Alpkarakush kyrgyzicus Rauhut, Bakirov, Wings, Fernandes & Hübner, 2024, sp. nov.
Creators: name: Rauhut, Oliver W. M.
name: Bakirov, Aizek A.
name: Wings, Oliver
name: Fernandes, Alexandra E.
name: Hübner, Tom R.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Metriacanthosauridae
Alpkarakush
Alpkarakush kyrgyzicus

description: Alpkarakush kyrgyzicus sp. nov.
ZooBank LSID: urn:lsid:zoobank.org:act: 2ABC5B5C-7119- 4D12-842F-1CB9D88CF2CB.
Etymology: The species epithet refers to the Kyrgyz Republic, the provenance of the type specimen.
Holotype: Partial skeleton (Fig. 3), including both postorbitals (IGB 2-1, 2-2), the left quadratojugal (IGB 2-9), two posterior dorsal vertebral centra (IGB 2-10,2-11), two partial dorsal neural spines (IGB 2-12, 2-13) and a fragment of the dorsal neural arch (IGB 2-22), five sacral vertebrae (IGB 2-14, 2-15), several dorsal ribs (IGB 2-16, 2-17, 2-18, 2-19, 2-20, 2-21), a manual phalanx (IGB 2-24), a manual ungual (IGB 2-47), a partial left ilium (IGB 2-25), a partial pubes (IGB 2-26, 2-27, 2-28, 2-29), both articulated ischia (IGB 2-30, 2-31), a complete left (IGB 2-32) and right (IGB 2-33) femora, a complete left (IGB 2-34) and right tibiae (IGB 2-35), an almost complete left fibula (IGB 2-36, 2-37), left and right astragalocalcanea (IGB 2-38, 2-39), a partial left distal tarsal IV (IGB 2-40), a left metatarsal II (IGB 2-41), a left and right metatarsal III (IGB 2-42, 2-43), a pedal phalanx (IGB 2-44), and two pedal unguals (IGB 2-45, 2-46).
Paratype: Fragmentary postcranium of a smaller individual, from the same locality as the holotype, including the left and right pubes (IGB 2-49, 2-50, 2-51, 2-52), a proximal fragment of the right ischium (IGB 2-53), and a right tibia (IGB 2-48).
Referred material: Several isolated teeth (IGB 2-3, 2-4, 2-5, 2-6, 2-7, 2-8, 2-27) and a furcula (IGB 2-23) from the type locality are referred to the same taxon. In the case of the teeth IGB 2-6, 2-7, 2-8, and 2-27, this referral should be regarded as tentative (see below).
Type locality and horizon: Locality FTU-1, just west of the town of Tashkumyr, Jalal-Abad Oblast, Kyrgyzstan (Figs 1, 2). The specimens were found in the higher part of the Balabansai Formation, Callovian.
Diagnosis: Alpkarakush kyrgyzicus can be diagnosed by the following combination of characters (autapomorphies are indicated by *): extremely developed supraorbital brow on the postorbital, overhanging the orbit; posterior dorsal vertebrae with a channel leading from the centroprezygodiapophyseal fossa posteromedially into pneumatic chambers in the neural arch*; sacral vertebrae with fused neural spines that are approximately as high as the combined height of the vertebral centrum plus neural arch; manual phalanx II-1 with a ventral sulcus proximally that is almost completely enclosed by medial and lateral ventral flanges*; dorsal margin of the ilium slopes steeply posteroventrally*; brevis fossa on ilium reduced to a small medial shelf; shaft of pubis strongly bowed anteriorly; well-developed longitudinal depression on the posterolateral side of the pubic shaft adjacent to the pubic boot (based on paratype); unusually high pubis/tibia ratio (1.22 or higher); articulated ischia with pronounced ischial boots that are convex distally and fused anteriorly, but separated posteriorly; ischium with small obturator flange that is offset from the pubic peduncle; pubic peduncle of ischium very long; iliac articulation in proximal ischium cup-shaped; narrow and deep intercondylar groove on the anterior side of distal femur*; robust and well-developed medial epicondylar crest on distal femur, considerably offset proximally from distal end*; tibia with robust, bulbous fibular flange; astragalus and calcaneum fused.
Description
Cranium
Only both postorbitals and the left quadratojugal are preserved of the cranium. In addition, several teeth were collected from the site, at least several of which represent the same taxon with high certainty.
Postorbital: Both postorbitals are preserved (Fig. 4). The left element (IGB 2-1) is complete (Fig. 4A–D), while the right postorbital (IGB 2-2) has been reconstructed from numerous fragments and misses several of the finer margins (Fig. 4E, F). As in most non-maniraptoriform theropods, the postorbital is a triradiate, T-shaped bone, in which the ventral (jugal) process is considerably longer than either the anterior (frontal) or posterior (squamosal) processes. The postorbital is c. 20 cm high and 17.5 cm long over the anterior and posterior processes, of which the anterior process is longer (c. 8 cm) than the posterior process (5 cm).
The most conspicuous character of the postorbital is an extremely developed, rugose orbital brow that covers the posterior two-thirds of the anterior process and the anterior half of the junction of the three processes (Fig. 4A, D, E). This brow forms a massive lateral and dorsal swelling with a rugose lateral surface, composed of numerous small tubercles, giving it a granulate texture. It considerably overhangs the posterodorsal part of the orbital cavity, thus forming a mediolaterally deeply concave dorsomedial surface of this opening at the junction of the processes.Strongly developed postorbital brows are generally present in carcharodontosaurids (e.g. Sereno et al. 1996, Coria and Currie 2006, Canale et al. 2022), and also in the metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Gao 1999). However, whereas carcharodontosaurids usually have an ornamentation of ridges and grooves, metriacanthosaurids show a similar granulated structure as it is also present in Alpkarakush. From the posterior margin of the brow, a marked oblique step extends posteroventrally over the lateral surface of the ventral process (Fig. 4A), as in Sinraptor (Currie and Zhao 1993). This step meets the convex posterior margin of this process at approximately its half height and separates a smooth posterodorsal surface from a more rugose anteroventral surface, which covers most of the lateral surface of the ventral process, similar to the condition in many abelisaurids (e.g. Sampson and Witmer 2007, Cerroni et al. 2020). Similar low rugosities also cover the lateral and dorsal surfaces of the anterior process anterior to the orbital brow. The anterior end of the anterior process is turned ventrally, as in Irritator (Schade et al. 2023). The anterior margin of the anterior process is massive (c. 15 mm thick dorsoventrally), but lacks an articular facet for the lacrimal/prefrontal, indicating that the latter elements did not contact the postorbital, in contrast to abelisaurids (Bonaparte and Novas 1985, Bonaparte et al. 1990, Sampson and Witmer 2007) and carcharodontosaurids (Sereno et al. 1996, Coria and Currie 2002, Sereno and Brusatte 2008). The posterior process is short and spike like, being wider mediolaterally than dorsoventrally over its entire length, as in Sinraptor (Currie and Zhao 1993), but in contrast to most theropods, where this process usually forms a dorsoventrally oriented, triangular sheet of bone (e.g. Madsen 1976, Britt 1991, Brusatte et al. 2012, Rauhut et al. 2016). In dorsal view, this process tapers posteriorly and shows weak longitudinal striations where it was overlapped by the squamosal (Fig. 4D). There is a marked asymmetry in the development of the lateral surface of the posterior process of the left and right postorbital; whereas it is dorsoventrally narrow in the former, and was probably underlain by the ventral anterior process of the squamosal, the right element shows a deep longitudinal groove on the lateral surface of this process (Fig. 4E), in which the anterior process of the squamosal might have fitted. A similar groove on the lateral side of the posterior process of the postorbital is found in Sinraptor (Currie and Zhao 1993).
In medial view, a marked ridge forms the posterior and posterodorsal margins of the orbit and extends from the anteromedial end of the anterior process to the ventral end of the ventral process (Fig. 4C, F). A stout flange of bone extends from the ventral end of this ridge towards the base of the posterior process, thus forming the markedly convex dorsal part of the anterior margin of the infratemporal fenestra; the ventral part of this margin was formed by the dorsal process of the jugal, which the postorbital overlapped anteriorly in the ventral third of its height. The facet for the jugal is developed as a broad, posteriorly opening groove on the posterior surface of the orbital ridge (Fig. 4B), as in megalosauroids (e.g. Britt 1991, Sereno et al. 1994, Sadleir et al. 2008, Rauhut et al. 2016, Schade et al. 2023). Anterior to the orbital ridge, a small anterior flange intruding into the orbit was obviously present, similar to the situation in Sinraptor (Currie and Zhao 1993) and other metriacanthosaurids (Gao 1999), but is damaged on both sides. At the junction of the three processes, a deep, oval depression dorsal to the orbital ridge marks the contact with the laterosphenoid. (Fig. 4C, F)
In dorsal view, the supratemporal fossa extends onto the medial side of the anterior process of the postorbital and forms a smooth, dorsally facing shelf anterior to the laterosphenoid articulation (Fig. 4D), as in Sinraptor (Currie and Zhao 1993). Anteromedial to this shelf, the relatively thin articular surface for the frontal is marked by a groove in the anteromedial margin of the bone. Behind the laterosphenoid articulation, the margin of the supratemporal fossa extends onto the dorsal surface of the medial side of the junction of the three processes and is offset posteroventrally by a marked step from the medial surface of the posterior flange on the ventral process. Posterodorsally, an abrupt flexure separates the margin of the supratemporal fossa from the dorsal surface of the base of the posterior process. This surface is flat and marked by a sharp dorsal rim laterally. Apparently, all of this surface was overlapped by the dorsal process of the squamosal, as indicated by the sharp lateral rim and a small raised lip at the anterior end of this surface (Fig. 4D). If this really was the case, the dorsal anterior process of the squamosal would have reached considerably more anteriorly than the ventral process and would have reached the level of the posterior margin of the orbit, an unusual condition in theropods, including metriacanthosaurids (Currie and Zhao 1993).
Quadratojugal: The left quadratojugal (IGB 2-9) is almost completely preserved, missing only minor parts at the anterior end (Fig. 5). It has a long and slender anterior (jugal) process and a ventrally wide, dorsally tapering triangular dorsal (squamosal) process. The element is considerably longer anteroposteriorly (> 180 mm) than high dorsoventrally (128 mm), indicating a large, at least ventrally anteroposteriorly wide, infratemporal fenestra, as in other metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Gao 1999). There is a medial, laterally facing shelf in the anterior part of the anterior process for the reception of the dorsal posterior process of the jugal (Fig. 5A), and a ventromedially facing groove on the ventromedial surface for the ventral posterior process of the jugal (Fig. 5C). A similar, but more ventrally placed groove is present in Allosaurus (Madsen 1976), and Currie and Zhao (1993) described a medial groove on the anterior process of the quadratojugal for Sinraptor, but interpreted it as the articulation for a seemingly unique, medial posterior process of the jugal in this taxon. Whereas the shelf for the dorsal jugal process ends approximately 25–30 mm anterior to the dorsal quadratojugal process, the medioventral groove continues to the level of the posterior margin of the infratemporal fenestra, indicating that the ventral jugal process was considerably longer than the dorsal process, as in many theropods. The anterior process and the ventral margin of the main quadratojugal process are somewhat thickened, as is the posterior margin of the dorsal process. In contrast, the lamina spanning between the two processes forms a thin sheet of bone that is slightly depressed in medial view. Posteriorly, a flat, oval surface at the posteroventral end of the bone that faces medially and slightly ventrally marks the contact with the lateral surface of the lateral condyle of the quadrate (Fig. 5C). Above this facet, the posterior margin flexes slightly medially, probably for continued contact with the quadrate shaft. There is no medial indentation in this margin, indicating that, if a quadrate foramen was present, it was mainly or entirely enclosed by the quadrate. In the dorsal third of the bone, there is a widened, posteriorly facing facet that was posteriorly overlapped by the lateral lamina of the quadrate (Fig. 5B), as e.g. in Yangchuanosaurus (Dong et al. 1983). In medial view, a very low, anterodorsally extending ridge delimits a medially facing, oval facet at the tip of the dorsal process that might have been overlapped by the ventral process of the squamosal. Overall, the quadratojugal of Alpkarakush seems to be more robust than that of Sinraptor or Yangchuanosaurus, especially in the more evenly tapering dorsal process (Dong et al. 1983, Currie and Zhao 1993, Gao 1999).
Teeth: Several partial to complete teeth have been collected from site FTU-1 (Fig. 6). Some are clearly theropod teeth, being labiolingually compressed, recurved, and showing serrated carinae (IGB 2-3, 2-4, 2-5). Only one of these teeth is complete (IGB 2-3; Fig.6D, E), with a fore-aft basal length of approximately 21.5 mm and a crown height of c. 40 mm, and are thus in the size range of maxillary teeth of Sinraptor (Hendrickx et al. 2020), but slightly smaller than the larger lateral teeth of Yangchuanosaurus hepingensis (Gao 1999). The other two teeth are partial and were considerably smaller. As tooth size varies considerably in theropods, both taxonomically and also in respect to their position within the jaw, these teeth might belong to the same individual as the rest of the bones, although this cannot be shown with certainty. Even more problematic are the remaining four teeth or tooth fragments (IGB 2-6, 2-7, 2-8, 2-27), which have a rather round cross-section, are notably recurved, and lack carinae or serrations. The best preserved of these teeth (IGB 2-6; Fig. 6A–C) has a fore-aft basal length of 16 mm and a crown height of also c. 40 mm, and is thus very similar in size to the largest theropod lateral tooth present, and also within the size range of premaxillary teeth of Sinraptor (Hendrickx et al. 2020). These teeth resemble teeth of certain crocodyliforms (e.g. pholidosaurids), and a similar tooth from the Balabansai Formation was indeed briefly described and figured by Nessov et al. (1989: pl. 2, fig. 14) as a probable thalattosuchian crocodyliform. However, most Jurassic terrestrial crocodiles are considerably smaller than the animal that these teeth were derived from, which is also the case for the vast majority of the crocodyliform remains described from the Balabansai Formation, in which teeth or alveoli generally show fore-aft basal lengths of less than 10 mm (Nessov et al. 1989, Averianov 2000). It should be noted that Averianov (2000: 778) also mentioned a large partial crocodyliform tibia, but he neither figured this specimen nor described it in detail. Furthermore, no crocodyliform remains were found at the FTU-1 locality. Thus, we consider the possibility that these teeth represent premaxillary teeth of Alpkarakush, as such teeth are often more rounded and differ in morphology from the lateral teeth in other theropods (e.g. in Ceratosaurus, see Madsen 1976, Madsen and Welles 2000), and longitudinal striations are also known in the premaxillary teeth of some theropods (e.g. Ceratosaurus; see Madsen 1976, Madsen and Welles 2000, Soto and Perea 2008) and in all teeth in baryonichine spinosaurs (e.g. Charig and Milner 1997). However, although Sinraptor also has more labiolingually thickened premaxillary teeth, these lack striations and bear serrations in this taxon, thus differing from the teeth described here (Hendrickx et al. 2020). Thus, the interpretation as possible premaxillary teeth of Alpkarakush should be seen as very tentative until future finds might show that such teeth are indeed present in theropods, or confirm a crocodyliform identification of these teeth. The following descriptions are mainly based on the two most complete specimens, IGB 2-6 and IGB 2-3.
The possible premaxillary tooth IGB 2-6 has an almost round cross-section at the base (16 mm long, 14.5 mm wide) and is notably recurved, so that the apical tip would lie distal to the tooth base when the tooth was placed in the jaw (Fig. 6A–C). The cross-section remains round throughout the height of the tooth, as there are no mesial or distal carinae. Very fine longitudinal striations are present throughout the labial and lingual sides.
The lateral tooth IGB 2-3 is considerably labiolingually compressed, being 21.5 mm long and 11 mm wide at the base, but only moderately recurved (Fig. 6D, E), as it is the case also in Sinraptor (Hendrickx et al. 2020). Its cross-section here is drop shaped, being widest towards the mesial end and gradually tapering distally into a sharp carina. Both labial and lingual sides are mesiodistally convex up to the carinae, the probably lingual side more notably so than the labial side, as the distal carina is somewhat displaced towards the labial side. Both carinae extend all the way to the root of the tooth, as in Sinraptor (Hendrickx et al. 2020) and several other theropods (Hendrickx et al. 2019). Small serrations are present on both the mesial and distal carina, with 11–12 denticles per 5 mm on both the mesial and distal carinae, as in the mid-section of the carinae of the largest lateral teeth in Sinraptor (Hendrickx et al. 2020). The individual denticles are chisel shaped, approximately as long mesiodistally as apicobasally and lack interdenticular sulci, in contrast to Sinraptor (Hendrickx et al. 2020) and several other basal tetanurans (Hendrickx et al. 2019). Likewise, pronounced marginal enamel wrinkles are also absent, although a few weak bandings are visible under oblique light on the probable labial side next to the distal carina. The enamel has a very finely developed braided texture (sensu Hendrickx et al. 2015).
Axial skeleton
The axial skeleton of Alpkarakush is represented by two posterior dorsal vertebral centra with fragmentary neural arches, several fragments of dorsal neural arches and spines, the central and partial neural arches (including spines) of the sacrum, and several partial to almost complete dorsal ribs (Figs 7–10). For vertebral measurements see Table 1.
Dorsal vertebrae: Two posterior dorsal vertebral centra are preserved (IGB 2-10, 2-11; Figs 7A–F, 8A). The centra are higher than wide, with oval to almost round articular surfaces, and very slightly higher than long. The articular facets are slightly amphicoelous, more notably posteriorly than anteriorly. The centra are notably constricted to almost half the width of the articular facets in ventral view (Fig. 7C), and are ventrally broadly rounded to flattened, especially in the probably more posterior element IGB 2-11. Pleurocoels are absent, but there are large, oval pleurocentral depressions on the dorsal part of the lateral sides (Fig. 7A, D), as in many basal tetanuran theropods, including metriacanthosaurids (Currie and Zhao 1993, Gao 1999). Internally, the dorsal half of the centrum is made up of spongy bone, while there is an asymmetrically developed larger hollow present on the ventral half in IGB 2-11, filled with large calcite crystals. Notable, but short, striations are present on the lateral side along the edges of the articular facets.
Only the base of the neural arch is present in both specimens. It was apparently slightly more than half the height of the centrum and enclosed a large neural canal that is somewhat constricted ventrally. In IGB 2-10, the neural canal slightly indents the dorsal margin of the centrum, but that did not seem to be the case in IGB 2-11. Anteriorly, the broken bases of the stout, posterodorsally directed centroparapophyseal laminae are preserved. Dorsal to these laminae, a large, round foramen led from the centroprezygodiapophyseal fossa posteromedially into the neural arch (Fig. 7B, E, F), which here obviously had large chambers above the neural canal, separated medially by a robust median lamina. Elaborate pneumatization of the neural arch is also present in Sinraptor, where a pneumatic foramen enters the neural canal from the postzygocentrodiapophyseal fossa in the posterior dorsal vertebrae (Currie and Zhao 1993). An isolated fragment of a neural arch (IGB 2-22, which might belong to IGB 2-10, but does not directly fit on the broken base of the centroparapophyseal lamina; Fig. 7B) shows the dorsal end of the thin centroparapophyseal lamina, the parapophysis, and the anteromedially directed stem of the prezygapophysis, which together form the lateral border of the large foramen leading into the neural arch. This fragment also shows that the parapophysis was placed anterodorsally on the neural arch and did not project strongly laterally, in contrast to abelisauroids (e.g. Bonaparte et al. 1990, O’Connor 2007, Carrano et al. 2011, Filippi et al. 2018) and piatnitzkysaurids (Bonaparte 1986). The prezygapophyses are preserved in IGB 2-11, though somewhat distorted (Fig. 7D, E). They are relatively large, standing at an angle of approximately 25–30° from the horizontal and have anterolaterally rounded articular surfaces that are approximately as wide as long. Anteriorly, the prezygapophyses form a robust, ventrally widening hypantrum (Fig. 7E). In contrast to some theropods, such as Condorraptor (Rauhut 2005), the interprezygapophyseal gap did not widen posterior to the hypantrum.
Parts of two probably dorsal neural spines are present (IGB 2-12, 2-13). They form thin, rectangular sheets of bone that slightly widen towards their dorsal end posteriorly. The anteroposteriorly larger and more complete spine (IGB 2-12; Fig. 7G) shows well-developed anterior and posterior ridges for the attachment of the interspinal ligaments, which almost reach the apex of the spine, as in Yangchuanosaurus hepingensis (Gao 1999).
Sacrum: Most of the sacrum is preserved, but in a very poor state of preservation (Fig. 8), as the articulated vertebrae were obviously collected from already largely eroded sediment at the surface. The anterior four sacral vertebrae were fused and collected in a jacket, together with their spines (IGB 2-14; Fig. 8), whereas the last sacral vertebra (IGB 2-15; Fig. 9) was isolated. The total length of the articulated sacrum is 54cm, with the length of the individual vertebrae being all of subequal length ranging between 10.5 and 11 cm, unlike the situation in Yangchuanosaurus hepingensis, where the first and last sacral vertebrae are notably longer than sacrals 3 and 4 (Gao 1999). The anterior articular surface of the sacrum is subequal in size to the articular surfaces of the posterior dorsal vertebrae, but the articulations between the sacral vertebrae are reduced in width, as in Yangchuanosaurus hepingensis (Gao 1999), although not to the degree seen in Siamotyrannus (Samathi 2013) and many ceratosaurs (e.g. Gilmore 1920, Bonaparte et al. 1990, Rauhut and Carrano 2016), and, in contrast to the latter, the boundaries between the vertebrae are still visible (Fig. 8B, C). The centra are strongly constricted in the middle, especially in sacrals 2 and 3, with narrow, transversely rounded to flattened ventral surfaces. Pleurocoels or marked lateral depressions are absent in the vertebral centra, as it is also the case in Sinraptor (Currie and Zhao 1993).
The neural arches of these vertebrae are very poorly preserved, and little can be said about their morphology. Only the transverse process of the first sacral is present; it is massive, round in outline distally and placed on the neural arch over the anterior end of the centrum, as in Sinraptor (Currie and Zhao 1993) or Piatnitzkysaurus (MACN CH 895), but in contrast to Allosaurus, where the transverse process is dorsoventrally elongate (Madsen 1976). Otherwise, only the neural spines can be discerned. At least from sacral one to three, the neural spines are anteroposteriorly long, plate-like and seem to have been completely fused to each other (Fig. 8A), as in Yangchuanosaurus hepingensis (Gao 1999), whereas the spines seem to have been fusedbasallybutremainedseparateddistallyin Yangchuanosaurus magnus (Dong et al. 1983) and are closely spaced but apparently separated over their entire height in Shidaisaurus (Wu et al. 2009). Their height approximately equals the height of the sacral centra plus the neural arch, and is thus similar to the relative height of the elongate posterior dorsal neural spines in other metriacanthosaurids (Huene 1926, Dong et al. 1983, Currie and Zhao 1993, Gao 1999, Wu et al. 2009).
The last sacral vertebra (IGB 2-15), which was found isolated from the rest of the sacrum, is largely preserved, but lacks the anterior end of the neural arch, the left postzygapophysis, and the neural spine (Fig. 9). The anterior articular surface is considerably smaller than the posterior surface, but the vertebra was obviously not fused to the rest of the sacrum. The centrum is considerably more massive than that of the middle sacrals, as in Yangchuanosaurus hepingensis (Gao 1999), and broadly rounded ventrally, although a very faint midline keel might be present in the anterior part; this keel is only visible under oblique light, but obvious to the touch of the surface. Laterally, relatively small, but marked pleurocentral depressions are present on the dorsal half of the centrum (Fig. 9A, B). However, although these depressions are rather sharply defined, they do not penetrate the cortex and do not lead into internal cavities. Such a sharply defined depression seems to be absent in Sinraptor (Currie and Zhao 1993) and Shidaisaurus (Wu et al. 2009).
The neural arch is narrow anteriorly and mainly formed by the massive bases of the transverse processes in lateral view. The latter are placed over approximately the half length of the centrum on the neural arch and they are higher dorsoventrally than long anteroposteriorly. No lateral lamina supporting the transverse process are present, but there is a very shallow depression anteriorly on the base of the transverse process, and posteriorly, a short, very stout ridge extends from the base of the postzygapophysis towards the roof of the transverse process. The processes itself seem to have been very short, and an anteroposteriorly expanded, largely vertically oriented bone that begins just a few centimetres lateral from the neural arch probably represent the fused sacral ribs, as in Sinraptor (Currie and Zhao 1993); the almost vertical dorsoventral orientation of these bones, especially on the better preserved left side, might be due to mediolateral compression of the obviously laterodorsally oriented ribs. A pronounced, sharp-edged ridge is present on the anteroventral part of the sacral rib on the left side, and fades rapidly posterodorsally into the surface of the bone (Fig. 9A); this part seems to be abraded on the right side. The neural canal is small anteriorly, but considerably wider posteriorly, where it slightly indents the dorsal margin of the articular facet posteriorly. The anterior side of the neural arch is largely damaged, but there did not seem to have been normal prezygapophyses developed, but the arch around the neural canal was narrow and probably in direct contact and possibly fused with the neural arch of the preceding vertebra. Above the neural canal, only a narrow (c. 10 mm), vertical strut of bone is present. Posteriorly, a large, triangular depression was present on the posterior side of the neural arch on either side of the base of the neural spine, above the neural canal and below the very dorsally placed postzygapophyses. This depression is subdivided by a thin lamina that extends from the lateral roof of the neural canal towards the dorsal base of the transverse process (Fig. 9C). An equivalent lamina seems to be also present in Sinraptor (Currie and Zhao 1993). The depression is bordered medially by a thin lamina that extends from the medial edge of the postzygapophysis ventrally and forms part of the hyposphene. The laminae extending down from the left and right postzygapophysis remain separated by the very narrow postspinal fossa over almost their entire length and only meet in a marked ventral mediolateral expansion of the hyposphene (Fig. 9C). The latter forms the ventral border of the triangular depression noted above, and its ventrolateral border is formed by a thin and low lamina that extends from the lateral end of the ventral expansion of the hyposphene dorsolaterally. Dorsally, the depression is bordered by the very stout postzygodiapophyseal lamina. The postzygapophysis is small (c. 23 by 17 mm), high oval in shape and placed high on the neural arch, at the level of the dorsal surface of the transverse process. As in the last sacral vertebra of Allosaurus (Madsen 1976) and Sinraptor (Currie and Zhao 1993), it is very steeply inclined, at an angle of c. 75° from the horizontal (Fig. 9C). The neural spine is missing. Its base is mediolaterally slender, and the spine was either anteroposteriorly short or largely placed over the anterior part of the centrum.
Dorsal ribs: Several partial to almost complete dorsal ribs are present, plus numerous isolated rib sections that cannot be fitted to any of the more complete elements. The following description is based on the more complete ribs IGB 2-16, 2-17, 2-18, 2-19, and 2-20, most, if not all of which seem to be derived from the right side (Fig. 10).
As in all dinosaurs, the dorsal ribs are double headed, with a short tuberculum and a long and slender capitulum. These two slightly thickened articular processes come together at the base of the rib shaft, but are connected above that by a thin web of bone that extends from the anteroventral edge of the tuberculum almost to the articular facet of the capitulum. Where the two proximal articular processes meet, the thickened bone extending distally from the tuberculum, which, at the rib head was more dorsoventrally oriented, twists to face more dorsally and thus forms a robust dorsal surface of the proximal rib shaft. Well-developed longitudinal furrows are present on both the anterior and posterior side of the proximal rib shaft (Fig. 10), resulting in a somewhat oblique, T-shaped cross-section of the proximal rib. The posterior one of these furrows extends somewhat further distally than the anterior one, but both disappear within 20 cm from the junction of the articular processes. Distal to this, the rib shaft becomes dorsoventrally high oval in cross-section, being slightly wider anteroposteriorly and dorsally flattened in its dorsal part. Even more distally, the ribs become thin and rod-like, and a very shallow longitudinal furrow appears again on the posterior and sometimes also the anterior side of the distal part of the rib. The proximal parts of the ribs have only a very moderate ventral curvature, indicating a rather deep ribcage in the posterior part of the body, as in Wiehenvenator (Rauhut et al. 2016).
Appendicular skeleton
The appendicular skeleton of Alpkarakush is mainly known from the pelvic girdle and hindlimbs. Of the forelimbs, only a furcula, a manual phalanx, and a manual ungual are preserved. The hindlimb is represented by a partial left ilium, partial pubes of the holotype and almost complete pubes of the paratype, complete ischia of the holotype and partial ischium of the paratype, both femora and tibiae of the holotype, the right tibia of the paratype, almost complete left fibula, both astragalocalcanea, a partial distal tarsal, left metatarsal II, left and right metatarsal III, and several pedal phalanges and unguals of the holotype. For measurements of the hindlimb elements see Table 2.
Furcula: A wide V-shaped bone represents a furcula (IGB 2-23; Fig. 11). With an overall transverse width of 17.5 cm (with maybe some 2–3 cm missing), the bone is rather small for an animal of the size as the holotype of Alpkarakush, but as there is considerable variation in the relative size (and shape) of furculae in theropods (Makovicky and Currie 1998, Nesbitt et al. 2009) and the vast majority of elements recovered from site FTU-1 seem to represent a single individual, we consider it likely that this element comes from the same animal. However, it cannot be excluded that it might represent the smaller, paratype individual. The furcula is formed by two largely symmetrical rami that meet each other at an angle of approximately 140° (Fig. 11A, C). The lateral ends are slender, but the bone becomes dorsoventrally wider towards the median junction. Whereas the rami are narrowly rounded dorsally and more sharp-edged ventrally, the central part is rounded both dorsally and ventrally. The cross-section of the rami is thus roughly teardrop shaped. A hypocleidium, as it is found in a variety of other non-avian theropods (Nesbitt et al. 2009), is absent. Towards the lateral ends, the bone becomes slightly wider anteroposteriorly and there is a laterally widening posterodorsally facing facet on the posterior side of the bone on either side (Fig. 11C). This facet extends over almost half of the length of the rami and is defined ventrally by a poorly developed ridge, which becomes slightly more conspicuous towards the lateral ends. Only a small piece of the very thin epicleideal process is preserved on the left side of the bone. On the posterior side of the right ramus, there is a conspicuous, obliquely oriented flat area of c. 15 mm width, which is bordered medially and laterally by sharp edges (Fig. 11C). Under magnification, the bone fibres are ripped at these edges, as if the bone was cut, and the upper bone layer seems to be stripped away. This area might thus represent a bite mark, where some of the bone was ripped away with the overlying flesh during scavenging of the carcass, or it might also be a trample mark (see Fiorillo 1987).
aLaterally incomplete. bDistorted. cSlightly damaged. dPosteriorly slightly incomplete. eSlightly rotated. fSomewhat eroded.
The element described here represents the first furcula known for a metriacanthosaurid. In general shape, especially in respect to the wide angle between the two rami, it is most similar to furculae of Allosaurus (Chure and Madsen 1996), whereas other non-avian theropods have more pronounced angles and/or a more rounded outline of the furcula angle (Norell et al. 1997, Makovicky and Currie 1998, Lipkin et al. 2007, Nesbitt et al. 2009).
Manus: The manus is represented only by a manual phalanx and a partial ungual (Fig. 12). Unfortunately, metriacanthosaurid forelimbs are generally poorly known, and only a few manual elements have been described for Sinraptor (Currie and Zhao 1993). A possible exception is Xuanhanosaurus, which has an almost complete forelimb, with a partial manus, and was found to be a metriacanthosaurid by Carrano et al. (2012). However, the phylogenetic position of this taxon is uncertain, as it was found as a possible piatnitzkysaurid in some recent phylogenetic hypotheses (Rauhut et al. 2016, Dai et al. 2020) and can take several positions within carnosaurs in our analysis (including as a metriacanthosaurid in the equal weights analysis, see below).
A rather short and robust manual phalanx (IGB 2-24) represent the first phalanx of the second digit of the left manus (Fig. 12A–F). It is 80 mm long, 38 mm wide, and 40 mm high proximally, and 32 mm wide and 27 mm high distally. As in some other theropods, e.g. Allosaurus (Madsen 1976), Asfaltovenator (MPEF-PV 3440), Xuanhanosaurus (IVPP V 6729), and Acrocanthosaurus (Currie and Carpenter 2000), the phalanx is markedly asymmetrical (Fig. 12A, D), though to a much greater extent than in these taxa. Thus, the proximal articular surface is divided into a large, high oval lateral concavity and a much smaller, triangular medial facet that is concave dorsoventrally, but slightly convex mediolaterally (Fig. 12E). Both facets are divided by a dorsoventrally oriented ridge, with the medial being somewhat offset ventrally from the lateral one. Whereas the dorsal margin of the proximal articulation forms a gently convex arch in dorsal view, the ventral margin extends further proximally and forms a pointed proximal end in elongation of the ridge subdividing the facets, which is slightly displaced medially from the midline of the bone. The shaft of the phalanx is constricted to a minimal height and width of 22 mm and slightly obliquely laterodistally oriented. Its cross-section is semioval to triangular proximal to the distal articular end, with a flattened ventral and a narrowing, dorsally-rounded dorsal margin. A markedly rugose tuberosity is found just distal to the proximal end on the ventral part of the medial side, as in Xuanhanosaurus (IVPP V 6729), Asfaltovenator (MPEV PV 3440), and Acrocanthosaurus (Currie and Carpenter 2000), and another, smaller one at about half-length of the bone on its ventrolateral margin. The most conspicuous feature, however, is a ventral sulcus that is placed just lateral to the pointed ventral margin of the proximal articular surface (Fig. 12D). This sulcus extends over half the length of the phalanx, slightly mediodistally directed and distally narrowing, and is defined by a marked step medially and an expanded, overhanging flange laterally. Although a shallow sulcus is also present on the ventral side of phalanx II- 1 in Allosaurus (USNM 4734; DINO 11541) and Acrocanthosaurus (NCSM 14345), it is much broader and less well defined in these taxa. The most similar morphology is found in Xuanhanosaurus (IVPP V 6729), which also has well-developed medial and lateral tubercles on the proximal end of the ventral side of phalanx II-1, in which, however, the ventral sulcus is less enclosed than in Alpkarakush.
The distal articular end is gynglimoidal, with the lateral condyle extending considerably more distally than the medial condyle. A small, triangular extensor groove is present dorsally just proximal to the gynglimus. Well-developed collateral ligament grooves are present on both sides. In lateral view, the lateral condyle is semicircular, whereas the medial condyle is only slightly lower but much shorter proximodistally in medial view.
The manual ungual (IGB 2-47) is notably short and curved (Fig. 12G, H). Based on its size and morphology, it is probably an ungual of the third digit. The proximal end is high oval, with a straight ventral margin. It is subdivided into lateral and medial concavities by a weakly developed dorsoventral ridge. The articular surface is almost twice as high as wide. A stout, but low and little projecting flexor tubercle is present just distal to the articular end on the ventral side. It is offset from the rim of the articulation by a shallow mediolateral groove. The flexor tubercle is less developed than in Allosaurus (Madsen 1976) or Sinraptor (Currie and Zhao 1993), although in the latter case, this might have to do with possible different positions of the unguals compared. Distal to the flexor tubercle, the ungual rapidly narrows dorsoventrally, considerably more so than in any ungual of Allosaurus, for example (Gilmore 1920, Madsen 1976). Dorsally, the dorsal margin of the ungual is not offset from the articular end by a groove or lip and curved directly ventrally from the articular end, in contrast to many coelurosaurian theropods, where the unguals first curve upwards, so that the highest point of the ungual is placed above the rim of the articular facet if the latter is held vertically. Well-developed and largely symmetrically arranged claw grooves are present on either side of the ungual. In comparison with the part dorsal to the claw groove, the part directly ventral to it bulges outwards. This is also the case in the area where the bone becomes dorsoventrally narrower from the flexor tubercle, so that here the area directly ventral to the claw groove was wider than the ventral margin of the ungual. More distally, the ventral side is broad and mediolaterally rounded. The ungual was strongly recurved, but the tip is missing, so nothing can be said about its total length or the position of the tip in relation to the articular end.
Ilium: Large parts of the left ilium are preserved, missing mainly the preacetabular process and most of the pubic peduncle (IGB 2-25; Figs 13, 14). The iliac blade is high above the acetabulum (c. 29 cm, excluding the supraacetabular crest), but slopes steeply posteroventrally (Fig. 13A), so that the posterior end of the iliac blade is only slightly more than a third of the height of this bone above the acetabulum (c. 11 cm). This slope is more pronounced than in Siamotyrannus (Buffetaut et al. 1996), Sinraptor (Currie and Zhao 1993), and Yangchuanosaurus hepingensis (Gao 1999), but similar to Yangchuanosaurus magnus (Dong et al. 1983). A lateral ridge on the acetabular blade above the acetabulum, as it is present in Siamotyrannus (Buffetaut et al. 1996), is absent. The acetabulum was large (c. 20 cm anteroposteriorly), almost two-thirds of the length of the postacetabular process (c. 33 cm). The supraacetabular crest is hood-like, strongly overhanging the acetabulum and covering its dorsal part in lateral view (Fig. 13A), as in non-tetanuran theropods and other metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Gao 1999, Wu et al. 2009), with the exception of Siamotyrannus (Buffetaut et al. 1996, Samathi 2013). Anteroventrally, it extends almost to the pubic articulation on the pubic peduncle, as in Metriacanthosaurus (OUM J 12144). The ischial peduncle is robust, wider mediolaterally than long anteroposteriorly and projects ventrally and slightly posteriorly. Its articular surface has a larger, marked convexity anteriorly and a small posterior flange that is somewhat offset medially from the lateral surface of the peduncle (Fig. 13B) and has a triangular outline in ventral view. The brevis fossa is strongly reduced, similar to the condition in Sinraptor (Currie and Zhao 1993); it is developed as a narrow medial shelf that extends from a flattened ventral area just posterior to the base of the ischial peduncle posterodorsally (Figs 13B, 14), first expanding slightly and then narrowing again towards the posterorodorsal edge of the iliac blade, which is broken off. The lateral brevis shelf is oriented strictly ventrally, and there is no exposure of the brevis fossa in lateral view, as it is found in most megalosaurids (e.g. Britt 1991, Benson 2010) and also in Sinraptor (Currie and Zhao 1993), but not in other metriacanthosaurids (Dong et al. 1983, Buffetaut et al. 1996, Gao 1999, Wu et al. 2009).
In medial view, the medial margin of the base of the pubic peduncle is expanded medially and forms a well-developed flange that overhangs the anterodorsal part of the acetabulum medially (Fig. 13B). Above the pubic peduncle, a deep, anteriorly opening depression is present for the attachment of the sacral rib of the second sacral vertebra (Fig. 13B), as in Allosaurus (Madsen 1976) and Sinraptor (Currie and Zhao 1993). Above the acetabulum, well-developed, dorsoventrally oriented striations indicate the insertion of the sacral rib of sacral vertebra three.
Pubes: The pubes are represented by an almost complete right element (IGB 2-26), missing the proximal peduncles and the distal boot (Fig. 15A–C), a part of the shaft of the left pubis (IGB 2-28; Fig. 15D, E), and the posterior end of the pubic boot (IGB 2-29; Fig. 15F–H) of the holotype and an almost complete left and partial right pubis of the paratype (Fig. 21F–H). The right pubic shaft of the holotype is complete from the proximal expansion towards the peduncles to the distal expansion towards the pubic boot (Fig. 15A–C). The preserved length is 73 cm, with approximately 10–15 cm missing at the distal end and some 5 cm missing at the proximal end. The almost complete left pubis of the paratype is 74 cm long. The shaft of the right pubis of the holotype is long and slender, being notably bowed anteriorly (Fig. 15A, C), more so than in Sinraptor (Currie and Zhao 1993) and Siamotyrannus (Buffetaut et al. 1996), but comparable to Allosaurus (Gilmore 1920, Madsen 1976), many carcharodontosaurs (e.g. Stromer 1931, Canale et al. 2022) or Tyrannosaurus (Brochu 2003), although the pubic shafts are considerably more slender than in all of these taxa. In other metriacanthosaurids, the pubic shafts are straight (Dong et al. 1983, Gao 1999, Wu et al. 2009). Proximally, the shaft is almost round in cross-section, being c. 58 mm deep anteroposteriorly and 56 mm wide mediolaterally. More distally, it becomes more slender, with a minimal anteroposterior depth of c. 48 mm, before it then gradually expands again in its distal half towards the distal boot (Fig. 15A), which is missing in the type, but partially preserved in articulation of the distal left and right pubic shafts in the paratype (Fig. 21F–H). A robust pubic apron was obviously present, but is largely broken away (Fig. 15B, C). It is developed as a flange that extends medially from the anterior edge of the medial side of the pubic shaft. The pubic apron ends considerably above the distal expansion for the pubic boot, so that that the pubes were separated distally anteriorly, as in Sinraptor (Currie and Zhao 1993) and Siamotyrannus (Samathi 2013).
The paratype shows that the pubic boot was steeply angled posteroproximally (Fig. 21G, H), as in Sinraptor (Currie and Zhao 1993), but more so than in Yangchuanosaurus shangyuensis (Dong et al. 1983), Siamotyrannus (Buffetaut et al. 1996), or Shidaisaurus (Wu et al. 2009). Yangchuanosaurus hepingensis seems to have an intermediate condition (Gao 1999). In the distal shaft, a wide longitudinal depression appears on the posterolateral side of the shafts, between the narrow lateral side of the latter and the posterior expansion of the boot. The distal shaft is expanded mediolaterally to form a wide, distal expansion that is rounded triangular in outline in distal view, being wider anteriorly, and has a flat medial side, where the expansions of the left and right sides met. Thus, the articulated pubic boots are heart shaped in outline in distal view (Fig. 21I), as in Sinraptor (Currie and Zhao 1993). Only the anteroproximal part of the posterior expansion of the pubic boots are preserved in the paratype. Here, the left and right boots are fused (Fig. 21H, I). A largely symmetrical, ventrally flattened bone is here interpreted as the posterior part of the conjoined pubic boots of the holotype (IGB 2-29; Fig. 15F–H). If correctly identified, the pubic boot would have been more prominently developed than in Sinraptor. At the proximal break, the bone is mediolaterally slender and anteroposteriorly elongate. No suture is visible, confirming that the posterior parts of the left and right pubic boots were completely fused, as in other basal tetanurans (e.g. Currie and Zhao 1993, Sereno et al. 2008). The lateral walls of the posterior pubic boots are slightly dorsoventrally concave so that the lateral margins of the boot form a slight lateral lip (Fig. 15G); this concavity is consistent with the development of a posterolateral depression in the distal pubic shafts in the paratype. A concavity of the lateral side of the pubic boot and the resulting lateral lip are also present in Sinraptor (Currie and Zhao 1993) and Siamotyrannus (Buffetaut et al. 1996, Samathi 2013). In ventral view, the cojoined boots are rather slender posteriorly but gradually expand anteriorly; the ventral surface is rugose and slightly indented in its midline.
Proximally, the flange of the pubic apron flexes posteriorly towards the obturator plate of the pubis, which is broken away. However,thereisnoflangeonthemoreproximalposteriormargin of the pubis of either the holotype or the paratype, indicating that the obturator foramen was either very large, or not enclosed by bone, as is also the case in other metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Buffetaut et al. 1996). The shaft becomes mediolaterally flattened and expands anteroposteriorly towards the proximal end. Anteriorly, a rather weakly developed pubic tubercle is present (Figs 15A, 21G). It is more proximally placed than the twist of the pubic apron into the obturator plate, and is developed as an elongate, anteriorly directed crest, as in other metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Buffetaut et al. 1996, Gao 1999). Although there is a notable flexure of the pubis at the level of the pubic tubercle (Figs 15B, 21F), it is not as marked as in some other theropods (e.g. Condorraptor; Rauhut 2005).
Ischia: Left and right ischia of the holotype are largely complete and preserved in articulation (IGB 2-30; Fig. 15I–M), with only a minor part of the shaft of the left ischium missing due to erosion, so that the iliac peduncle of the left ischium is preserved in isolation (IGB 2-31). Of the paratype, only a part of the proximal portion of the right ischium is preserved (IGB 2-53; Fig. 21J), missing both peduncles and most of the shaft. The holotype ischium is 68 cm long from the iliac peduncle to the distal extremity. The proximal end is 25 cm long anteroposteriorly and wasdividedintothedorsallydirected,short,butanteroposteriorly broad iliac peduncle and the much longer, slender, anteriorly directed pubic peduncle (Fig. 15I, K). The pubic peduncle is longer than the articular surface between the ischium and ilium, and about twice as long as the iliac peduncle, which contrasts with the much shorter condition of this peduncle in Sinraptor (Currie and Zhao 1993), Yangchuanosaurus hepingensis (Gao 1993), and Monolophosaurus (Zhao and Currie 1993), but is similar to Shidaisaurus (Wu et al. 2009) and Allosaurus (Madsen 1976). The iliac articulation is maximally 8 cm wide and has a large, semioval, cup-shaped concavity posteriorly for the reception of the convex ventral surface of the ischial peduncle of the ilium (Fig. 15L), thus forming a peg-in-socket articulation, as in many abelisauroids (Carrano and Sampson 2008) and carcharodontosaurids (Carrano et al. 2012), but apparently not in Sinraptor (Currie and Zhao 1993). This articulation expands far lateral to the lateral surface of the ischium more distal to it. Anterior to the concavity, a broad, anterodorsally facing and laterally tapering surface represents the posteroventral margin of the acetabulum; towards the pubic peduncle, this margin becomes rapidly narrower, and while the mediolateral width of the acetabulum is c. 85 mm at the iliac-ischium articulation, it is only c. 15 mm at the dorsal rim of the pubic peduncle (Fig. 15L). At the posterior margin of the base of the iliac peduncle, a marks depression on the lateral side marked the insertion area of the m. flexor tibialis internus (Hutchinson 2001a), as it is also present in Siamotyrannus (Buffetaut et al. 1996). The paratype ischium furthermore shows a marked, large, oval tubercle with a slightly rugose surface below this depression just above the base of the shaft; this area is poorly preserved in the holotype.
Whereas the iliac peduncle extends only for c. 6 cm above the ventral margin of the acetabulum, the pubic peduncle extends anterior for c. 14 cm from the posterior margin of this structure. It is semioval in cross-section, with a minimal height of c. 7 cm, although small parts of the thin ventral margin might be missing. The pubic articulation forms an anteriorly directed flat surface with a semioval, ventrally slightly tapering outline that is 7 cm high and 3.5 cm wide. The obturator plate is largely reduced to only a relatively small anteroventrally projecting obturator process at the base of the ischial shafts (Fig. 15I, K), as in Allosaurus (Madsen 1976), Sinraptor (Currie and Zhao 1993), Yangchuanosaurus magnus (Dong et al. 1983), and Yangchuanosaurus hepingensis (Gao 1999). The obturator process is roughly rectangular in shape, with a straight anteroventral edge and a notable incision between its ventral margin and the ischial shaft, as it is typically present in non-coelurosaurian theropods (Rauhut 2003), including Sinraptor (Currie and Zhao 1993) and Yangchuanosaurus hepingensis (Gao 1999).
The shaft of the ischium is long and slender, and slightly curved anteroventrally in lateral view, as in Megalosaurus (Benson 2010) and Siamotyrannus (Buffetaut et al. 1996, Samathi 2013). It is semioval in outline, being deeper anteroposteriorly than wide mediolaterally, and with a flattened medial surface for the contact with the opposite element. This flat contact extends over almost the entire length of the ischial shafts (Fig. 15J), so there is no ischial apron, as it is present in some megalosaurids (Benson 2010). From the beginning of the interischial suture to slightly distal of the half length of the shafts, the posterodorsal margin of each ischium thins out, so that the joined ischia form a distinct, well developed posterodorsal median flange (Fig. 15K). Such a flange on the posterodorsal surface of the articulated ischia is also present in Shidaisaurus (Wu et al. 2009), Yangchuanosaurus (Dong et al. 1983, Gao 1999), Sinraptor (Currie and Zhao 1993), and Siamotyrannus (Buffetaut et al. 1996, Samathi 2013), and thus probably represents a synapomorphy of metriacanthosaurids, as it is absent in Allosaurus (Gilmore 1920, Madsen 1976) and carcharodontosaurs (Coria and Currie 2006, Brusatte et al. 2008, Canale et al. 2022). Distally, the shafts become more compressed mediolaterally, but gradually expand anteroposteriorly, until a marked expansion at the distal end forms a large ischial boot (Fig. 15I, K), as it is also present in other metriacanthosaurids (Dong et al. 1983, Currie and Zhao 1993, Gao 1999, Wu et al. 2009). This boot is more anteriorly than posteriorly expanded and has a gently convex ventral outline in lateral view. In ventral view, the anterior ends of the ischia ae fused into a pointed anterior end of the boot, whereas there is a notable groove between the left and right ischium in the posteriorly widening posterior two-thirds of the boot (Fig. 15M). Whereas the minimal anteroposterior width of the ischial shaft is c. 5.5 cm just distal to its proximal base, the ischial boot expands to a maximal anteroposterior length of c. 22.5 cm.
Femur: Both femora of the holotype are completely preserved (IGB 2-32, 2-33; Fig. 16), with the head of the left femur having been found in the acetabulum of the left ilium. The femur is a robust bone and only moderately curved in lateral or medial view (Fig. 16B, D, H), similar to the situation in Metriacanthosaurus (OUM J 12144), Sinraptor (Currie and Zhao 1993), Yangchuanosaurus hepingensis (Gao 1999), and other basal tetanurans. The femoral head is directed anteromedially at an angle of approximately 25°, as in many non-neotetanuran theropods, but in contrast to the strictly medially directed femur in most coelurosaurs and allosauroids. In anterior view, the femoral head of the left femur is distinctly proximomedially directed (Fig. 16A), as in carcharodontosaurids (Brusatte and Sereno 2008), Metriacanthosaurus (OUM J 12144) and, to a lesser degree, Yangchuanosaurus hepingensis (Gao 1999). In the right femur, the femoral head has been tectonically sheared off and glued back in a position appearing to indicate a horizontal orientation of the head (Fig. 16G). However, this position probably derives from the reconstruction of that part of the proximal femur, and we consider the left femur to show the original orientation. In proximal view, the femoral head is gently convexly rounded mediolaterally, and widens gradually from the greater trochanter laterally towards the medial side of the head (Fig. 16E). The greater trochanter narrows to a slightly posterolaterally directed point. Its proximal surface is very gently convex anteroposteriorly and slightly posteroproximally directed. The medial end of the head is expanded both anteriorly and posteriorly, with the anterior side forming a rounded angle of approximately 90° between the anterior and medial margin of the head, while the posterior edge forms the medial boundary of a well-developed, oblique ligament groove on the posterior side of the femoral head (Fig. 16C). Between these edges, the medial margin of the head is convexly rounded in proximal view. A well-developed oblique groove (‘proximal articular groove’ of Carrano et al. 2002) is present on the proximal surface of the femur (Fig. 16E), in contrast to most allosauroids and coelurosaurs (Benson 2010), although a similar groove might be present in Sinraptor (based on fig. 22C in Currie and Zhao 1993). In posterior view there is a narrow incision between the head and the shaft at the distal end of the head, in continuation of the oblique ligament groove.
The lesser trochanter is aliform (‘wing-like’) and separated from the proximal shaft by a small incision in lateral view (Fig. 16D). Its proximal end is placed at the level of the distal margin of the femoral head, in contrast to the more proximally placed lesser trochanter in Yangchuanosaurus shangyuensis (Dong et al. 1983), Sinraptor (Currie and Zhao 1993), Metriacanthosaurus (OUM J 12144), Allosaurus (Madsen 1976), and coelurosaurs (Rauhut 2003), but as in Yangchuanosaurus hepingensis (Gao 1999). A small, triangular, slightly anterolaterally directed accessory trochanter is present at the base of the lesser trochanter (Fig. 16B, D, H, I), as in species of Yangchuanosaurus (Dong et al. 1983, Gao 1999), whereas such an accessory trochanter seems to be absent in Sinraptor (Currie and Zhao 1993). This position bears a large, low, mound-like swelling on the lateral side at the distal end of the lesser trochanter, probably for the insertion of the m. iliofemoralis externus, as in other tetanurans (Hutchinson 2001b). Proximal and slightly posterior to this mound, a flat area with marked anterior and posterior edges and a rectangular cross-section extends from the lesser trochanter towards the shaft, into which it fades just at the level of the proximal end of the mound. The fourth trochanter is developed as a robust, elongate semioval flange on the posteromedial side of the femoral shaft, just above the mid-length of the bone (Fig. 16B, D, H, I). In contrast to some theropods (e.g. Baryonyx; Charig and Milner 1997; Suchomimus, MNN Gad 500), there is no groove or depression medial to the fourth trochanter, but notable, posteriorly directed striations are present on the lateral side of the trochanter. Distal to the fourth trochanter, the shaft is mediolaterally compressed; although this might be somewhat exaggerated by compaction, this is observed in both femora. In Sinraptor (Currie and Zhao 1993), Yangchuanosaurus hepingensis (Gao 1999), and Metriacanthosaurus (OUM J 12144), the femoral shaft is wider mediolaterally than anteroposteriorly.
Distally, the shaft becomes more massive and rounded triangular in cross-section, before it then notably expands towards the distal articular end. A well-developed, semioval medial epicondylar crest (sensu Carrano et al. 2002, medio-distal crest of other authors, e.g. Juárez-Valieri et al. 2007) is present above the distal end (Fig. 16A, G). This crest is more prominent than in most other theropods, with the exception of abelisauroids (e.g. Carrano et al. 2002, Juárez-Valieri et al. 2007), but including other metriacanthosaurids, such as Sinraptor (Currie and Zhao 1993), Yangchuanosaurus hepingensis (Gao 1999), and Metriacanthosaurus (OUM J 12144). However, it is more robustly developed than in abelisaurids and notably offset from the distal end (by c. 15 cm), more so than in e.g. Masiakasaurus (Carrano et al. 2002), a morphology that seems to be unique for the new taxon. An elongate, flat triangular surface on the medial side, as it is found medial to the epicondylar crest in many theropods(see RauhutandCarrano2016), isabsent.Theanterior side of the femur is completely flat at the level of the epicondylar crest, but just distal to it, a narrow, deep, sharply defined groove begins and extends to the distal end of the femur, where it is confluent with the intercondylar groove on the distal surface of the bone (Fig. 16A, G). This groove is markedly different from the large medial depression present on the anterior side of the distal femur for the attachment of the m. femorotibialis (Hutchinson 2001b) that is present in numerous theropods (Rauhut 2003), including Sinraptor (Currie and Zhao 1993), as it is considerably narrower, deeper, and defined by an abrupt step from the anterior surface medially and a sharp ridge laterally. The distal end of the femur is well rounded in both medial and lateral view, and subdivided by an anteroposteriorly oriented intercondylar groove into a slightly broader lateral and a narrower medial condyle. In posterior view, the distal condyles are separated by a broad and deep intercondylar groove that extends proximally up to the level of the proximal end of the crista tibiofibularis, where it ends rather abruptly. The crista tibiofibularis is narrower than the tibial condyle and extends more proximally than the latter. An unusual feature of this crest is a medial expansion distally, which gives the crest a triangular outline in distal view, with a medially directed tip that overhangs the intercondylar groove posteriorly (Fig. 16F).
Tibia: Both tibiae of the holotype are preserved (IGB 2-34, 2-35; Fig. 17A–H), and the right tibia of the paratype is also present (IGB 2-48; Fig. 21A–E). Apart from differences in robustness and a slight difference in the development of the distal end (see below), the holotype and paratype tibiae are virtually identical, so the following description is mainly based on the holotype elements. The holotype tibiae are stout and considerably shorter than the femur. The shaft is slightly curved laterally, so that the medial side is gently concave and the lateral side convex over almost their entire length (Fig. 17B, D), as in Condorraptor (Rauhut 2005) and Sinraptor (Currie and Zhao 1993). The proximal end is strongly anteroposteriorly expanded. Anteriorly, the well-developed cnemial crest arises gradually from the tibial shaft (Fig. 17A). Proximally, it accounts for c. 9.5 cm of the total anteroposterior length of the proximal end of c. 21 cm and extends slightly further proximally than the posterior part of the tibia. In lateral view, its outline is subrectanguar. A well-developed, oblique ridge is present on the lateral side of the cnemial crest proximally (Fig. 17A, E), being slightly offset from the anterior end. In anterior view, the cnemial crest flexes slightly laterally, although to a lesser degree than in Sinraptor (Currie and Zhao 1993). Posteriorly, the expansion of the proximal tibia is more abrupt, forming an overhanging lip over the proximal part of the posterior side of the shaft. The medial proximal trochanter and the laterally placed fibular trochanter are separated by a shallow concavity on the proximal surface, and a small incision at the posterior rim, as it is often present in theropods (Rauhut 2003). Anteriorly, the fibular condyle is separated from the cnemial crest by a moderately developed incisura tibialis (Fig. 17E). In proximal view, the fibular condyle is semicircular posteriorly and has a slightly lower, angular edge anteriorly that is confluent with a bulge extending proximally from the fibular flange. The latter is proximally placed on the lateral side of the tibia shaft, ending no more than 26 cm from the proximalmost point of the bone (Fig. 17A, B, G, H). In contrast to many tetanuran theropods, including Sinraptor (Currie and Zhao 1993), where it is often clearly demarcated, it arises rather gradually out of the shaft distally. As in Megalosaurus, Piatnitzkysaurus, and Sinraptor (see Benson 2010), it is anteroposteriorly widened and bulbous, rather than developed as a thin lamina, as is the case in most other theropods. The flange is rounded in anterior or posterior outline and is only about 10 cm long. Proximally, it again fades gradually into a notable swelling on the lateral side of the proximal end of the tibia, which continues up to the proximal articular end (Fig. 17A, B), as is also the case in many megalosauroids (Benson 2010) and Sinraptor (Currie and Zhao 1993), but unlike the fibular flange that is completely offset from the proximal end in many neotetanurans. In lateral view, the fibular flange and the bulge are flanked by notable longitudinal depressions both anteriorly and posteriorly.
Distal to the fibular flange, the shaft of the tibia has a rounded, rather massive outline, before it becomes gradually more anteroposteriorly flattened distally. However, the shaft remains considerably more robust than the more anteroposteriorly compressed shafts of many coelurosaur tibiae throughout its length. Although the anterior side is only slightly convex mediolaterally in the distal half of the shaft, the posterior side remains strongly convex, and the ratio of mediolateral width to anteroposterior depth is c. 1.35 at the mid-shaft, which is higher than in Neovenator (1.22; Brusatte et al. 2008) and Acrocanthosaurus (1.29; Stovall and Langston 1950), but less than in Sinraptor (1.45; Currie and Zhao 1993). Throughout the length of the shaft, a narrow, flattened to slightly mediolaterally concave area on the lateral margin of the anterior side marks the contact between tibia and fibula.
Distally, the tibia expands notably mediolaterally, from a minimal shaft width of c. 8.5 cm to a maximal distal width of 19.5 cm. The expansion is slightly more marked laterally than medially, and the lateral malleolus reaches slightly further distally than the medial, as in Sinraptor (Currie and Zhao 1993). Whereas the lateral expansion forms a gentle curve from the shaft into the malleolus, there is a more marked flexure point on the medial side, where the proximally concave margin of the expansion flexed into an almost straight medial margin. This is opposite to the condition in Sinraptor, where the medial side expands gradually and the lateral expansion is slightly offset from the shaft (Currie and Zhao 1993). In distal view, the articular surface of the tibia is broad triangular in outline, with a slightly anteriorly flexed anteromedial edge and a much shorter posteromedial than posterolateral side (Fig. 17F). From the posteromedial angle of the distal outline, a broad and well-defined groove extends anterolaterally, but does not reach the anterior end distal surface. A depression in this position is present in many theropods, but it is often less well defined as a sharp and deep groove, with the exception of Sinraptor (Currie and Zhao 1993). Medial to this groove, the distal end is notably convex in anterior view, whereas it remains rather flat over two-thirds of its width laterally, and then flexes proximolaterally. On the anterior side of the distal tibia, an oblique step extending from the mediodistal corner laterally marks the bracing for the ascending process of the astragalus (Fig. 17B, F), as is present in most basal tetanurans (Rauhut 2003). In the holotype, this step extends to approximately the mid-width of the distal tibia and becomes more prominent up to this point. It then flexes proximally and rapidly fades into the shaft. In the paratype tibia, the shelf is notably shorter and flexes proximally in the medial third of the distal expansion (Fig. 21D); apart from relative robustness, this is the only notable difference between the holotype and paratype tibiae. In Sinraptor, the flexure of the step is more marked and even further laterally placed than in Alpkarakush (Currie and Zhao 1993).
Fibula: The left fibula is almost completely preserved in two pieces (IGB 2-36, 2-37; Fig. 17I–M), with only an estimated 1–2 cm of the shaft missing. The fibula is a very slender bone, with a minimal anteroposterior shaft diameter of 30 mm. The proximal end is considerably expanded, more posteriorly than anteriorly, to a maximal anteroposterior width of 135 mm. The proximal articular surface is widest anteriorly, where it forms a both anteroposteriorly and mediolaterally convex condyle that is c. 5.5 cm wide mediolaterally and 4.5 cm long anteroposteriorly. A small, triangular process extends from this condyle anteromedially in proximal view (Fig. 17L). Posterior to this convex area, the proximal articular surface narrows rapidly and becomes notably anteroposteriorly concave. In its posterior third, it is again anteroposteriorly convex and narrows to a posteromedially directed point. The lateral surface of the proximal end showsd notable longitudinal striations. It is flat in its central part, curves abruptly anteriorly towards the anterior margin and forms a thin, medially positioned posterior flange posteriorly. Anteriorly, there is a small, thickened flange at the anteromedial edge where the expanded part of the fibula fades into the shaft. Just proximal to the iliofibularis tubercle, there is a notable deflection in the shaft, distal to which it extends slightly more posterodistally. Such a deflection is absent in Sinraptor (Currie and Zhao 1993) and Yangchuanosaurus shangyuensis (Dong et al. 1983). The iliofibularis tubercle is developed as an elongate, slightly obliquely curved, broad, and low ridge on the anterolateral margin of the fibular shaft, starting c. 20 cm distal to the proximal end (Fig. 17I, J). The ridge is c. 8 cm long and borders a very shallow depression on the anterior side of the shaft laterally (Fig. 17J). This depression is c. 17 mm wide, proximomedially-distolaterally oriented, and bordered medially by a small, longitudinal rugosity. Distal to the iliofibularis tubercle, the fibular shaft narrows notably, from an anteroposterior width of c. 5 cm proximal to it to a width of 3.5 cm distal from the tubercle. This notable change in anteroposterior width is absent in Sinraptor (Currie and Zhao 1993), but seems to be present also in Yangchuanosaurus shangyuensis (Dong et al. 1983). From here, the shaft gradually narrows to its minimal width of 30 mm, until it then expands again towards the distal end in its distalmost fifth. This expansion is first moderate and gradual, until the appearance of an anteromedial flange some 11.5 cm above the distal end marks an anteromedial twist of the long axis of the distal end in relation to the proximal end (Fig. 17I). Such a flange is also present in Sinraptor (Currie and Zhao 1993). Further distally, the fibula also expands posteriorly and laterally, to form a rather massive, distally well-rounded distal articular end, which is 7.5 cm long anteroposteriorly and up to 5 cm wide mediolaterally. In distal view, the articular surface is triangular to teardrop shaped, tapering posterolaterally and with a flat posteromedial surface (Fig. 17M).
In medial view, a large depression is present in the expanded proximal end of the fibula (Fig. 17K), as in many tetanuran theropods (Rauhut 2003), including Sinraptor (Currie and Zhao 1993). This depression covers almost the entire medial surface of the proximal end and is bordered anteriorly by a thickened rim, whereas it gradually fades out posteriorly, unlike the both anteriorly and posteriorly clearly defined depression in some theropods, such as ornithomimosaurs (e.g. Rauhut 2003: fig. 47b) or Deltadromeus (SGM Din 2). Distally, the depression tapers and ends in a rather well-defined point, just proximal to the level of the iliofibularis tubercle. Distal to this point, the medial surface is very slightly concave anteroposteriorly up to the level where the anteromedial flange starts distally. Here, a low, obliquely posterodistally extending step separates this concave surface from a more flat, only posteriorly very slightly concave distal medial surface.
Astragalocalcaneum: Both astragalocalcanei are completely preserved (IGB 2-38, 2-39; Fig. 18A–G). The astragalus and calcaneum are largely fused, only in the left element is a suture still visible anteriorly (Fig. 18D), but becomes difficult to follow posteriorly. The elements are generally very similar to the corresponding but unfused elements in Sinraptor (Currie and Zhao 1993). The astragalocalcaneum has a maximum width of c. 19 cm, of which 14.7 cm are accounts for by the astragalus, so that the calcaneum only accounted for 22.6% of the width of the compound element, very similar to this value in Sinraptor (22.5%, Currie and Zhao 1993). The astragalar condyle expands anteroproximally, so that the body of the astragalus extends also anterior to the tibia (Fig. 18G), as is typical in tetanurans, and not only distal to it, as in non-tetanuran theropods. Most of the proximal articular surface of the astragalus is made up by the facet for the distal tibia, which forms a marked, mediolaterally elongate and anteroposteriorly concave facet along the posterior side of the bone (Fig. 18A). This facet is anteroposteriorly widest on the medial side, where it also curves slightly anteromedially and gradually narrows laterally. At the posteromedial edge of the tibia facet, a stout, anterolaterally directed tubercle is present (Fig. 18A, C), which fits into the groove in the distal articular surface of the tibia. A similar tubercle is also present in Sinraptor, where it seems to be less angular, though (Currie and Zhao 1993). Anterior to the tibia facet, the well-developed, laminar ascending process of the astragalus is found. It is triangular in outline in anterior view, with a vertical lateral margin, and approximately as high as the astragalar body below it, as in Sinraptor (Currie and Zhao 1993). As in most non-coelurosaurian theropods, the base of the ascending process is restricted to the lateral part of the astragalar body. The process is slightly offset anterior from the anterodistal articular condyle by a low step, and a large, mediolaterally elongate oval groove is present at its base (Fig. 18D), similar to the condition in Sinraptor (Currie and Zhao 1993). The proximomedial margin of the ascending process is flattened, where it abuts the oblique step on the anterior side of the distal end of the tibia. Lateral to the ascending process, a both anteroposteriorly and mediolaterally concave, laterally widening facet marks the contact between the astragalus and the fibula.
Posterolaterally, the tibia facet extends onto the posterior side of the calcaneum, where it is bordered anteriorly by an oblique ridge that extends from the posterolateral edge of the ascending process towards the posterolateral corner of the calcaneum and separates the tibia facet from the fibular facet. The latter occupies most of the proximal surface of the calcaneum and widens lateroposterioly. Medially it is confluent with the fibular facet on the astragalus.
The distal condyles of the astragalocalcaneum are strongly convex anteroposteriorly. In anterior view, there is a constriction between the medial part of the astragalus and the lateral astragalocalcaneal condyles. A well-developed groove is present across the anterior face of the distal condyles over the medial two-thirds of the width of the astragalocalcaneum (Fig. 18D, G), as in many basal averostrans (Rauhut 2003). The level of this groove roughly coincides with the astragalar articulation with the medial malleolus of the distal tibia, as in other tetanurans (Rauhut and Pol 2017). In distal view, there is also a constriction between the medial part of the astragalar condyle and the lateral astragalocalcaneal condyle. However, this constriction is restricted to the anterior margin of the condyles, the posterior margin is largely straight (Fig. 18E). Furthermore, the medial side is considerably more and more abruptly anteriorly expanded than the lateral side, in contrast to Sinraptor, where the two expansions are subequal (Currie and Zhao 1993). The maximum anteroposterior width of the astragalocalcaneum condyles is c. 95 mm medially, this width is minimally c. 67 mm in the constricted part, before the condyles expand again to a width of c. 75 mm laterally. The medial surface of the astragalus is more or less flat proximodistally and very slightly convex anteroposteriorly, whereas there is a large, semioval depression on the lateral side of the calcaneum (Fig. 18B).
Pes: The foot is represented by a partial left distal tarsal IV (IGB 2-40; Fig. 18H), metatarsals II (IGB 2-41; Fig. 19A–F) and III (IGB 2-42; Fig. 19G, H) of the right and metatarsal III of the left side (IGB 2-43; Fig. 19I–N), a pedal phalanx (IGB
2-44; Fig. 20A–E), and two pedal unguals (IGB 2-45, 2-46; Fig. 20F–L). The distal tarsal IV is represented by its anterolateral half (Fig. 18H). As in other theropods (e.g. Madsen 1976, Currie and Zhao 1993, Rauhut and Pol 2017), this tarsal seems to have been roughly trapezoidal in outline, with a mediolaterally wider anterior part. The anterolateral corner is rounded and forms an angle of approximately 8590°, whereas the anteromedial corner of the bone forms a sharp angle of approximately 60°. In between these two corners, the anterior margin of the bone is gently convexly rounded. In anterior view, the anterolateral side of the bone is proximomedially thickened to almost double the thickness of the anteromedial corner. The proximal surface of the tarsal is gently concave, whereas the distal surface is largely flat, with slightly proximally flexed margins towards the anterolateral and anteromedial corners. Low, but well defined, parallel anteroposteriorly oriented ridges are present on the distal surface, these are most prominent on the lateral side of the bone.
The metatarsus is elongate and slender. The length of metatarsal III is about 56% of the length of the tibia, which is slightly longer than in Sinraptor (53%; Currie and Zhao 1993), considerably longer than in Allosaurus (47%; Gilmore 1920), and only slightly less than in Albertosaurus (60%; Parks 1928), all theropods with a roughly comparable femoral length. Metatarsal II is about 87% of the length of metatarsal III. The shaft is almost straight in anterior and posterior view, being only very slightly flexed medially in its distal part (Fig. 19A, C), similar to the condition in Sinraptor (Currie and Zhao 1993). In medial or lateral view, it is slightly flexed anterodistally, especially along its posterior surface (Fig. 19B, D); this flexure is more pronounced than in Sinraptor (Currie and Zhao 1993). The proximal end is considerably expanded both anteriorly and posteriorly, and also towards the medial side. Thus, whereas the minimal anteroposterior shaft width is c. 46 mm, the maximal anteroposterior expansion of the shaft is c. 100 mm. In medial view, the anterior expansion forms a gradual concave arch, whereas the posterior expansion is only slightly concave at its base and then forms a rather sharp and straight posterior edge. The expansion towards the medial side is restricted to the proximalmost part of the metatarsal and here forms a marked medial concavity in anterior view. In relation to the anteroposterior expansion, this expansion is less marked, reaching a maximal mediolateral width of 60 mm, in contrast to a minimal mediolateral shaft width of c. 40 mm. In proximal view, the proximal articular surface is semioval in outline, with a flat lateral margin, a slightly drawn-out anterolateral corner, and a robust posterior flange laterally (Fig. 19E), similar to the condition in Sinraptor (Currie and Zhao 1993) and Allosaurus (Gilmore 1920, Madsen 1976), although the flexure from the main part of the surface into the posterior flange is more angular in the latter. The articular surface is very slightly anteroposteriorly concave in its central portion, whereas the surface flexes slightly distally on the anterolateral corner and the posterolateral flange. The lateral side of the proximal metatarsal is flattened where it would have contacted metatarsal III. It shows weakly developed rugosities on this contact surface, which are developed as low, rugose proximodistal ridges.
The shaft of metatarsal II is deeper anteroposteriorly than wide mediolaterally. The lateral side is flattened over most of its length, with a slight twist in the flattened surface just below mid-length, with the distal part facing slightly more anterolaterally than the proximal part. Only the distalmost section of the lateral side, adjacent to the distal articular condyle, becomes anteroposteriorly convex. The cross-section of the shaft is thus semioval at mid-length, narrowing slightly posteriorly. The anterior surface is flattened proximally and towards the distal articular end, with a transversely convex section in between. An elongate oval, slightly rugose patch is on the anterolateral side of the bone, just below the proximal expansion (Fig. 19A). A similar patch seems also to be present in Sinraptor (Currie and Zhao 1993). The medial surface is generally convex anteroposteriorly and becomes narrower towards the distal end. The ventral side of the shaft bulges posteriorly in the mid-shaft section. Proximally, below the posterior flange of the articular surface, the posterior side met the lateral side in a sharp posterolateral edge, which disappears into the shaft at the level where the proximal expansion starts. Just below, above the mid-length of the shaft, there is a posteriorly flattened, elongate, slightly rugose patch on the posterior side of the shaft (Fig. 19C). This patch ends at about the mid-length of the metatarsal, distal to which, a rugose, elongate oval is found on the posterolateral side of the bone; its proximal end coincides with the twist in orientation of the lateral side described above. This rugose area flexes more onto the posterior side distally and ends some 3 cm proximal to the expansion of the distal condyles.
The distal end forms a notably mediodistally directed articular condyle, with the medial orientation resulting from the fact that the distal articular surface extends more distally laterally than medially, not from a medial flexure on the distal shaft (Fig. 19A, C). The distal condyle forms an anteroposteriorly strongly convex arch that extends approximately as far proximally on the anterior and on the posterior side, similar to the condition in Sinraptor (Currie and Zhao 1993). The actual articular surface is marked by slight steps on the anterior, medial, and posterior surfaces, distal to which the surface is notably smooth, indicating the presence of articular cartilage. In distal view, the articular surface widens posteriorly, where it is split into a wide lateral and a much narrower lateral part by a deep, wide U-shaped posterior embayment (Fig. 19F), as in Sinraptor (Currie and Zhao 1993). In contrast to some theropods, the narrow medial part of the articular surface does not flay medially in its posterior part. The lateral margin of the articular surface is markedly concave anteroposteriorly, and several marked, but shallow and narrow grooves extend from that margin medially over the lateral surface of the articular end. Well-developed collateral ligament fossae are present both medially and laterally, with the lateral one being larger and placed into a funnel-shaped depression that covers the entire anteroposterior height of the lateral side of the distal end (Fig. 19D). On the anterolateral side, a low, slightly mediolaterally elongate tubercle is present at the proximal end of this depression. Medially, the dorsal margin of the collateral ligament groove bulges medially and is slightly rugose.
Left and right metatarsals III are extremely similar in their size, relative dimensions, and morphology (Fig. 19G–N). The shaft is straight over most of its length in anterior view, but shows a slight, but notable lateral flexion in its distal fourth (Fig. 19G, K), as in Sinraptor (Currie and Zhao 1993). In medial or lateral view, the shaft remains straight over its entire length. Proximally, the bone is notably flattened anteromedially-posterolaterally, and the proximal end strongly expands anterolaterallyposteromedially, with the expansion being more marked anterolaterally than posteromedially. Anterolaterally, there is an abrupt expansion of the proximal shaft into a small, rectangular flat anterior surface just distal to the proximal articular surface, with the latter slightly expanding onto the anterolateral side, especially at the anteromedial corner of this area. This rectangular surface shows low, but notable longitudinal striations, and it continues to the lateral surface, where there is a small flat area showing the same striations and being offset distally from the shaft by an oblique step. Posteromedially, the expansion of the proximal end of the metatarsal forms a sharp posteromedial edge. The proximal articular surface shows the hourglass shape that is typical of basal tetanuran metatarsal III, having widened anterolateral and posteromedial portions that are separated by a constricted middle part (Fig. 19I). However, in comparison with taxa such as Allosaurus (Gilmore 1920, Madsen 1976) or Acrocanthosaurus (Stovall and Langston 1950), the proximal end is very slender, and the posteromedial expansion is much less marked, resembling Sinraptor in that respect (Currie and Zhao 1993). The proximal articular surface thus has a long and very slightly concave anteromedial margin that would have contacted metatarsal II and anteriorly flexes into the anterolateral margin at an angle of approximately 70°. The anterolateral margin is relatively short and meets the lateral margin at an angle of approximately 120°. The lateral margin has a straight anterior half and a strongly concave, indented posterior half where metatarsal II would have overlapped the posteromedial process of metatarsal IV. Posteriorly, this margin meets the posterior margin at an angle of approximately 105°. The posterior margin is the shortest margin of the proximal articular surface and meets the anteromedial margin in a sharp angle of approximately 40°. The proximal articular surface is flat mediolaterally, but slightly concave in its central part anteroposteriorly. Anteromedially and posteromedially, the surface flexes slightly distally.
The shaft of metatarsal III is marked by the strong anteromedial-posterolateral flattening of its proximal half, which is twisted in respect to the more conventionally oriented distal half, as in Sinraptor (Currie and Zhao 1993) and Allosaurus (Madsen 1976). Thus, the proximal shaft has a very narrow, anteroposteriorly rounded posteromedial side below the sharp posteromedial crest towards the articular end described above. The anteromedial side is large, expands proximally and is very slightly anteroposteriorly convex (Fig. 19K, N). The anterolateral side is flattened and shows a low and broad, anterolaterally directed tubercle some 9 cm below the proximal end (Fig. 19K). Below this tubercle, a marked edge separates the anterolateral from the posterolateral side; this edge persists into the distal half of the shaft, extending posteriorly as the anterolateral side twists to become the lateral side in the distal shaft. The posterolateral side of the proximal shaft is similar in expansion as the anteromedial side, and has a marked anteroposterior concavity in its posterior two-thirds, but becomes slightly transversely convex towards the distal half of the shaft. Towards the distal half of the shaft, the anteromedial side twists to become the anterior side of the bone. Distal to its mid-length, metatarsal III is wider mediolaterally (c. 47 mm) than deep anteroposteriorly (c. 38 mm) and has a flattened anterior side. The medial side gradually flexes into the posterior and then the posterolateral side, only laterally, this side remains separated from the narrow, flat lateral side by the posterolateral edge described above. Where the shaft flexes laterally, the anteromedial edge is thickened and forms a notable edge that slightly overhangs the medial side (Fig. 19N). Towards the distal end, the shaft becomes more markedly anteroposteriorly flattened, more markedly so than in Sinraptor (Currie and Zhao 1993).
The distal articular end of metatarsal III is mediolaterally broadened. The roller-like distal articular surface extends approximately as far proximally both dorsally and ventrally, and, as in metatarsal II, the area covered by articular cartilage is marked by a slight step and a markedly smooth surface distal to it. In distal view, the articular surface is trapezoidal in outline, being dorsoventrally higher medially than laterally (Fig. 19H, J), as in Sinraptor (Currie and Zhao 1993). It is flat mediolaterally, without a distinction of medial and lateral condyles. The ventral margin is markedly concave, mainly due to the posteromedial edge, which is drawn out posteriorly into a point.Well-developed collateral ligament grooves are present both medially and laterally, placed in large depressions on either side at the level of the articular end. On the medial side, the anterior margin of the ligament groove is thickened and overhangs the groove. Only a very faint, broad extensor groove is present on the anterior side directly distal to the articular surface.
The single non-ungual pedal phalanx recovered (IGB 2-44; Fig. 20A–E) most probably represents phalanx II-2 of the right foot, as it closely resembles this element in Sinraptor (Currie and Zhao 1993). It is a robust element, maximally 95 mm long and with a minimal mediolateral width of 40 mm. The proximal articular end is trapezoidal in outline, being wider ventrally than dorsally, and with rounded edges (Fig. 20E), missing the sharply angled lateroventral corner seen in this phalanx in Sinraptor (Currie and Zhao 1993). It is subdivided by a low vertical ridge into two dorsoventrally notably concave facets of nearly subequal size. The articular end is 54 mm wide and 43 mm high. In dorsal view, the shaft is constricted, more so from the lateral than from the medial side. On the medial side, a marked, rugose tubercle is present ventral from the half height of the proximal articular end. The distal articular surface forms a well-developed gynglimus, with only a very faint extensor groove on the dorsal surface proximal to it. The distal articular surface extends proximally considerably more ventrally than dorsally and is ventrally mediolaterally broader than dorsally (Fig. 20D), as in Sinraptor (Currie and Zhao 1993). The articular surface is subdivided into a medial and lateral condyle by a deep, broad dorsoventrally oriented groove. The lateral condyle is slightly broader mediolaterally than the medial condyle and extends further distally, so that the articular end is asymmetrical in dorsal view. The medial condyle is considerably narrower, but slightly higher and more ventrally expanded than the lateral condyle. Large, well-developed collateral ligament grooves are present on either side of the phalanx, being displaced dorsally from the mid-height of the distal articular end (Fig. 20B). The posterior margin of the medial ligament groove is somewhat thickened and projects medially. The distal articular end is maximally 45 mm wide and 42 mm high.
The pedal unguals are broad and ventrally flattened, with only a moderate curvature in medial or lateral view. The larger ungual IGB 2-45 (Fig. 20F–I) most probably represents the ungual of the right digit II, whereas IGB 2-46 (Fig. 20J–L) is most likely an ungual of digit IV. As in some theropods, such as Sinraptor (Currie and Zhao 1993), the ungual of digit II is markedly asymmetrical and slightly curved medially in dorsal view (Fig. 20H). The proximal articular surface is round to subquadrangular in outline (Fig. 20G), with a considerably proximally expanded dorsal edge that forms a proximally rounded point in dorsal view (Fig. 20H). A ridge subdividing the facet into two concavities is only rudimentarily developed dorsally. The cross-section of the claw at mid-length is semioval, tapering medioventrally. Well-developed, single claw grooves are present, with the lateral one being placed slightly higher than the medial one. Proximally, a small tubercle is present at the end of the claw groove on either side. Ventrally, only a slightly elevated, rugose patch at the proximal end indicates the insertion of the flexor tendon (Fig. 20I); a flexor tubercle or a ventral groove (as it is present in abelisaurids) is absent, as in Sinraptor (Currie and Zhao 1993). This ungual is maximally 40 mm wide and 44 mm high proximally. Its length is 115 mm as measured in a straight line from the tip to the end of the dorsal proximal process (maybe missing 5 mm at the damaged tip), or 130 (+ c. 5) mm along the dorsal curve.
The smaller ungual IGB 2-46 is symmetrical and has a round proximal articular facet, with slight indentations on either side just above the mid-height. The dorsal proximal expansion is less prominently developed than in the other ungual. The claw grooves are symmetrically arranged and face dorsolaterally and dorsomedially, respectively (Fig. 20K). Ventral to them the ungual is markedly broadened in respect to the dorsal part; this broadening disappears proximally at the end of the claw grooves, giving the ungual an arrowhead shape in ventral view. As in the other ungual, the flexor tubercle is only developed as a small raised patch with a mildly rugose surface ventrally (Fig. 20L). This ungual is maximally 43 mm wide and 38 mm high proximally, and 90 mm long in a straight line (also missing maybe 5 mm at the tip), or 95 (+ 5) mm along the dorsal curve.
Bone histology
The analyses with the fluorescence spectrometer revealed a high content of manganese, barium and calcium. Manganese is certainly the main source for the dark staining of the bone.
Femur IGB 2-33: The bone wall thickness is 14.4 mm in the thinnest part of the bone wall and 21.1 mm in the lateral corner. Including the potentially pathological tissue in the medullary cavity (inner unit, see below), the maximum thickness increased to 25.1 mm in the lateral corner.
The observable bone tissue could be separated into an inner, middle, and outer unit (Fig. 22). The inner unit, developed within the medullary cavity, consisted of a highly irregularly vascularized tissue with only sparse compaction of the wide vascular canals (Fig. 23A, B). The osteocyte lacunae were very abundant and mostly irregularly shaped, except within bands of lamellar tissue, where they were mostly spindle shaped. There were at least three such bands present in the lateral side (Fig. 23B), but the middle and outer band merged together into a single band and then split again towards the posterior side (Fig. 22). The outer band clearly corresponded to an inner circumferential layer (ICL), although it became subsequentially less distinct and more interrupted by vascular canals towards the posterior side of the bone wall. The outer edge of the ICL was clearly resorptive laterally, whereas there was an almost continuous transition between coarse and slightly compacted cancellous endosteal bone tissue and external periosteal bone tissue in the area of the posterolateral wall (Fig. 23A).
The middle unit, which represents the inner part of the periosteally deposited bone wall, comprised about half the thickness of the latter posterolaterally and more than two-thirds of the thickness laterally (Fig. 22). Although the middle unit was partly obscured by dark manganese-rich mineral, it was clear that this unit consisted of highly vascularized, fibrolamellar bone tissue. The vascular canals were mainly oriented laminar to plexiform, rarely reticular, with well-developed primary osteons (Fig. 23C). The degree of organization slightly increased externally as well as from the lateral corner to the posterolateral wall, as the usual mode of variation between bone wall units (Hübner 2012). Secondary osteons were present, but scattered widely and almost never overlapped each other. The overall number of secondary osteons was higher in the lateral corner than in the posterolateral bone wall. The middle unit was almost azonal, without lines of arrested growth (LAG) or annuli. The first traceable growth mark represented the border to the outer unit, although weak zonation in terms of variability in vascular orientation was already present internal to it.
The outer unit, extending from the first clearly visible growth mark internally to the outer edge of the periosteally deposited bone wall (Fig. 22), also consisted mainly of fibrolamellar bone tissue.However, it was strongly separated into zones with laminar to reticular, well-developed primary osteons in the fast-growing zones. Parallel-fibred annuli and/or lines of arrested growth (LAGs) werepresentinbetween (Fig.23D). Thelatterwererarely interrupted by vascular canals. Secondary osteons were isolated and scarce, but were more abundant in the lateral corner. None reached the periosteal surface, however. The innermost growth mark was mostly blackish and obscured posteriorly (Fig. 22) but could be followed towards the lateral corner, where it was developed as a double LAG. The spacing of the fast-growing zones was irregular, but there was a clear decrease in average thickness about halfway towards the surface posterolaterally (Fig. 23D). However, the growth zones became generally thinner from the posterolateral wall to the lateral corner and growth marks even merged together. The best account for growth marks was visible in the posterolateral bone wall. Depending on the area of the count, between 15 and 17 growth marks were present, developed as annuli and LAGs, in one case even a triple LAG (Fig. 23D). Where preserved, the outer edge of the bone wall lacked a typical External Fundamental System (EFS), but the vascularization was already very scarce with mostly longitudinal primary osteons in a laminar order arranged in between thicker, avascular, parallel-fibred tissue (Fig. 23E).
An unusual feature in the outer unit was the termination of two of the fast-growing zones at the transitional area between the posterolateral wall and the lateral corner, combined with a distinct increase in thickness of two other zones in the same area (Fig. 23F). In addition, these fast-growing zones, together with at least a third zone, consist almost exclusively of woven-fibred tissue and contain unusually highly disorganized vascular canals with mainly reticular and sometimes even radial orientations (Fig. 23F). In the lateral and posterior sides of the bone wall, there was at least a single area with this fast-growing tissue present, respectively.
Tibia IGB 2-48: The bone wall in the sampled posterior area is c. 13.5 mm thick. There was no clear separation between the main histological units (Fig. 24) as in the femur of the holotype. Only a distinct band of ICL was present internally along the whole course of the sampled area (Fig. 24A). The ICL was penetrated occasionally by mostly radial simple vascular canals.
The periosteal bone wall consisted of fibrolamellar bone tissue with numerous, well-developed primary osteons and abundant osteocyte lacunae. The vascular pattern was mainly laminar to plexiform with local occurrences of reticular or even short radial canals (Fig. 24B). The spatial density of vascular canals decreased towards the outer periphery, whereas the percentage of longitudinal canals increased (Fig. 24C). Secondary osteons were completely absent.
Three growth marks were present, widely spaced from each other (Fig. 24). The innermost growth mark was a LAG, which was partially resorbed by the expanding medullary cavity, and the middle and outer growth marks were developed as annuli. There was no EFS developed at the outer edge (Fig. 24C).
description: Published as part of Rauhut, Oliver W. M., Bakirov, Aizek A., Wings, Oliver, Fernandes, Alexandra E. & Hübner, Tom R., 2024, A new theropod dinosaur from the Callovian Balabansai Formation of Kyrgyzstan, pp. 1-51 in Zoological Journal of the Linnean Society 201 (4) on pages 7-40, DOI: 10.1093/zoolinnean/zlae090, http://zenodo.org/record/13774241