豊田ホタルの里ミュージアム研究報告書 第 11 号 : 9-23 頁 , 2019 年 3 月
Bull. Firefly Museum of Toyota Town. (11): 9-23, Mar. 2019

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formationin the Ischimachi district of Toyota Town, Yamaguchi Prefecture, Japan
Benjamin T. BREEDEN III1) and Kentaro IZUMI2)
1)

Department of Geology & Geophysics, University of Utah, 115 S 1460 E, Salt Lake City, UT 84112-

0102, U.S.A., b.breeden@utah.edu
2)Faculty & Graduate School of Education, Chiba University, 1-33 Yayoi Town, Inage Ward, Chiba City,

Chiba Prefecture 263-8522, Japan, izumi@chiba-u.jp

Abstract: The Lower Jurassic Nishinakayama Formation in the Ishimachi district of Toyota
Town in the Toyora region of Shimonoseki City, Yamaguchi Prefecture has yielded a small but
important assemblage of vertebrate fossils from shallow-marine rocks. This vertebrate fossil
assemblage includes the body fossils of teleost fish (Leptolepidiformes?), crocodylomorphs(Crocodylomorpha), and turtles (Testudinata), but no vertebrate trace fossils have yet been
identified. The crocodylomorph and turtle specimens, together with a turtle shell fragmentfrom the Kuruma Group in the Toyama Prefecture, represent the oldest records of theirrespective clades within Japan, and the turtle specimen is particularly significant because itis the only articulated Early Jurassic turtle known from marine strata. Although limited, thevertebrate fossil assemblage of the Nishinakayama Formation is internationally significant
because it is both comparable with other Early Jurassic vertebrate fossil assemblages
worldwide and among the only records of Early Jurassic tetrapod fossils in Japan.
Key words: Vertebrate palaeontology, Early Jurassic, Toyora Group, Toarcian Ocean Anoxic
Event.
INTRODUCTION
The shallow marine Lower to Middle Jurassic sediments of the Toyora Group are exposed in the vicinity of Toyota
Town in Shimonoseki City, Yamaguchi Prefecture, Japan (Fig. 1), and these strata represent one of the best Toarcian
palaeoenvironmental records from northwestern Panthalassa (Izumi et al., 2012, in press). The Toarcian is characterized by aninterval of marked global environmental change and an associated turnover of marine invertebrate faunas; however, it is unclear
how vertebrate animals were affected by these changes, if at all. To better understand this, it is necessary to critically assess theglobal fossil record of Early Jurassic vertebrates.
Within the Toyora Group, vertebrate fossils are known only from the Nishinakayama Formation in the Ishimachi Districtof Toyota Town (Fig. 2), and the material from the Nishinakayama Formation represents one of the only significant recordsof Lower Jurassic tetrapod vertebrate body fossils in Japan. The Nishinakayama Formation is important because it is the only
stratigraphic unit in East Asia from which a stable carbon-isotope perturbation associated with the Toarcian ocean anoxic
event has been identified (Izumi et al., 2012, 2018a, b; Kemp & Izumi, 2014; Them et al., 2017). Furthermore, it is the only
stratigraphic unit in East Asia to have yielded articulated vertebrate body fossils of definitively Toarcian age. Although theLower Jurassic Lower Lufeng Formation in the Yunnan Province of southwestern China has yielded a diverse assemblage of9

B. T. BREEDEN III and K. IZUMIFigure 1. Geographic location of the study area (Ishimachi district, Toyota Town) within (A) Japan and(B) Yamaguchi Prefecture.
well-preserved vertebrate body fossils, including reptiles (e.g., Young, 1941; Luo & Wu, 1994; Barrett & Xu, 2005; You et al.,2014), those strata are likely Hettangian to Pliensbachian in age based on biostratigraphic correlation (Luo & Wu, 1995). It is
possible that the Lower Lufeng Formation may be as young as Toarcian in age based on magnetostratigraphy (Huang et al.,2005); however, that age assignment is not supported by other methods of stratigraphic correlation.
Despite the inherent significance of the vertebrate fossils from the Nishinakayama Formation, vertebrate palaeontologicalresearch focused on this formation is scarce, particularly over the past two decades. This is partially owing to the relatively
limited nature of exposures of the Nishinakayama Formation; however, despite the limited outcrop exposure and rarity ofvertebrate fossils, several potentially significant vertebrate fossils have been reported from the formation beginning during the
1980s (e.g., Tanabe et al., 1982; Manabe & Hasegawa, 1998; Hasegawa et al., 1998; Nakada & Matsuoka, 2012). Herein,
we summarize the published record of vertebrate fossils from the Nishinakayama Formation, provide new observations on
the taxonomy of these fossils based on recent phylogenetic systematics, and compare the vertebrate fossil assemblage of the
Nishinakayama Formation to other Early Jurassic vertebrate fossil assemblages both within Japan and globally.
Institutional Abbreviations
FPDM, Fukui Prefectural Dinosaur Museum ( 福井県立恐竜博物館 ); GMNH, Gunma Museum of Natural History (群馬県立自然史博物館 ); MMHF, Mine Museum of History and Folklore ( 美祢市歴史民俗資料館 ); NU, Niigata
University ( 新潟大学 )GEOLOGIC SETTING
The Nishinakayama Formation is a stratigraphic unit within the Lower to Middle Jurassic Toyora Group, which crops out
around the towns of Toyota and Kikugawa in the western Yamaguchi Prefecture of southwestern Japan (Kobayashi, 1926;
Takahashi et al., 1965). The Toyora Group is a package of siliciclastic continental shelf sedimentary strata with sporadic
horizons of volcanic tuffs (Tanabe et al., 1982). Outcrop of the Toyora Group is distributed across two districts of the Tabe
Basin separated by the NW-SE-trending Tabe Fault. These districts are the Ishimachi (northern) district and the Tabe (southern)
district. The study area comprises the northern Ishimachi District along distributary channels of the Koya River where the
Nishinakayama Formation is primarily exposed along incised valleys and streambeds (Fig. 2).
In the study area of the Ishimachi district, the Toyora Group unconformably overlies the metamorphosed pre-Jurassic rocksof the Nagato Tectonic Zone (Kawamura, 2010) and itself comprises Lower to Middle Jurassic (Sinemurian to Bathonian)10

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

Figure 2. Geologic map of the study area in the Ishimachi district of Toyota Town, Shimonoseki City,
Yamaguchi Prefecture. Vertebrate fossil-bearing localities discussed within are highlighted by name.
Modified after Nakada and Matsuoka (2011) and Izumi et al. (2012).

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B. T. BREEDEN III and K. IZUMI

shallow marine siliciclastic deposits of the Higashinagano Formation (Sinemurian-Pliensbachian), Nishinakayama Formation(Pliensbachian-Toarcian), and Utano Formation (Toarcian-Bathonian), listed in ascending stratigraphic order. These strata
have been interpreted to represent transgressive, inundative, and regressive phases of a major sedimentary cycle, respectively(Hirano, 1971). The Nishinakayama Formation comprises silty shale, mudstone, and sandstone horizons which were depositedin a shallow marine environment with terrestrial influence (Tanabe et al., 1982; Kemp & Izumi, 2014). The formation in thisregion is approximately 300 meters in maximum stratigraphic thickness, and multiple informal lithostratigraphic nomenclaturalschemes for dividing it have been used over the past several decades. Hirano (1971, 1973a, 1973b) divided the Nishinakayama
Formation into a lower Nm member and an upper Na Member; Tanabe et al. (1982) divided it into Na, Nb, and Nc members,
listed in ascending stratigraphic order; and Nakada and Matsuoka (2011, 2012) utilized the divisions of Hirano (1971,
1973a, 1973b) but renamed those divisions the Sakuraguchidani Mudstone Member (=”Nm member”) and the Ischimachi
Sandstone and Mudstone Member (=”Na member”). Herein, we will utilize the lithostratigraphic framework of Tanabe etal. (1982) unless otherwise noted. In ascending stratigraphic order, the Nishinakayama Formation sensu Tanabe et al. (1982)
comprises the Na member (silty shale, ~90 m thick), Nb member (dark silty shale with intercalated fine-grained sandstone
and laminated black shales, ~160 m thick), and Nc member (alternating sandstone and mudstone, ~20-60 m thick). The “Nbmember” is relatively well-exposed in the region surrounding Toyota Town, primarily in valleys formed by both permanentrivers and ephemeral mountainside streams; however, continuous exposures are rare. It is also the most fossiliferous memberof the Nishinakayama Formation in the Ishimachi district, and it is from this interval that the vertebrate fossils discussed herein
originated. The uppermost “Nc member” is considered a barren interval for macrofossils, and no ammonite biozones havebeen established within it. The Toyora Group is unconformably overlain by the Cretaceous Kwanmon (=Kanmon) Group in
the Ishimachi district.
The Toyora Group in the Ishimachi district is characterized by abundant marine macrofossils such as ammonites, whereas
it is predominantly characterized in the Tabe district by the presence of abundant plant fossils and a relative lack of marine
fossils (Yamada & Ono, 2005). As a result of this, the biostratigraphy is better established in the Ishimachi district. The
biostratigraphic framework of the Toyora Group in the Ishimachi district was first established by Hirano (1971, 1973a, b), andthat framework was recently revised for the Sakuraguchi-dani section by Nakada and Matsuoka (2011). The biostratigraphic
framework recognized by Hirano (1973b) comprises three ammonite biozones within the “Nb member” of the Nishinakayama
Formation. In ascending stratigraphic order, those are the Fontanelliceras fontanellense, Protogrammoceras nipponicum, andDactylioceras helianthoides zones.
Nakada and Matsuoka (2011) recognized four ammonite assemblage biozones within the “Nb member” of the
Nishinakayama Formation in the Sakuraguchi-dani valley section. In ascending stratigraphic order, those ammonite biozones
are the Canavaria japonica, Paltarpites paltus, Dactylioceras helianthoides, and Harpoceras inouyei zones. The Canavariajaponica Zone is only exposed along the southern branch of the Sakuraguchi-dani valley and is characterized by the occurrenceof Canavaria japonica and the Pliensbachian index taxon Amaltheus margaritatus. The same morphotype of Amaltheus
margaritatus occurs in Tethyan marine deposits of northwestern Europe, which supports correlation between the Canavariajaponica Zone of Japan with the Pleuroceras apyrenum Subzone of Europe (Meister, 1988). Nakada and Matsuoka (2011)
placed the boundary between the Pliensbachian and Toarcian stages at the base of the Paltarpites paltus Zone; however, thecontact between the Canavaria japonica and Paltarpites paltus zones is ambiguous owing to a paucity of ammonite fossils
from that interval. The biostratigraphic framework of Nakada and Matsuoka (2011) has not yet been established beyond the
Sakuraguchi-dani section.
In the Sakuraguchi-dani valley section of the Ishimachi district, a ~3‰ negative organic stable carbon isotope excursion
13(δ Corg) spanning ~30 m within the “Nb member” was recognized via δ13Corg isotope ratio mass spectrometry (IRMS) analysis,12

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

which was interpreted as a diagnostic expression of the Toarcian ocean anoxic event (Izumi et al., 2012; Kemp & Izumi, 2014;
Izumi et al., 2018a, b). That interpretation was supported by ammonite biostratigraphy. The negative carbon isotope excursion
observed in the Sakuraguchi-dani valley section occurs within the Paltarpites paltus and Dactylioceras helianthoides Zoneof Nakada and Matsuoka (2011) within the lower Toarcian Stage (Izumi et al., 2018a, b) and was chemostratigraphicallycorrelated to lower Toarcian sections in Europe.
MATERIALS AND METHODS
The published record of vertebrate fossils from the Nishinakayama Formation is summarized herein. Comments pertaining
to MMHF specimens are made on the basis of firsthand observations by the authors unless otherwise noted by a citation.
Additionally, the turtle (MMHF 5-00001) and crocodylomorph (MMHF-5-00002) specimens are briefly reassessed in amodern phylogenetic systematic context.
VERTEBRATE BODY FOSSILS
Actinopterygii
The earliest mention of fossil actinopterygiian fish from the Nishinakayama Formation was made by Tanabe et al. (1982),
who noted the presence of specimens tentatively referred to the teleost clade Leptolepiformes within the Protogrammoceras
nipponicum and Dactylioceras helianthoides ammonite zones of Hirano (1973b); however, no further data were provided for
those specimens. Tanabe (1991) also noted the presence of fossil fish scales in the Protogrammoceras nipponicum Zone fromthe Ishimachi and Sakuraguchi-dani Valley localities and in the Dactylioceras helianthoides Zone from the Nishinagato Valley
locality. Additionally, Tanabe (1991) noted a specimen referred to Leptolepis? sp. from the Nishinagato Valley locality in the
Dactylioceras helianthoides Zone. Again, no further data were provided for those specimens. The observations of Tanabe(1991) followed the ammonite biostratigraphy established by Hirano (1973b).
Nakada and Matsuoka (2012) reported a partial fish skeleton (NU-MV0001; see Nakada & Matsuoka, 2012: Fig. 4) fromthe Sakuraguchi-dani Mudstone Member of the Nishinakayama Formation in the northern branch of the Sakuraguchi-daniValley locality. The Sakuraguchi-dani Member, which roughly correlates to the “Nb member,” is characterized by laminatedblack silty mudstones (Nakada & Matsuoka, 2011), and the horizon which yielded NU-MV0001 is further characterized byparallel lamination, pyrite granules, ammonoid fossils, and fossil wood fragments (Nakada & Matsuoka, 2012: horizon 23-1).
The specimen was collected from the Harpoceras inouyei Zone of the revised ammonite biostratigraphy of the Nishinakayama
Formation established by Nakada and Matsuoka (2011). NU-MV0001 comprises a 15 mm portion of the postcranial skeleton
including vertebrae, ribs, and a caudal fin, and it is preserved in the same slab as an individual of the ammonite Fuciniceras
nakayamense (Nakada & Matsuoka, 2012).
In addition to the published record of fossil actinopterygiian fish from the Nishinakayama Formation, previously
unpublished teleost specimens are reposited at the Mine Museum of History and Folklore in Mine City, Yamaguchi Prefecture.
One specimen tentatively referred to Leptolepis (MMHF 5-00003; Fig. 3A) comprises the anterior end of an individual
including the skull, pectoral girdle, and a portion of the vertebral column. This specimen was collected in Ishimachi, Toyota
Town. Two other specimens (MMHF 5-00004; MMHF 5-00005) comprise partial postcranial skeletons tentatively referred
to Leptolepiformes. MMHF 5-00004 (Fig. 3B) was collected in Ishimachi, Toyota Town by a Mrs. or Mr. Shiraishi. MMHF
5-00005 (Fig. 3C) was discovered in the Nishinakayama Quarry in Kikugawa Town by Mr. Kenro Hironaka. No further
locality or occurrence data are documented for any of these specimens. The anatomy of the fish fossils from the Nishinakayama

13

B. T. BREEDEN III and K. IZUMIFigure 3. Fossil teleost actinopterygian fish from the Lower Jurassic Nishinakayama Formation,
Toyota Town, Yamaguchi Prefecture, Japan. A, Leptolepiformes? (MMHF 5-00003). B,
Leptolepiformes? (MMHF 5-00004). C, Leptolepiformes? (MMHF 5-00005). Scale bars equal
10 mm.

14

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

Formation is superficially consistent with leptolepiform teleosts; however, further detailed study of these specimens is required
before their taxonomy may be rigorously tested, and they are herein tentatively referred to Leptolepiformes.
Crocodylomorpha
A single crocodylomorph specimen (MMHF 5-00002; Fig. 4) has been reported from the Nishinakayama Formation (Manabe& Hasegawa, 1998). MMHF 5-00002 was collected as an isolated slab near the Era River in Toyota Town, but more detailed
locality data are not available for the specimen. The crocodylomorph specimen comprises associated caudal and trunk (i.e.,
dorsal) vertebrae, the distal portion of the left tibia, the distal portion of the left fibula, the left tarsus, the left pes, and several
postcranial osteoderms from a single individual. The specimen is preserved mostly as external molds of bones on a part andcounterpart of a split slab of mudstone, and a specimen of the ammonite Harpoceras inouyei is preserved on the same slab.
Given that Harpoceras inouyei is only known to occur within the “Nb member” of the Nishinakayama Formation (Nakada &Matsuoka, 2011), it follows that the MMHF 5-00002 slab itself originated from the “Nb member.”Manabe and Hasegawa (1998) tentatively referred MMHF 5-00002 to Atoposauridae, a clade of crocodylomorphs longconsidered important components of Mesozoic ecosystems; however, they emphasized the tentative nature of their assignmentof the specimen to Atoposauridae and the need for further study of the specimen. Recent phylogenetic revisions by Tennant etal. (2016) have rendered Atoposauridae a much more taxonomically and chronostratigraphically exclusive clade restricted to
the Upper Jurassic of Europe. Manabe and Hasegawa (1998) noted similarities between MMHF 5-00002 and Alligatorellus
beaumonti, an atoposaurid from the Upper Jurassic of France, including anteroposteriorly elongated caudal centra, elongatedmetatarsals, and morphologically comparable osteoderms; however, these features are widespread among crocodylomorphs,
and the specimen has not yet been referred to a taxon on the basis of apomorphies (e.g, Bell et al., 2004; Nesbitt & Stocker,
2008) or placed into a rigorous phylogenetic analysis.

Figure 4. Fossil crocodylomorph (Crocodylomorpha, MMHF 5-00002) from the Lower Jurassic
Nishinakayama Formation, Toyota Town, Yamaguchi Prefecture, Japan. Photograph and line
drawing. Abbreviations: am, ammonite fossil; cav, caudal vertebrae; fib, fibula; fsh, fish fossil;
ha, haemal arch; mt, metatarsal; na, neural arch; os, osteoderm; ph, phalanges; tar, tarsals; tib,tibia; II-IV, digit number.

15

B. T. BREEDEN III and K. IZUMI

Although fairly incomplete, MMHF 5-00002 possesses a limited number of character states that are consistent with areferral to Crocodylomorpha. The specimen possesses four or fewer phalanges on pedal digit IV, which is diagnostic of
Crocodylomorpha under accelerated transformation (“ACCTRAN”) parsimony parameters (Nesbitt, 2011: Character 3961). Furthermore, MMHF 5-00002 possesses a compact metatarsus (at least half as wide as long), which is unambiguously
diagnostic of a more exclusive clade comprising (Sphenosuchus + Crocodyliformes) within Crocodylomorpha (Nesbitt, 2011:
Character 382-1). Given both the geographically and temporally restricted nature of the revised Atoposauridae sensu Tennant
et al. (2016) and the combination of apomorphic taxonomically informative morphological features preserved in MMHF
5-00002, a referral to Crocodylomorpha rather than Atoposauridae is therefore perhaps the most conservative taxonomic
assignment possible for MMHF 5-00002 at this time. Regardless, MMHF 5-00002 represents the oldest record of a fossil
archosaur in Japan, and further study of the specimen will elucidate the evolution and distribution of crocodylomorphs during
the Early Jurassic in East Asia.
Testudinata
Fujinaga (1990) reported a single turtle specimen comprising an associated skull, shell, and forelimb (MMHF 5-00001; Fig.
5) from the Nishinakayama Formation near the mouth of the Era River in Toyota Town. MMHF 5-00001 was collected as an
isolated slab of mudstone, which was split into a part and counterpart to expose the skeleton. MMHF 5-00001 was described in
more detail by Hasegawa et al. (1998), who formally referred it to Testudines but noted that further preparation of the specimen
was necessary before a detailed description or further taxonomic assignment could be given. A specimen of the ammonoid
Cleviceras chrysanthemum is preserved on the same slab, and Cleviceras chrysanthemum is only known to occur within the“Nb member” of the Nishinakayama Formation (Nakada & Matsuoka, 2011), so it is most likely that the MMHF 5-00001 slab
originated from the “Nb member.”
MMHF 5-00001 preserves at the least a skull, cervical vertebrae, a partial right forelimb comprising the zeugopodial andautopodial elements, a carapace, and plastral elements from an individual partially articulated turtle skeleton; however, thespecimen is split into part and counterpart through the carapace, and the entire skeleton is not exposed. The skull and cervicalvertebrae remain too poorly exposed for detailed description. The zeugopodium and autopodium of the right forelimb are
seemingly complete, preserving the ulna, radius, carpus, and manus. The ulna and radius are 8 mm in length, and articulate
with one another via a ridge on the ulna. At least eight flattened carpal elements are preserved. The manus is not elongated andmodified into a paddle or flipper. The phalanges are taphonomically offset from their respective metacarpals, but Hasegawa etal. (1998) estimated a phalangeal formula of 2-3-3-3-3 for the digits of the manus.
Hasegawa et al. (1998) estimated the complete carapace to be 70 mm in length and 62 mm in width. The nuchal is broadand ventrally flat. There appear to be seven neural osteoderms and eight costal osteoderm pairs (= “coastal” osteoderms in
Hasegawa et al, 1998). Nine peripheral osteoderms are exposed; however, the exact number of peripheral osteoderms present
in the carapace remains unknown. Hasegawa et al. (1998) noted the presence of large fontanelles between the costal andperipheral osteoderms. Of the plastron, Hasegawa et al. (1998) tentatively identified two elements as the entoplastron and a
fragment of the plastron tentatively identified as the hypoplastron. The entoplastron is a cruciform element 15 mm in lengthwith a prominent keel on one side. The tentative hypoplastron fragment was identified only based on its position relative to thecarapace, and it is too incomplete to describe its morphology.
Recent phylogenetic revisions of early turtle relationships have rendered the clade Testudines (i.e., the crown group ofliving turtles) more taxonomically exclusive in the time since the original publication of MMHF 5-00001 (e.g., Joyce, 2007;
Anquetin, 2012; Joyce et al., 2016). The fossil record of Testudines begins in the Upper Jurassic, and all known Lower Jurassic
turtles belong to the clade Testudinata outside of Testudines. Hasegawa et al. (1998) noted superficial similarities between the16A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

Figure 5. Fossil turtle (Testudinata, MMHF 5-00001) from the Lower Jurassic Nishinakayama
Formation, Toyota Town, Yamaguchi Prefecture, Japan. (A) Photograph and line drawing ofpart slab and (B) close up photograph and line drawing of partial forelimb from part slab.
Abbreviations: car, carpals; crv, cervical vertebrae; C#, costal plate of carapace; ent,
entoplastron; hyp, hypoplastron; mc, metacarpals; per, peripheral plate of carapace; ph,phalanges; pyg, pygal plate of carapace; ra, radius; sk, skull; trv, trunk vertebrae; ul, ulna; vt,
vertebra; I-V, digit number. Scale bars equal 10 mm.

carapace of MMHF 5-00001 and Toxochelyidae; however, Toxochelyidae is a derived clade of sea turtles within Testudines
known only from the Cretaceous, and MMHF 5-00001 lacks other diagnostic features of Toxochelys, so any similarities
present between the two are likely the result of evolutionary convergence rather than shared ancestry. Pending further study of
the anatomy and phylogeny of MMHF 5-00001, a referral to Testudinata is the most conservative assignment possible at thistime. Although a fragment of turtle shell has been reported from the Lower Jurassic Kuruma Group in central Japan (Sonoda
et al., 2015), MMHF 5-00001 represents the oldest relatively complete fossil record of turtles in Japan, and given the rarity
low taxonomic diversity of turtle fossils from the early Mesozoic (i.e., the Triassic and Early Jurassic), further study of thisspecimen has great potential for elucidating the early evolution of turtles.
DISCUSSION
Aside from the Toyora Group, only the brackish to shallow-marine Lower to Middle Jurassic Kuruma Group which crops17B. T. BREEDEN III and K. IZUMI

out in the Toyama, Nagano, and Niigata prefectures (e.g., Goto & Tazaki, 1996) has also produced significant vertebrate fossilmaterial from Lower Jurassic strata in Japan. Perhaps the earliest reference to vertebrate material from the Kuruma Group
was a teleost fish skull tentatively referred to the genus Leptolepis (Oe & Chiba, 1988). Subsequent discoveries of vertebrate
fossils from the Kuruma Group include chondrichthyan teeth (Takakuwa & Hasegawa, 1992); plesiosaurid teeth (Tanimotoand Okura, 1989; Takakuwa & Hasegawa, 1992); and dinosaur trackways (Hatakeyama, 1995; Matsukawa et al., 2005). Most
recently, Sonoda et al. (2015) reported a partial isolated costal osteoderm plate of a turtle (FPDM-V9543), a ganoid fish scale(FPDM-V9544), and several other indeterminate bone fragments. Putative vertebrate coprolites have also been discoveredfrom the Kuruma Group (pers. obs.); however, these remain unpublished. Additionally, an acrodontid chondrichthyan tooth
was reported from the marine Lower Jurassic Iwamuro Formation near Numata City, Gunma Prefecture (GMNH-PV 2406;
Takakuwa & Gunma Fossil Club, 2011).
To date, neither chondrichthyan nor plesiosaurid fossils have been identified from the Nishinakayama Formation; however,their eventual discovery would be unsurprising given the occurrence of both taxa in Lower Jurassic marine strata elsewhere
both within Japan and globally. The presence of fish fossils referred to the genus Leptolepis from both the Nishinakayama
Formation and the Kuruma Group is intriguing; however, none of these specimens have yet been described in enough detail
to allow morphological or systematic comparison. The carapace of MMHF 5-00001 is not exposed such that individual costal
plates can be directly compared to the material from the Kuruma Group, but together, these specimens represent the oldest
record of turtles in Japan. No crocodylomorph fossils have been discovered from Lower Jurassic strata within Japan outsideof the Nishinakayama Formation; however, their eventually discovery elsewhere would be unsurprising given the abundanceof crocodylomorph taxa present in both non-marine and marine Lower Jurassic strata globally. The absence of vertebrate
trace fossils within the Nishinakayama Formation is conspicuous given the shallow marine environment and the abundanceof vertebrate trace fossils in the Kuruma Group; however, this lack of a vertebrate trace fossil record may be a result of limited
outcrop and sampling rather than a natural indication of absence. The presence of dinosaur footprints within the Kuruma
Group indicates that it was at least occasionally deposited at or near the interface between marine and non-marine depositional
environments (Kumon et al., 1999). Although no dinosaur material has been identified from the Nishinakayama Formation, theichnofossil record in the Kuruma Group indicates that dinosaurs were present in Japan by the Early Jurassic, and it would beconceivable to eventually discover the remains of dinosaurs from the Nishinakayama Formation. The abundance of terrestrial
input in both the Nishinakayama Formation and the Kuruma Group strata and the resulting juxtaposition of both marine andnon-marine fossils preserved within those strata is encouraging for further palaeontological exploration.
Despite an overall paucity of vertebrate fossils within the Nishinakayama Formation, the presence of teleost fish,
crocodylomorphs, and turtles is consistent with other Early Jurassic assemblages worldwide. At present, Early Jurassic turtles
are known only from non-marine deposits, including Kayentachelys from the Kayenta Formation in Arizona, U.S.A (Gaffney
et al., 1987); Australochelys from the Elliot Formation in South Africa (Gaffney & Kitching, 1995); and Indochelys fromthe Kota Formation in India (Datta et al., 2000). In contrast to turtles, crocodylomorphs and teleost fish are somewhat more
ubiquitous in both marine and non-marine Lower Jurassic strata worldwide (e.g., Simmons, 1965; Clark & Fastovsky, 1986;
Arratia & Thies, 2001; Tykoski et al, 2002; Tykoski, 2005; Konwert & Stumpf, 2017; Ősi et al., 2018).
Interestingly, the cooccurrence of Early Jurassic testudinatans and crocodylomorphs is unique to non-marine deposits outsideof Japan; however, the Nishinakayama Formation is itself unquestionably a shallow marine deposit (Hirano, 1973b; Tanabe etal., 1982; Tanabe, 1991). The superficial similarities between the vertebrate assemblages of the Nishinakayama Formation andnon-marine Lower Jurassic strata elsewhere in the world is perhaps unsurprising given near-shore depositional environmentof the Nishinakayama Formation and the high concentration of terrestrial matter preserved alongside marine fossils within thedeposit, including plant fragments (e.g., Nakada & Matsuoka, 2011; Kemp & Izumi, 2014; Izumi et al., 2018a, in press). Both18

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

the turtle and crocodylomorph specimens from the formation were initially interpreted as representing marine taxa (Hasegawa
et al., 1998; Manabe & Hasegawa, 1998), but it is also possible that both are in fact allochthons washed out to sea prior todeposition and instead represent non-marine taxa. Further scrutiny of the anatomy of these two reptile specimens may allow amore rigorous evaluation and interpretation of their lifestyle.
CONCLUSIONSThe shallow marine strata of the Nishinakayama Formation preserve a limited but important Early Jurassic assemblageof vertebrate body fossils, and this is particularly true of tetrapod vertebrates. Although limited fragmentary and isolated
plesiosaurid and testudinatan body fossils are known from the Lower to Middle Jurassic Kuruma Group in central Japan,
the testudinatan and crocodylomorph specimens from the Nishinakayama Formation are the only articulated tetrapod
skeletons of Early Jurassic age known in Japan. These specimens (MMHF 5-00001, Testudinata indet.; MMHF 5-00002,
Crocodylomorpha indet.), together with the turtle shell fragment from the Kuruma Group, represent the oldest records of theirrespective clades within Japan, and the testudinatan specimen is particularly important because it is both relatively complete
the only Early Jurassic turtle known from marine strata. The presence of teleost fish, testudinatans, and crocodylomorphs
from strata deposited in northwestern Panthalassa is broadly comparable with other vertebrate fossil assemblages from Lower
Jurassic strata worldwide and is thus consistent with age estimates assigned to the Toyora Group on the basis of ammonite
biostratigraphy. Given the overall paucity of Early Jurassic vertebrate fossils in Japan, further study of the specimens discussed
herein will certainly provide new insight about both the evolutionary and biogeographic histories of vertebrates in East
Asia and the homogeneity of Early Jurassic vertebrate assemblages globally. This also encourages further palaeontologicalreconnaissance in the Nishinakayama Formation.
ACKNOWLEDGEMENTS
Fumio Takahashi (MMHF) accommodated specimen access to BTB and KI and provided photographs and valuable historicalinformation about specimens discussed herein. Makoto Manabe (National Museum of Nature and Science, Tokyo) and Erin
Maxwell (Staatliches Museum für Naturkunde Stuttgart) provided valuable insight to BTB about Japanese fossil vertebrates
and Early Jurassic fossil fishes, respectively. Wakako Akamine (University of Utah) graciously assisted BTB with thetranslation of Japanese literature and correspondence. BTB received funding from the American Philosophical Society (Lewis
and Clark Fund for Exploration and Field Research), the Geological Society of America (Graduate Student Research Grant),
and the University of Utah Department of Geology & Geophysics (The David S. and Inga M. Chapman Fund). KI received
funding from Fujiwara Natural History Foundation and Asahi Group Foundation. This study contributes to IGCP 655.
REFERENCES
Anquetin, J. (2012) Reassessment of the phylogenetic relationships of basal turtles (Testudinata). J. Syst. Palaeontol., 10: 3-45.
Arratia, G., Thies, D. (2001) A new teleost (Osteichthyes, Actinopterygii) from the Early Jurassic Posidonia shale of Northern
Germany. Mitt. Mus. Nat.kd. Berl., Geowiss. Reihe, 4: 167-187.
Barrett, P. M., Xu, X. (2005) Reassessment of Dianchungosaurus lufengensis Yang, 1982, an enigmatic reptile from the Lower
Lufeng Formation (Lower Jurassic) of Yunnan Province, People's Republic of China. J. Paleontol., 79: 981-986.
Bell, C. J., Head, J. J., Mead, J. I. (2004) Synopsis of the herpetofauna from Porcupine Cave. pp. 117–126 in Barnosky, A. D.

19

B. T. BREEDEN III and K. IZUMI(ed.), Biodiversity Response to Climate Change in the Middle Pleistocene: The Porcupine Cave Fauna from Colorado.
University of California Press, Berkeley.
Clark, J. M., Fastovsky, D. E. (1986) Vertebrate biostratigraphy of the Glen Canyon Group in northern Arizona. pp. 285-301 in
Padian, K. (ed.), The Beginning of the Age of Dinosaurs. Cambridge University Press, Cambridge.
Datta, P. M., Manna, P., Ghosh, S. C., Das, D. P. (2000) The first Jurassic turtle from India. Palaeontology, 43: 99-109.
Fujinaga, T. (1990) Japan’s oldest turtle. Karst, 10: 2. (In Japanese, translated by authors)Gaffney, E. S., Hutchinson, J. H., Jenkins, F. A Jr., Meeker, L. J. (1987) Modern turtle origins: the oldest known cryptodire.
Science, 237: 289–291.
Gaffney, E.S., Kitching, J.W. (1995) The Morphology and Relationships of Australochelys, an Early Jurassic Turtle from South
Africa. Am. Mus. Nov., 3130, 29pp.
Goto, M., Tazaki, K. (1996) Clay mineral analysis of paleo-environment – Sedimentary environment of the Kuruma Group.
Journal of the Clay Science Society of Japan, 38: 91-102.
Hasegawa, Y., Manabe, M., Hirano, H., Takahashi, F. (1998) A turtle from the Early Jurassic Toyora Group, Yamaguchi, Japan.
Mem. Natn. Sci. Mus. Tokyo, 31: 67-72.
Hatakeyama, K. (1995) Dinosaur footprints from the Lower Jurassic Kuruma Group, Otari village, Nagano Prefecture. Bull.
Nagano City Mus., 3: 1-10.
Hirano, H. (1971) Biostratigraphic study of the Jurassic Toyora Group, Pt. 1. Mem. Fac. Sci., Kyushu Univ. Ser. D, 21: 93–128.
Hirano, H. (1973a) Biostratigraphic study of the Jurassic Toyora Group, Pt. 2. Trans. Proc. Palaeont. Soc. Japan N.S., 89: 1-14.
Hirano, H. (1973b) Biostratigraphic study of the Jurassic Toyora Group, Pt. 3. Trans. Proc. Palaeont. Soc. Japan N.S., 90: 4571.
Huang, B. -C., Li, Y. -A., Fang, X. -S., Sun, D. -J., Pang, Q. -Q., Cheng, Z. -W., Li, P. -X. (2005) Magnetostratigraphy of theJurassic in Lufeng, central Yunnan. Geol. Bull. China, 24: 322–328.
Izumi, K., Miyaji, T., Tanabe, K. (2012) Early Toarcian (Early Jurassic) oceanic anoxic event recorded in the shelf deposits inthe northwestern Panthalassa: evidence from the Nishinakayama Formation in the Toyora area, west Japan. Palaeogeogr.
Palaeoclimatol. Palaeoecol., 315–316: 100–108.
Izumi, K., Kemp, D. B., Itamiya, S., Inui, M. (2018a) Sedimentary evidence for enhanced hydrological cycling in response to
rapid carbon release during the early Toarcian oceanic anoxic event. Earth Planet. Sci. Lett., 481: 162-170.
Izumi, K., Endo, K., Kemp, D. B., Inui, M. (2018b) Oceanic redox conditions through the late Pliensbachian to early Toarcian
on the northwestern Panthalassa margin: Insights from pyrite and geochemical data. Palaeogeogr. Palaeoclimatol.
Palaeoecol., 493: 1-10.
Izumi, K., Suzuki, K., Kemp, D., Iizuka, T. (in press) Palaeogeographic and tectonic setting of the Lower Jurassic(Pliensbachian-Toarcian) Nishinakayama Formation, Toyora Group, SW Japan. Geological Journal xxx: xxx-xxx.
Joyce, W. G. (2007) Phylogenetic relationships of Mesozoic turtles. B. Peabody Mus. Nat. Hi., 48: 3-102.
Joyce, W. G., Rabi, M., Clark, J. M., Xu, X. (2016) A toothed turtle from the Late Jurassic of China and the global
biogeographic history of turtles. BMC. Evol. Biol., 16: DOI:10.1186/s12862-016-0762-5.
Kawamura, H. (2010) Stratigraphic revision of the Jurassic Toyora Group of the southern part of the Tabe Basin, Yamaguchi
Prefecture, southwest Japan. J. Geol. Soc. Jpn. 116: 27–44. (In Japanese with English abstract)Kemp, D. B., Izumi, K. (2014) Multiproxy geochemical analysis of a Panthalassic margin record of the early Toarcian oceanic
anoxic event (Toyora area, Japan). Palaeogeogr. Palaeoclimatol. Palaeoecol., 414: 332-341.
Kobayashi, T. (1926) Note on the Mesozoic Formations in Prov. Nagato, Chugoku, Japan. Jour. Geol. Soc. Japan, 33: 1-9.
Konwert, M., Stumpf, S. (2017) Exceptionally preserved Leptolepidae (Actinopterygii, Teleostei) from the late Early Jurassic

20

A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

Fossil-Lagerstätten of Grimmen and Dobbertin (Mecklenburg-Western Pomerania, Germany). Zootaxa, 4243: 249-296.
Kumon, F., Ura, M., Akiyama, M. (1999) The Kuruma Group. pp. 11-21 in The Academic Research Group of Dinosaur Fossil
Footprints of Otari Village (ed.), Report of the Research of Dinosaur Footprints. (In Japanese)
Luo, Z. -X., Wu, X. -C. (1994) The small tetrapods of the Lower Jurassic Lufeng Formation, Yunnan, China. pp. 251-270 in:
Fraser, N. C., Sues, H. -D. (eds), In the Shadow of Dinosaurs—Early Mesozoic Tetrapods. Cambridge University Press,
Cambridge.
Luo, Z. -X., Wu, X. -C. (1995) Correlation of vertebrate assemblage of the Lower Lufeng Formation, Yunnan, China. pp.
83–88 in: Sun, A., Wang, Y. (eds.), Sixth Symposium on Mesozoic Terrestrial Ecosystems and Biota, Short Papers. China
Ocean Press, Beijing.
Manabe, M., Hasegawa, Y. (1998) A crocodile from the Early Jurassic Toyora Group, Yamaguchi, Japan. Mem. Natn. Sci. Mus.
Tokyo, 31: 73-77.
Matsukawa, M., Shibata, K., Kukihara, R., Koarai, K., Lockley, M. G. (2005) Review of Japanese dinosaur track localities:
implications for ichnotaxonomy, paleogeography, and stratigraphic correlation. Ichnos, 12: 201-222.
Meister, C. (1988) Ontogenèse et èvolution des Amaltheidae (Ammonoidea). Eclogae Geol. Helv., 81: 763-841.
Nakada, K., Matsuoka, A. (2011) International correlation of the Pliensbachian/Toarcian (Lower Jurassic) ammonoid
biostratigraphy of the Nishinakayama Formation in the Toyora Group, southwest Japan. Newsl. Stratigr., 44: 89-111.
Nakada, K., Matsuoka, A. (2012) Early Jurassic fish from the Nishinakayama Formation of the Toyora Group, Yamaguchi
Prefecture, southwest Japan. Sci. Rep., Niigata Univ. (Geology), 27: 1-7.
Nesbitt, S. J. (2011) The early evolution of archosaurs: relationships and the origin of major clades. B. Am. Mus. Nat. Hist., 352:
292 pp.
Nesbitt, S. J., Stocker, M. R. (2008) The vertebrate assemblage of the Late Triassic Canjilon Quarry (northern New Mexico,USA), and the importance of apomorphy-based assemblage Comparisons. J. Vertebr. Paleontol., 28: 1063-1072.
Oe, F., Chiba, M. (1988) On a teleost fish from the Daira River, Asahi Town, Toyama Prefecture (Kuruma Group). Abstractsof the 1988 Annual Meeting of the Palaeontological Society of Japan: 83. (In Japanese with English title translated byauthors)
Ősi, A., Young, M. T., Galácz, A., Rabi, M. (2018) A new large-bodied thalattosuchian crocodyliform from the Lower Jurassic(Toarcian) of Hungary, with further evidence of the mosaic acquisition of marine adaptations in Metriorhynchoidea.
PeerJ, 6: e4668.
Simmons, D. J. (1965) The non-therapsid reptiles of the Lufeng Basin, Yunnan, China. Fieldiana: Geol., 15: 1-93
Sonoda, T., Goto, M., Terada, K.（2015）Discovery of the Early Jurassic turtle from the Kuruma Group, Central Japan. Mem.
Fukui Pref. Dinosaur Mus. 14: 19-24. (In Japanese with English abstract)
Takahashi, E., Uto, S., Mihara, A., Takahashi, H. (1965) Palaeozoic and Mesozoic Formations of the Eastern and the Southern
Parts of the Tabe Basin, Yamaguchi Prefecture. Sci. Rep. Yamaguchi Univ., 15: 33-49. (In Japanese)
Takakuwa, Y., Gunma Fossil Club. (2011) New occurrence of acrodontid shark (Chondrichthyes) from the Lower Jurassic
Iwamuro Formation in Numata City, Gunma Prefecture, central Japan. Bull. Gunma Mus. Natu. His., 15: 147-152. (InJapanese with English abstract)
Takakuwa, Y., Hasegawa, Y. (1992) A plesiosaurid fossil from the Kuruma Group, Otari Village, Nagano Prefecture, and its
paleogeographic. Abstracts of the Regular Meeting of the Palaeontological Society of Japan, 141: 14. (In Japanese withEnglish title translated by authors)
Tanabe, K. (1991) Early Jurassic macrofauna of the oxygen-depleted epicontinental marine basin in the Toyora area, west
Japan. Saito Ho-on Kai Spec. Pub., 3: 147-161.

21

B. T. BREEDEN III and K. IZUMITanabe, K., Inazumi, A., Ohtsuka, Y., Katsuta, T., Tamahama, K. (1982) Litho- and biofacies and chemical composition of theLower Jurassic Nishinakayama Formation (Toyora Group) in west Japan. Mem. Ehime Univ. Sci. Ser. D (Earth Sci.), 9:
1-16. (In Japanese with English abstract)
Tanimoto, M., Okura, M. (1989) Plesiosaurid teeth from Daire-gawa, the Kuruma Group (Early Jurassic) Toyama Prefecture,
Japan: A preliminary report. Bull. Hobetsu Mus., 3: 27-32. (In Japanese)
Tennant, J. P., Mannion, P. D., Upchurch, P. (2016) Evolutionary relationships and systematics of Atoposauridae(Crocodylomorpha: Neosuchia): implications for the rise of Eusuchia. Zool J Linn Soc-Lond, 177: 854-936.
Them, T. R., III, Gill, B. C., Caruthers, A. H., Gröcke, D. R., Tulsky, E. T., Martindale, R. C., Poulton, T. P., Smith, P. L. (2017)
High-resolution carbon isotope records of the Toarcian Oceanic Anoxic Event (Early Jurassic) from North America andimplications for the global drivers of the Toarcian carbon cycle. Earth Planet. Sci. Lett., 459: 118-126.
Tykoski, R. S., Rowe, T. B., Ketcham, R. A., Colbert, M. W. (2002) Calsoyasuchus valliceps, a new crocodyliform from the
Early Jurassic Kayenta Formation of Arizona. J. Vertebr. Paleontol., 22: 593-611.
Tykoski, R. S. (2005) Vertebrate paleontology in the Arizona Jurassic. pp. 72-93 in McCord, R. D. (ed.), Vertebrate
Paleontology of Arizona, Mesa Southwest Museum Bulletin Number 11.
You, H. -Y., Azuma, Y., Wang, T., Wang, Y. -M., Dong, Z. -M. (2014) The first well-preserved coelophysoid theropod dinosaur
from Asia. Zootaxa, 3873: 233-249.
Young, C. -C. (1941) A complete osteology of Lufengosaurus huenei Young (gen. et. sp. nov.) from Lufeng, Yunnan, China.
Palaeontol. Sinica, New Ser. C, 7: 1-53.

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A review of the vertebrate fossil assemblage from the Lower Jurassic Nishinakayama Formation

豊田町石町の下部ジュラ系西中山層産の脊椎動物化石
ブリーデン・ベンジャミン 1), 泉 賢太郎 2)
1)〒 84112-0102 米国ユタ州ソルトレイクシテイ 115 S 1460 E ユタ大学地質学・地球物理学科

2)〒 263-8522 千葉県千葉市稲毛区弥生町 1-33 千葉大学教育学部

要約：山口県下関市豊田町石町の下部ジュラ系西中山層（海成層）は，小規模だが，重要な脊椎動物の化石を産出している．この脊椎動物の化石群集には魚類（Teleostei: Leptolepidiformes?），ワニ類（Crocodylomorpha）やカメ類（Testudinata）といった爬虫類の体化石がある．しかしながら脊椎動物の生痕化石は未だ確認されていない．西中山層のワニ類やカメ類と来馬層群のカメ類の甲羅の断片化石は，それぞれのクレードにおいて日本最古の化石記録である．西中山層のカメ類は，下部ジュラ系海成層のカメ類として関節状態の報告は唯一で，特に重要である．
以上のことから化石は少数であるものの，西中山層の脊椎動物化石の群集は，世界のジュラ紀前期の脊椎動物群集と並ぶ国際的に重要な群集である．また日本におけるジュラ紀前期の四肢動物の化石の数少ない貴重な報告である．
キーワード：古脊椎動物学，ジュラ紀古世，豊浦層群，トアルシアン海洋無酸素
事変 .

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