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Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China

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Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/5339479 Osteology and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China Article in Journal of Anatomy · July 2008 Impact Factor: 2.1 · DOI: 10.1111/j.1469-7580.2008.00907.x · Source: PubMed CITATIONS READS 14 20 2 authors, including: Ke-Qin Gao Peking University 63 PUBLICATIONS 1,710 CITATIONS SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Ke-Qin Gao letting you access and read them immediately. Retrieved on: 13 June 2016 J. Anat. (2008) 212, pp747–768 doi: 10.1111/j.1469-7580.2008.00907.x OsteologyBlackwell Publishing Ltd and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China Ke-Qin Gao1,2 and Daniel T. Ksepka2 1School of Earth and Space Sciences, Peking University, Beijing, China 2Division of Paleontology, American Museum of Natural History, New York, New York, USA Abstract Although the long-necked choristodere Hyphalosaurus is the most abundant tetrapod fossil in the renowned Yixian Formation fossil beds of Liaoning Province, China, the genus has only been briefly described from largely unprepared specimens. This paper provides a thorough osteological description of the type species Hyphalosaurus lingyuanensis and the con-generic species Hyphalosaurus baitaigouensis based on the study of fossils from several research institutions in China. The diagnoses for these two species are revised based on comparison of a large sample of specimens from the type area and horizon of each of the two species. The skull, better known in H. baitaigouensis, exhibits key choristodere synapomorphies including an elongate contact between the prefron- tals and posteriorly expanded supratemporal fenestrae that strongly support the placement of the highly derived hyphalosaurids within Choristodera. Both species of Hyphalosaurus share a proportionally small head, an elongate neck, a relatively unspecialized appendicular skeleton and a long, dorsoventrally heightened tail. Soft tissue preservation in several specimens provides rare insight into the integument of an extinct group. The integument of Hyphalosaurus is made up of small polygonal scales with several parasagittal rows of large, keeled, ovoid scutes. These possibly ornamental scutes bear a strong resemblance to the rows of large scutes in the monjurosuchid choristodere Monjurosuchus splendens. Observations from a variety of growth stages reveal that significant ontogenetic change in the proportions of the body and limb bones occurred in both species of Hyphalosaurus. Key words anatomy and taxonomy; Choristodera; Early Cretaceous; Hyphalosauridae; western Liaoning of China. and eutherian mammals (Hu & Wang, 2002; Ji et al. Introduction 2002). The second species, Hyphalosaurus baitaigouensis, The genus Hyphalosaurus includes two species of highly is known from far more abundant material from the specialized aquatic choristoderan reptiles with unusually Yizhou area (Fig. 1), where multiple localities exposing long necks. Fossils of this genus are known only from the strata from the upper part of the Yixian Formation (Wang Early Cretaceous Yixian Formation of western Liaoning et al. 2004; but see also Ji et al. 2004a) have yielded at least Province, China. The type species, Hyphalosaurus ling- several thousand specimens of this species, including soft- yuanensis, is known from multiple specimens from a single shelled fossil eggs (Ji et al. 2004b). These two species, locality in the Lingyuan area (Fig. 1), where the fossil beds along with Shokawa ikoi from the Early Cretaceous Okuro- of the lower Yixian Formation have yielded a radiometric dani Formation of Japan (Evans & Manabe, 1999), form the date of 123–126 Ma (Smith et al. 1995; Ji et al. 2004a). The family Hyphalosauridae (Gao & Fox, 2005). In addition to fossil beds exposed at the Lingyuan area are also known features such as dorsally positioned orbits and pachyostotic for producing superbly preserved fossils including feathered ribs that are widely distributed within Choristodera, hypha- dromaeosaurs (Ji et al. 2001), a juvenile enantiornithine bird losaurids are characterized by a small head, a long neck (Hou & Chen, 1999; Zhang et al. 2003), and multituberculate consisting of more than 16 cervical vertebrae, and a greatly elongated and heightened tail. This combination of traits suggests that hyphalosaurids were the most completely aquatic choristodere clade. Taphonomy also supports this Correspondence inference, as Hyphalosaurus is common in deep-water Dr K.-Q. Gao, School of Earth and Space Sciences, Peking University, lacustrine facies bearing the large chondrostean fish Pro- Beijing 100871, China. E: [email protected] topsephurus in the Lingyuan area and co-occurs with the Accepted for publication 12 February 2008 abundant large shrimp Liaoningogriphus in the Yizhou © 2008 The Authors Journal compilation © 2008 Anatomical Society of Great Britain and Ireland 748 Cretaceous reptile Hyphalosaurus from China, K.-Q. Gao and D. T. Ksepka Fig. 1 Map showing distribution pattern of Hyphalosaurus fossils in western Liaoning. Note the absence of Hyphalosaurus from the Chaoyang area (satellite image of the area from TAGEO.com). area (Shen, 2003). Interestingly, Hyphalosaurus is absent V11075) was incompletely exposed when the original descrip- from the Chiufotang Formation exposed near the city of tion of the species was completed. Further preparation Chaoyang (Fig. 1). These deposits appear to represent a of the holotype and examination of additional previously shallow-water swampy environment and contain the undescribed specimens of H. lingyuanensis allow us to blunt-snouted monjurosuchid choristodere Philydro- provide new observations on the osteology of the type saurus proseilus (Gao & Fox, 2005; Gao et al. 2007) and species, including previously unknown information on gavialiform neochoristoderes (unpublished specimens LPMC the skull roof and integument of this species. Based on R00070, 00071), as well as birds and non-avian dinosaurs. examination of a large number of well-preserved speci- Thousands of Hyphalosaurus specimens are currently mens in several institutions, we also provide a revision of deposited in research collections or in private hands. Despite H. baitaigouensis, including an emendation of the diagno- the surfeit of material, the anatomical details of these sis and clarification of its cranial and postcranial anatomy. animals have scarcely been touched upon. The first account This study provides essential information for understanding of Hyphalosaurus briefly described the type species H. the evolution of Hyphalosauridae in Asia as a highly lingyuanensis, at that time considered Diapsida incertae specialized family of aquatic diapsids, and the phylogeny sedis (Gao et al. 1999). Following the recognition of the of the Choristodera, now recognized as an important choristoderan affinities of the genus (Gao et al. 2000), a clade in Mesozoic terrestrial ecosystems throughout the second species, H. baitaigouensis, was described (Ji et al. Northern Hemisphere. 2004b). However, the original publication of the second species lacks essential anatomical treatment of the skull, and, more unfortunately, several skull elements are misidentified Abbreviations and mislabeled in the published figure (see discussion Institutional: AMNH – American Museum of Natural History, below). The two species are most easily distinguished by New York, NY, USA; BMNHC – Beijing Museum of Natural the number of cervical vertebrae, but several other features History, Beijing, China; CAGS – Chinese Academy of Geo- (listed below) are also diagnostic, as recognized in this logical Sciences, Beijing, China; GMC – Geological Museum study. In this paper, we present a detailed description of of China, Beijing, China; IVPP – Institute of Vertebrate the osteology of Hyphalosaurus, primarily from newly Paleontology and Paleoanthropology, Beijing, China; prepared specimens from several institutions including the LPMC – Liaoning Paleontological Museum of China, BMNHC, IVPP, LPMC, and Peking University Paleontological Shenyang, China; PKUP – Peking University Paleontological Collections. The holotype specimen of H. lingyuanensis (IVPP Collections, Beijing, China. © 2008 The Authors Journal compilation © 2008 Anatomical Society of Great Britain and Ireland Cretaceous reptile Hyphalosaurus from China, K.-Q. Gao and D. T. Ksepka 749 Anatomical: ar, articular; as, astragalus; bo, basioccipital; Genus Hyphalosaurus Gao et al. 1999 ca, calcaneum; ch, choana; cl, clavicle; co, coronoid; cor, coracoid; cv, cervical vertebra; den, dentary; dc, distal Type species: Hyphalosaurus lingyuanensis Gao et al. 1999. carpal; dt, distal tarsal; dv, dorsal vertebra; ect, ectopterygoid; Constituent species: The type species and H. baitai- ectf, ectepicondylar foramen; fem, femur; fi, fibula; fr, gouensis (see discussion below). frontal; hu, humerus; icl, interclavicle; il, ilium; intv, interp- Generic diagnosis: The genus is diagnosed by derived terygoid vacuity; isc, ischium; j, jugal; lac, lacrimal; m, character states including an elongated neck consisting of maxilla; mc, metacarpal; mt, metatarsal; na, nasal; neo, 19–24 cervical vertebrae and subequal length of the third neomorph; pa, parietal; pal, palatine; palf, palatal and fourth metatarsals. Because the skull of Shokawa foramen; par, parasphenoid;
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