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(Diapsida: Saurosphargidae), with Implications for the Morphological Diversity and Phylogeny of the Group

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(Diapsida: Saurosphargidae), with Implications for the Morphological Diversity and Phylogeny of the Group Geol. Mag.: page 1 of 21. c Cambridge University Press 2013 1 doi:10.1017/S001675681300023X A new species of Largocephalosaurus (Diapsida: Saurosphargidae), with implications for the morphological diversity and phylogeny of the group ∗ CHUN LI †, DA-YONG JIANG‡, LONG CHENG§, XIAO-CHUN WU†¶ & OLIVIER RIEPPEL ∗ Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China ‡Department of Geology and Geological Museum, Peking University, Beijing 100871, PR China §Wuhan Institute of Geology and Mineral Resources, Wuhan, 430223, PR China ¶Canadian Museum of Nature, PO Box 3443, STN ‘D’, Ottawa, ON K1P 6P4, Canada Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL 60605-2496, USA (Received 31 July 2012; accepted 25 February 2013) Abstract – Largocephalosaurus polycarpon Cheng et al. 2012a was erected after the study of the skull and some parts of a skeleton and considered to be an eosauropterygian. Here we describe a new species of the genus, Largocephalosaurus qianensis, based on three specimens. The new species provides many anatomical details which were described only briefly or not at all in the type species, and clearly indicates that Largocephalosaurus is a saurosphargid. It differs from the type species mainly in having three premaxillary teeth, a very short retroarticular process, a large pineal foramen, two sacral vertebrae, and elongated small granular osteoderms mixed with some large ones along the lateral most side of the body. With additional information from the new species, we revise the diagnosis and the phylogenetic relationships of Largocephalosaurus and clarify a set of diagnostic features for the Saurosphargidae Li et al. 2011. Largocephalosaurus is characterized primarily by an oval supratemporal fenestra, an elongate dorsal ‘rib-basket’, a narrow and elongate transverse process of the dorsal vertebrae, and the lack of a complete dorsal carapace of osteoderms. The Saurosphargidae is distinct mainly in having a retracted external naris, a jugal–squamosal contact, a large supratemporal extensively contacting the quadrate shaft, a leaf-like tooth crown with convex labial surface and concave lingual surface, a closed dorsal ‘rib-basket’, many dorsal osteoderms, a large boomerang-like or atypical T-shaped interclavicle. Current evidence suggests that the Saurosphargidae is the sister-group of the Sauropterygia and that Largocephalosaurus is the sister-group of the Saurosphargis–Sinosaurosphargis clade within the family. Keywords: marine reptile, morphology, phylogeny, Triassic, China. 1. Introduction the Saurosphargidae and represents a new morphotype of the family. This is supported by the discovery The marine vertebrate fauna of the Triassic Guanling of a new species Largocephalosaurus qianensis (see Formation has become well known recently, occurring Section 3) from the same horizon of southwestern mainly in boundary area between Yunnan and Guizhou Guizhou, about 100 km northeast of the type locality provinces in southwestern China (Jiang et al. 2006; of L. polycarpon in Luoping County, Yunnan. 2008, 2009; Motani et al. 2008; Wang et al. 2009; L. polycarpon was originally established primarily Wu et al. 2011; Liu et al. 2011; Shang, Wu & based on an incomplete skull because the preparation Li, 2011). Conodonts dated the fossil yielding strata of the postcranial skeleton of the specimen, WIGM (Upper Member) of the Guanling Formation to the (Wuhan Institute of Geology and Mineral Resources) Pelsonian substage of the Anisian, Middle Triassic SPC V 1009, was then incomplete. It was considered (Sun et al. 2006; Zhang et al. 2009). This fauna has to be an eosauropterygian and closely related to been recently enriched by the discovery of a new the Nothosauroidea–Pachypleurosauria clade within member, Sinosaurosphargis yunguiensis Li et al. 2011, the Sauropterygia (Cheng et al. 2012a). Whilst this of the Saurosphargidae Li et al. 2011 from Luoping specimen was being studied, research groups at County, Yunnan. Early in 2012, a new marine reptile, GMPKU (Geological Museum of Peking University) Largocephalosaurus polycarpon Cheng et al. 2012a and IVPP (Institute of Vertebrate Paleontology and was briefly described and attributed to the Eosaur- Paleoanthropology, Chinese Academy of Sciences) opterygia. Our further examination of the specimen were working on three similar specimens collected suggests that this reptile is most probably referable to from the Upper Member of the Guanling Formation †Authors for correspondence: [email protected]; xwu@mus- in Panxian, Guizhou Province in 2008. The GMPKU nature.ca specimen was briefly reported as Saurosphargis cf. http://journals.cambridge.org Downloaded: 20 May 2013 IP address: 124.16.246.40 2 C. LI AND OTHERS S. volzi (Jiang et al. 2011). These new specimens are (15) humerus not expanded at both ends; (16) nine carpals; better preserved than that of the type species, and and (17) four tarsals. clearly show the presence of dorsal osteoderms and a closed dorsal ‘rib-basket’ formed by the underlying Remarks. Saurosphargis, the type genus of the Saurosphar- gidae, from the Middle Triassic of Europe, is a poorly ribs. Our further examination demonstrates that those known taxon; it is only represented by a section of features are also present in L. polycarpon as shown by 12 incomplete dorsal vertebrae with ribs (see Nosotti & the partly prepared postcranial skeleton. Therefore, it Rieppel, 2003, fig. 11). The diagnosis of the family was is evident that Largocephalosaurus is clearly not an not defined when the family was established and is here eosauropterygian but instead may be closely related largely based on two Chinese genera: Sinosaurosphargis and to the Saurosphargidae. In morphology, the three new Largocephalosaurus. specimens are different enough from L. polycarpon Largocephalosaurus Cheng et al. 2012a and represent a new species; the most striking of those differences are: the presence of three premaxillary Type species. Largocephalosaurus polycarpon Cheng et al. teeth, an oval orbit, a large parietal foramen, two 2012a. (rather than three) sacral vertebrae, a unique pattern of osteoderms, and a short retroarticular process. Here, Revised diagnosis. A saurosphargid genus differing from we describe the osteological anatomy of the new the others of the family in the following combination of apomorphies: (1) closed dorsal ‘rib-basket’ elongate oval; species, revise the systematics of Largocephalosaurus, (2) oval supratemporal fenestra present, much smaller than and further test the phylogenetic relationships of the orbit; (3) premaxilla excluded from external naris; (4) three Saurosphargidae with other marine reptilian groups. or four premaxillary teeth; (5) posterolateral process of frontal elongate; (6) transverse process of dorsal vertebrae 2. Material and methods and proximal portion of the dorsal ribs slender, not much wider than inter-process or inter-rib spaces, respectively; (7) All three specimens were preserved in blocks of presence of a median row of large osteoderms on the top limestone or muddy limestone. One is a nearly of neural spines; (8) coracoid and pubis similarly round complete skeleton, housed at the IVPP. As for the in outline, separately with an open coracoid foramen and an open obturator fenestra; and (9) dorsal osteoderms not other two specimens housed at the GMPKU, one is forming a complete carapace. represented by a skull and a cervical vertebra and the other is an incomplete skeleton with the skull and most Remarks. Most of the diagnostic characters for Largo- of the tail missing. All specimens were mechanically cephalosaurus are unknown in Saurosphargis owing to the prepared using airscribes with tips of various sizes. fragmentary nature of the latter. However, characters (1) The IVPP specimen was exposed in ventral view but its and (6) distinguish the two genera. In Saurosphargis the skull was prepared on both dorsal and ventral sides. The body should have been broader than in Largocephalosaurus in terms of a large turning angle at the shoulder region two GMPKU specimens were exposed only in dorsal of the dorsal ribs; this condition is comparable to that of view. Sinosaurosphargis. Similarly, the transverse process of the dorsal vertebrae and the proximal (dorsal) portion of the 3. Systematic palaeontology dorsal ribs are very similar between Saurosphargis and Sinosaurosphargis in morphology; they are much broader Order DIAPSIDA Osborn, 1903 than the inter-process or inter-rib spaces. Family SAUROSPHARGIDAE Li et al. 2011 Largocephalosaurus qianensis sp. nov. Type genus. Saurosphargis Huene, 1936. Figures 1, 2, 3, 4a–g, 5a–d, 6 Referred genera. Sinosaurosphargis Li et al. 2011; Largo- Etymology. Specific name refers to the simplified name cephalosaurus Cheng et al. 2012a (originally described as ‘Qian’ for Guizhou Province where the specimens were an eosauropterygian). collected. Diagnosis. Aquatic diapsids characterized by the following Holotype. IVPP V 15638, a nearly complete skeleton in combination of apomorphies: (1) dorsal ribs forming a ventral view, with posterior-most section of the tail missing. closed basket; (2) presence of dorsal osteoderms; (3) external naris retracted, much closer to orbit than rostral tip; (4) Referred specimens. GMPKU-P-1532-A, a skull with a median elements of gastral ribs often with a two-pronged cervical vertebra in dorsal view. GMPKU-P-1532-B, an lateral process on one side;
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