenoid recess divided into two foramina (also in
Citipati, Chirostenotes and some eudromaeosaurs);
14(0) solid basipterygoid processes (also in Paraves);
19(2) exits for cranial nerves X–XII located in a shallow
depression (also in some eudromaeosaurs, late-diverging
avialans and Incisivosaurus);27(2) large maxillary fenestra (also in troodontids diverging
later than Almas and IGM 100/1126);
38(1) lacrimal foramen (or fenestra) (also in Citipati andRinchenia);
39(2) small posterodorsal process of lacrimal (also introodontids diverging later than Jinfengopteryx and Mei,

In addition, Fukuivenator exhibits eight unambiguous
synapomorphies for Therizinosauria:

72

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMAFIGURE 58. Strict consensus tree of coelurosaurs including Fukuivenator paradoxus. A, Coelurosauria; B, Tyrannosauroidea; C, Tyrannosauridae; D,
Tyrannosaurinae; E, Compsognathidae; F, Maniraptoriformes; G, Orninthomimosauria; H, Ornithomimidae; I, Maniraptora; J, Alvarezsauria; K,
Alvarezsauridae; L, Therizinosauria; M, Therizinosauroidea; N, Therizinosauridae; O, Pennaraptora; P, Oviraptorosauria; Q, Oviraptoridae; R, Paraves; S,
Avialae, T, Pygostylia; U, Deinonychosauria; V, Troodontidae; W, Anchiornithinae; X, Dromaeosauridae; Y, Unenlagiinae; Z, Eudromaeosauria.
Numbers at the bottom left of each node represent the decay index (Bremer support; left) and the bootstrapping score (right) for the node. Hyphen (-)
indicates that the node has not been recovered for the respective test.

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

Austroraptor and some alvarezsaurs);
42(1) sinusoidal anterior emargination of supratemporal fossa
on frontal (also in most dromaeosaurids);
44(1) notched lacrimal suture on frontal (also in somedromaeosaurids and Ichthyornis);46(0) unfused parietals (also in Avialae andEudromaeosauria);
56(1) twisted paroccipital process (also in Dromaeosauridae
and Haplocheirus);
91(1) large epipophyses of axis (also in Eudromaeosauria,
Mei and some avialans);
100(0) absence of hypapophyses in anterior dorsal vertebrae(also in Alvarezsaurus, Microvenator, Microraptor, Sinovenatorand basal avialans);
101(1) distinctly projected parapophyses in posterior dorsal
vertebrae (also in Alvarezsauria, Dromaeosauridae, Mei and
Confuciusornis);107(1) interspinous ligament scar terminating below apex of
dorsal neural spine (also in Deinonychosauria and Conchoraptor);
116(1) caudal neural spines separated anteroposteriorly (alsoin Rahonavis and Jeholornis);118(0) relatively short prezygapophyses of distal caudals(also in avialans diverging later than Archeopteryx and sometroodontids);
146(1) metacarpal I longer than the half of metacarpal II (alsoin Ornithomimosauria and a late-diverging avialan Chauna);
241(1) lateral lamina of maxilla reduced to small triangle(also in some dromaeosaurids, Anchiornithinae, sometroodontids and Jeholornis);318(1) fused distal caudal vertebrae (also in Pygostylia,
Citipati, Conchoraptor, Beipiaosaurus and Sapeornis);337(1) flat or convex dorsal suture on coracoid against
scapula (also in alvarezsaurs diverging later than Ceratonykus,
Albertonykus, Xixianykus and Albinykus, and some avialans);
345(1) flat medial surface around coracoid foramen (also in
Parvicursorinae, Sinovenator, and some avialans);
351(1) ulna approximately long as humerus (also in
Alvarezsauria, Microraptor, Aurornis, Eosinopteryx andavialans diverging later than Archaeopteryx);359(1) deltopectoral crest dorsoventrally thick as humeral
shaft (also in Unenlagiinae, some avialans, Oviraptor,
Heyuannia, Mononykus and Shuvuuia);373(1) pit on distolateral margin of humerus (also in
Ornithurae);377(0) indistinct dorsal cotyla of ulna (also in Citipati);
409(0) slightly developed posterior trochanter (also in
Paraves);448(1) anterior tympanic recess confluent with prootic recess(also in late-diverging troodontids);
607(1) ventral flange at distal end of paroccipital process(also in Dromaeosauridae);
630(1) dorsoventrally shallow meckelian groove (also in
Zanabazar);
687(1) medial condyle of humerus expanded well medially(also in Patagonykus);

73713(1) distally situated fourth trochanter (also intherizinosauroids diverging later than Erliansaurus);
766(1) deeply concave proximal articular surface on
metacarpal III (also in Mononykus and Shuvuuia);
777(0) horizontal groove separating distal condyles fromascending process of astragalus (also in Alvarezsaurus);
797(1) expanded medial aspect of distal humerus (also intherizinosauroids diverging later than Alxasaurus);
829(0) meckelian groove centered at midheight of dentary(also in Austroraptor and Erlikosaurus);
837(0) antorbital fossa occupying almost entire lateral surfaceof lacrimal body (also in Sapeornis and Oviraptorosauria);
841(1) horizontal ridge between femoral head and neck (alsoin Linhenykus).
Azuma et al. (2016) previously argued that these features
highlight unusual mosaic condition of Fukuivenator. Revised
features of Fukuivenator and its position in the current
phylogenetic analyses in the present study seem to support theprevious hypothesis.
Emendation of diagnosis
With aids of CT analyses in the present study, 5 of 14
characters are retracted from the original diagnosis in Azuma
et al. (2016). Particularly, those on the premaxilla and the
postorbital (the first and fifth of the original diagnoses) arewithdrawn due to misidentification of the elements. The large
promaxillary fenestra subequal in size to maxillary fenestra,
the second of the original diagnosis (Azuma et al., 2016), is infact the dorsal part of the maxillary fenestra (Fig. 4C). In
relation to the lacrimal, the element originally described as theleft lacrimal is re-identified as the right lacrimal, where theanterior and descending rami being identified conversely in
Azuma et al. (2016). The present study identifies the groove
present between the lateral and medial laminae of thedescending ramus (Fig. 6K), and the ridge on the laterodorsal
margin of the anterior ramus (Fig. 6J) as in most theropods.
Contrary to Azuma et al. (2016), no additional projection on
the posterior surface of basal tubera of the braincase is present
except for the ridges extending mediodorsally from it (Fig.
12D). Such feature is shared with other theropod such as
Piatnitzkysaurus (Rauhut, 2004) and Velociraptor (Norell etal., 2004), although it is more distinctive in Fukuivenator.
Two additional diagnostic characters of Fukuivenator withinmaniraptorans are also identified: unusually thick dorsal marginof the jugal and bifid posterior end of the ectopterygoid. The
pronounced thickness results from the inflation of the bone by alarge jugal sinus. Although the jugal sinus is also present intyrannosauroids and Pelecanimimus (Gold et al., 2013) among
coelurosaurs, such thickness at the dorsal margin of thesuborbital part is unknown with most maniraptorans completely
lacking the sinus. Therefore, this character highlights an unusual
pneumaticity of the jugal of Fukuivenator within the clade. In
the ectopterygoid, the posterior margin of the main body is

74

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

bifurcated by a notch in lateral view. This notch is adjacent to
the medial margin of the main body of ectopterygoid that is
usually attached to the pterygoid, so that a complex, relatively
robust connection may have been present between the two bones
in Fukuivenator.
Remarks on maniraptoran anatomy
Maxillary fenestra and other openings on antorbital fossa
One of the notable characters of Fukuivenator, the large
maxillary fenestra occupying more than half the area between
the anterior margins of the antorbital fossa and fenestra, is alsopresent in most troodontids (character 27). Such condition of themaxillary fenestra is scored as a second derived state forTroodontidae by Currie and Varricchio (2004; character 22(2))
and incorporated into the TWiG dataset by Turner et al. (2012;
character 27). However, this character scoring was modified in
the latter study into the first derived state (state 1) for mosttroodontids as possessing the smaller maxillary fenestra, except
for jinfengopterygines, although the maxillary fenestra isapparently larger also in other troodontids (Norell et al., 2009;
Pei et al., 2017). To assess the phylogenetic affinity of
Fukuivenator, scorings of the character in troodontids wererevised in the present study based on published descriptions andfigured photographs. Specifically, scorings for the following
taxa are modified from the first (Turner et al., 2012) to thesecond derived states (state 2): Saurornithoides (Norell et al.,2009), Zanabazar (Norell et al., 2009), Xixiasaurus (Lü et al.,2010), Byronosaurus (Makovicky et al., 2003; Bever and Norell,2009), Aurornis (Godefroit et al., 2013) and Sinovenator (Xu etal., 2002b). In addition, the scorings for Troodon (interpreted as
a composite of Stenonychosaurus and Latenivenatrix after van
der Reest and Currie, 2017) is modified into unknown due toinsufficient description in the literature referred in Turner et al.
(2012).
In addition, the scorings for the location of maxillary fenestra(character 28) are revised for some troodontids. The locations of
the maxillary fenestra in Jinfengopteryx and Almas is apparently
situated posterior to the border (Shuan and Qiang, 2007:fig. 2;
Pei et al., 2017) and their condition is scored as the derived state(state 1) in the present study, contra to Turner et al. (2012) thatscores them as the primitive state (state 0).
Fukuivenator and some late-diverging troodontids such as
Saurornithoides and Zanabazar exhibit similarly large maxillaryfenestra located anteriorly in antorbital fossa (Norell et al.,2009). Yet, despite the apparent resemblance, the phylogenetic
position of Fukuivenator is recovered somewhat distant from
Troodontidae. Indeed, the maxillary fenestra of Fukuivenator israther quadrangular than elliptical, unlike those in thelate-diverging troodontids. In addition, the maxilla of
Fukuivenator bears a large pneumatic excavation (excavatio
pneumatica sensu Witmer, 1997) posterodorsal to the maxillaryfenestra as well as a small promaxillary fenestra, both of which
are absent in troodontids. The antorbital fossa of Fukuivenator is
strikingly similar to a hatchling of allosauroid IPFUB Gui Th 4

(Rauhut and Fechner, 2005) in exhibiting both the quadrangularmaxillary fenestra and the excavatio pneumatica. Therefore, it ishypothesized that such features of antorbital fossa in
Fukuivenator possibly resulted from the retention of apaedomorphic condition widely shared among avetheropods
including basal maniraptorans.
Incisiform tooth and loss of denticles
Fukuivenator exhibits highly heterodont condition with fourmorphotypes of teeth within one individual, all of which are
unserrated and approximately conical. As mentioned by Azuma
et al. (2016), the mesialmost tooth is somewhat similar to theincisor-like teeth of Incisivosaurus (Xu et al., 2002a; Balanoff etal., 2009) in having subcircular section at the base and the
lingual surface becoming flatter apically. However, theincisiform tooth of Fukuivenator does not exhibit apparent wear
surface nor large size, features that are extensive inIncisivosaurus. According to Hendrickx et al. (2019), such
incisiform teeth are also seen in Ornitholestes, Falcarius andPelecanimimus. Therefore, such condition is possibly
plesiomorphic to a clade containing Ornitholestes andManiraptoriformes and inherited in basal ornithomimosaur
Pelecanimimus and basal therizinosaurs Fukuivenator andFalcarius.
The loss of denticles in the whole dentition is another dental
feature of Fukuivenator. Hendrickx et al. (2019) hypothesizes
that the complete loss of the denticles is apomorphy of Paraves
so that the denticles are plesiomorphically present in
Therizinosauria. However, the present phylogenetic position of
Fukuivenator suggests an alternative hypothesis where the teethof Therizinosauria are plesiomorphically unserrated and theirdenticles are re-acquired within the clade. In the reduced strict
consensus tree of the present study, unserrated conditions forboth the mesial (character 81(2)) and lateral (character 782(1))
dentitions are optimized as ancestral for Maniraptoriformes. In
this scenario, the unserrated dentitions are inherited to basal
paravians, so that the partially serrated (character 81(1)) andserrated lateral dentitions are acquired in Haplocheirus andwithin Therizinosauria, respectively (gained twice). This
scenario is more plausible than the alternative in accordancewith Hendrickx et al. (2019), in which the loss of the serration
independently occurs in Fukuivenator, Oviraptorosauria andbasal paravians (lost three times). On the other hand, the sharedconidont/folidont tooth condition in basal therizinosaurs
Fukuivenator and Falcarius and basal oviraptorosaurs
suggests that such dentition is synapomorphic to the clade
Therizinosauria + Pennaraptora in support of Hendrickx et al.
(2019).
Fused posteriormost caudals
Fused posteriormost caudals is generally known as a shared
trait in avialan pygostylians. On the other hand, such feature isalso known in the ornithomimosaur Deinocheirus (Lee et al.,2014), therizinosaur Beipiaosaurus (Xu et al., 2003),
and oviraptorosaurs Nomingia (Barsbold et al., 2000b),
Similicaudipteryx (He et al., 2008), Citipati and Conchoraptor(Persons et al., 2014), as well as the non-pygostylian avialan

OSTEOLOGY OF FUKUIVENATOR PARADOXUS

75

TABLE 9. Encephalization quotients (REQs) of Fukuivenator and otherselected theropods.

Note: 1Cerroni & Paulina-Carabajal (2019); 2Franzosa (2004); 3Hurlburt (1996);
4
Lautenshlager et al. (2012).

Fukuipteryx (Imai et al., 2019). This feature appears to beconvergently acquired at least in Therizinosauria andOviraptorosauria because it is absent in their basal members(Persons et al., 2014) and anatomically different between thetwo clades (Wang and O’ Connor, 2017). The number of caudal
vertebrae contributing to the structure varies from two to five.
As in Similicaudipteryx and possibly Deinocheirus, the fused
posteriormost caudal vertebrae in Fukuivenator are composed oftwo. However, the anatomy of the structure as a whole is rather
similar to the one in Beipiaosaurus than to the ones inoviraptorosaurs and Fukuipteryx in exhibiting the concave
ventral margin and a blunt posterior end in lateral view.
Although it is assumed that the pygostyle functioned tomaneuver fan-forming rectrices in oviraptorosaurs, the presenceof such rectrices is only known in the taxa lacking the fused
posteriormost caudals (Ji et al., 1998) and rectrices are evidently
absent in the posterior end of the tail in Beipiaosaurus (Xu et al.,2003). Therefore, there is no evidence that the fused
posteriormost caudals of Fukuivenator supported the rectrices inlife. In addition, the large total number of caudal vertebrae in
Fukuivenator suggests that the fusion of the posteriormost
vertebrae is not associated with the reduction of the tail incontrast to the pygostyle of most avialans. With lacking amodern analogue possessing a long tail ending with fusedvertebrae, and highly convergent nature of the feature,functional role of the fused posteriormost caudal vertebrae in

FIGURE 59. Relationship between olfactory ratio and body mass forselected dinosaurs. Bold and dashed lines represent independent contrast
least-squares regression and 95% CI, respectively. Star, Fukuivenator;
diamond, oviraptorid; squares, ornithomimosaurs; circles, othertheropods. Modified from Zelenitsky et al. (2009).

non-pygostylian theropods remains unknown.
Paleobiology of Fukuivenator
Calculations of Sensory and Cognitive Capabilities
The preserved portion of the brain endocast of Fukuivenatormeasures 7.94 cm3 in volume, and the minimum volume of thecomplete endocast is estimated 12.24 cm3 based on the existing
part. Following Hurlburt et al. (2013), the reptile
encephalization quotients (REQs) at 100%, 70%, and 50% of theendocranial volume of Fukuivenator was calculated using thefollowing equation: REQ = MBr/ (0.0155 × MBd0.553), where
MBr is the brain mass and MBd is the body mass (Hurlburt etal., 2013). In the present analysis, MBr was calculated from theestimated minimum volume of the complete endocast (12.24 g),
and MBd was adopted from Azuma et al. (2016) as 2,500 g. It
turns out that the REQ of Fukuivenator is barely different fromthose of basal maniraptorans (Table 9), and close to the REQ of
a late-diverging therizinosaurid Erlikosaurus. The similar REQs
between an early-diverging therizinosaur Fukuivenator and alate-diverging therizinosaurid Erlikosaurus suggest thattherizinosaurs did not acquire a large brain throughout theirevolution.
The size of the olfactory bulb relative to the width of thecerebrum is considered an indicator of olfactory acuity(Zelenitsky et al., 2009, 2011). The maximum width of the

76

SOKI HATTORI, SOICHIRO KAWABE, TAKUYA IMAI, MASATERU SHIBATA, KAZUNORI MIYATA, XING XU AND YOICHI AZUMA

cerebrum and the length of the olfactory bulb were measured on
Fukuivenator and the relative size of the olfactory bulb was
calculated. Then, following Zelenitsky et al. (2009), a regression
analysis was conducted between the relative olfactory bulb size
and the body weight for theropods (Fig. 59), which suggests that
Fukuivenator had the most acute sense of smell among them.
Presence of the relatively large olfactory bulb with unusuallyacute sense of smell in early-diverging therizinosaur
Fukuivenator supports Lautenschlager et al. (2012) arguing thattherizinosaurs had a well-developed sense of olfaction as aplesiomorphic feature.
As demonstrated by Azuma et al. (2016), the semicircular
canals of Fukuivenator are also quite sophisticated among
theropods, indicating an acute sense of balance equilibrium of theanimal. Following Gleich et al. (2005), the best frequency ofhearing range and highfrequency hearing limit were estimated1477 Hz and 3766 Hz, respectively, based on the length of thecochlear duct. These values are within the range of values
calculated for other therizinosaurs. These values are close to
those of a tyrannosaurid Gorgosaurus (Witmer and Ridgely,
2009; Sakagami and Kawabe, 2020), indicating that
Fukuivenator was able to perceive a wide range of sound among
dinosaurs.
Omnivory of Fukuivenator
As noted by Azuma et al. (2016), Fukuivenator exhibits theheterodont dentition with incisiform mesialmost teeth, unserrated
mesial teeth, symmetrical lateral teeth and non-ziphodont wholedentition. Such dental features, as well as the increased numberof cervical vertebrae, are consistent with the traits correlated withherbivory (Zanno and Makovicky, 2011). The elongated cervicalregion is indeed common in purportedly herbivorous
therizinosaurs (Falcarius, Nanshiungosaurus, Neimongosaurus),
oviraptorosaurs (Osmólska et al., 2004; Balanoff and Norell,2012), Jinfengopteryx (Ji et al., 2005), and numerous avialans.
On the other hand, the recurved, well-pointed posterior maxillaryteeth of Fukuivenator are somewhat reminiscent of carnivorous
theropods, despite the lack of denticles. Additionally, many
herbivorous dental traits, such as elongate premaxillary teeth,lack of mesial teeth, lanceolate lateral teeth, and densely packed
dentition, are absent in the dentition of Fukuivenator. These lines
of evidence probably indicate the omnivorous diet of the animal.
If such condition reflects the dietary shift from the carnivory(Azuma et al., 2016), the carnivory is likely plesiomorphic in theclade Therizinosauria + Pennaraptora and possibly retained in
Deinonychosauria, while the omnivory and herbivory are
acquired in the rest of the clade.
The higher olfactory ratios seen in carnivorous tyrannosaurids
and dromaeosaurids suggest that they largely relied on olfaction
than in other theropods, probably to accommodate for thediminished vision effectiveness in low-light conditions, to locate
food sources, and/or to navigate larger home ranges (Zelenitsky
et al., 2009). On the contrary, ornithomimosaurs and oviraptorids
exhibit lower olfactory ratios suggesting that they relied more onsight than on olfaction, possibly reflecting adaptations for theomnivorous diet (Zelenitsky et al., 2009), although herbivory is

inferred in some of them (Zanno and Makovicky, 2011). The
large olfactory acuity is also known in Erlikosaurus and ishypothesized as an ancestral feature of Therizinosauria, which is
widely regarded as herbivorous (Lautenschlager et al., 2012).
The unusually large olfactory ratio of Fukuivenator not only
supports the idea, but also suggests that the exceptional olfactory
ability is a synapomorphic feature of Therizinosauria, possibly
evolved as a response to a dietary shift.
CONCLUSIONS
Description of the holotype specimen of Fukuivenatorparadoxus with aids of CT techniques clarifies its notable
morphological features unknown in the original description.
Among them, re-identification of some cranial elements causes
both the retraction of previously suggested diagnostic charactersas well as the discovery of additional ones. The phylogenetic
analyses using an additional set of morphological features of
Fukuivenator reveals that the animal is an unambiguous memberof Maniraptora placed at the base of Therizinosauria. Alongwith some therizinosaurian synapomorphies, Fukuivenator
exhibits morphological features related to the herbivorous dietwhile retaining reminiscence of the carnivorous diet, indicatingthat the animal is likely adapted to the omnivory as a transitional
stage of the dietary shift that occurred at the earliest stage oftherizinosaurian evolution. In addition, the revised brain
endocast with newly identified parietals reveals the highest
olfactory ratio of Fukuivenator among theropods, indicating thatthe exceptional olfactory ability could be a synapomorphy of
Therizinosauria.
Although dinosaurs are widely popular in Japan among the
public, their fossils are fairly limited in quality and quantity
compared with dinosaur-rich areas in the world such as North
America, China and Argentina. Among the dinosaur remains
from Japan, the occurrence of Fukuivenator is notable due to its
outstanding preservation, unusually unique morphologicalfeatures and its unexpected phylogenetic position within
theropods. Due to its close relationship to the stem-lineage,
future studies on Fukuivenator will substantially contribute tounderstand the evolutionary history of maniraptoran theropods
toward birds in numerous perspectives, including the functional,
ecological and biogeographical aspects. Additionally, this study
highlights the importance of further exploration at the Kitadani
Dinosaur Quarry to comprehend the evolutionary history ofdinosaurs in the Early Cretaceous.
ACKNOWLEDGEMENTS
The authors thank all people who contributed to the firstthrough fourth Dinosaur Excavation Projects in Fukui since
1989, as well as to the preparators at FPDM for their patient andskillful work. The authors are grateful to the late Junchang Lü
for his considerable support to the earlier phase of theinvestigation on the specimen. The authors also thank Stephen
Brusatte (University of Edinburgh, UK) and an anonymousOSTEOLOGY OF FUKUIVENATOR PARADOXUS

reviewer who provided detailed and insightful reviews that
greatly improved this manuscript.
APPENDIX
Character-state codings for Fukuivenator paradoxus for thecharacter-taxon matrix of Brusatte et al. (2014) updated by
Nesbitt et al. (2019).
??????11101010011011?????00201??????0012??10100?????00
00110??0?????000????????0?0200??0??0110000111001111001
0110000110010010???????0002001010?0???10001???????????
??????????????100?000100?00?00?010000000?0?000??00???0?00?0?0???0?????001?0000?10??00??0???0?????010200???0??
0?0??00????0??????00?000????????????00100100000120?????
????????20010?000001010101??????1100???0100000010000?0
00???00000??00000??????????0?0000000000000000000000?0?1000000?0001201000?00???0010?????????000000??????????
0110100?0000000?000???0???00??00000011111???10??0??01
000?0??????????????????????0?00???000?0000000?0000?0011
00?000???0??10100010100???????????01?????????????????01
0?001000000101100001?001000000000010?10?0100????????0??????0?001???0?01?00000020000???00101?00??01?10??1??0000100?0?100000????00002??1??0001100000010000?????????
0000??00000000000?0??100?000111?0100?????01
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