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The Early Cretaceous Lizard Genus Yabeinosaurus from China: Resolving an Enigma

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The Early Cretaceous Lizard Genus Yabeinosaurus from China: Resolving an Enigma Journal of Systematic Palaeontology ISSN: 1477-2019 (Print) 1478-0941 (Online) Journal homepage: http://www.tandfonline.com/loi/tjsp20 The early Cretaceous lizard genus Yabeinosaurus from China: Resolving an enigma Susan E. Evans , Yuan Wang & Chun Li To cite this article: Susan E. Evans , Yuan Wang & Chun Li (2005) The early Cretaceous lizard genus Yabeinosaurus from China: Resolving an enigma, Journal of Systematic Palaeontology, 3:4, 319-335, DOI: 10.1017/S1477201905001641 To link to this article: http://dx.doi.org/10.1017/S1477201905001641 Published online: 09 Mar 2010. Submit your article to this journal Article views: 107 View related articles Citing articles: 21 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tjsp20 Download by: [National Science Library] Date: 22 March 2016, At: 02:17 Journal of Systematic Palaeontology 3 (4): 319-335 Issued 30 November 2005 doi:10.1017/S1477201905001641 Printed in the United Kingdom © The Natural History Museum THE EARLY CRETACEOUS LIZARD GENUS Yabeinosaurus FROM CHINA: RESOLVING an enigma Susan E. Evans* Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, England Yuan Wang Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 142 Xi-Zhi-Men-Wai St, P.O.Box 643, Beijing 100044, China Chun Li Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 142 Xi-Zhi-Men-Wai St, P.O.Box 643, Beijing 100044, China SYNOPSIS The lizard genus Yabeinosaurus was originally described more than 60 years ago from deposits of the Early Cretaceous Jehol Group of northeastern China. Its morphology and phylogenetic position have remained enigmatic for several reasons, not least the inaccessibility of the type and figured material, now lost, and the immaturity of all previously documented specimens. Recent work on the Jehol Biota has yielded a series of well-preserved Yabeinosaurus skeletons, ranging from juvenile to adult. Contrary to previous interpretations, Yabeinosaurus was not a small, weakly ossified lizard with gekkotan affinities. The adult reached a size in excess of 300 mm snout-pelvis length and had a large, heavily ossified and strongly sculptured skull. Phylogenetic analysis highlights some of the difficulties involved in determining the systematic positions of early squamate taxa. Nevertheless, the overall results suggest a position close to the iguanian-scleroglossan dichotomy (just above or just below). As such, Yabeinosaurus may represent a relict taxon in the Jehol Biota, a survivor of the Pan-Laurasian lizard fauna of the Jurassic. KEYWORDS Jehol Biota, Squamata, Reptilia, Asia Contents Introduction 320 Geological background 320 Downloaded by [National Science Library] at 02:17 22 March 2016 Systematic palaeontology 321 Squamata Oppel, 1811 321 Yabeinosaurus Endo & Shikama, 1942 321 Description 322 Skull 322 Mandibles 328 Postcranial skeleton 328 Soft tissue traces 331 Discussion 331 Implications 332 Acknowledgements 333 References 333 Appendix: Character lists and data matrices used 335 * Corresponding author. E-mail address: [email protected] 320 S.E. EVANS ETAL. INTRODUCTION Rocks of the Jehol Group (Yixian and Jiufotang formations) outcrop over a large area of northeastern China from north- ern Hebei Province to western Liaoning Province, across the southeast part of the Inner Mongolia Autonomous Region. Although there remains some controversy over aspects of the dating, most scientists now regard the deposits as Early Cretaceous in age (Valanginian to Aptian: Smith et al. 1995; Wang & Zhou 2003; Zhou etal. 2003). These fossil horizons have yielded a spectacular array of plant, invertebrate and vertebrate fossils, including fish, salamanders, frogs, turtles, lizards, choristoderes, pterosaurs, dinosaurs, mammals and birds. The first lizard fossil described was Yabeinosaurus tenuis (Endo & Shikama 1942), but other taxa have been ad- ded more recently (Ji 1998; Ji & Ren 1999). Endo & Shikama (1942) referred Yabeinosaurus to a new family, Yabeinosaur- idae, an attribution accepted by Young (1948). Young (1958) later recorded a second specimen (IVPP V961) from Ket- zutung (Gezidong), Lingyuan, Liaoning, placing it in the type species. However, Hoffstetter (1964, 1967a) referred the specimen to a new species, Y. youngi, on the basis of minor proportional differences. This specimen remains in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, but the preservation is extremely poor and its status is indeterminate. Hoffstetter (1964) placed Yabeinosaurus close to the European Ardeosaurus and Eichstaettisaurus, within the family Ardeosauridae. This position was adopted by Estes (1983a) in his brief review of the taxon. Both Hoffstetter (1964, 1967a) and Estes (1983a) classified Ardeosauridae within the Gekkota, but on pre-cladistic criteria. Neither the original descriptions, nor Estes' review, provide any significant morphological information on Yabeinosaurus beyond small size, paired frontals and rel- atively short limbs. There is thus little basis for a discussion of its phylogenetic position. As noted by Estes (1983a: 63), the original description implies that the holotype specimen was a very young animal since the scapula and coracoid and the astragalus and calcaneum remained unfused. This holo- type is now lost. Ji et al. (2001) recently described a further specimen of Yabeinosaurus tenuis (YFM R002) and erected Figure 1 Yabeinosaurus tenuis. IVPP V13285 (adult skull) in this as a neotype. Unfortunately, this specimen is also very comparison with IVPP V12641 (juvenile skeleton). Eph, Ephemeropsis immature and the description provides no phylogenetically trisetalis (a common Jehol insect larva). Downloaded by [National Science Library] at 02:17 22 March 2016 useful data. Four new specimens in the collections of the IVPP in Beijing have permitted a detailed reassessment of Yabeino- Group to the Yixian and Jiufotang formations. Most Chinese saurus. They provide a growth series (Fig. 1), showing that workers recognise four major divisions of the Yixian Form- the adult Yabeinosaurus was a large, well-ossified lizard ation: Lujiatun, Jianshangou, Dawangzhangzi and Jingang- with a long period of post-hatchling growth. This explains shan. Overlying the last of these is the Boluchi Bed of the the difficulties encountered by previous workers. The adult Jiufotang Formation. Multiple tuff horizons have yielded a Yabeinosaurus shows no paedomorphic traits and is not re- series of consistent dates: Lujiatun Bed (c. 129-128 Ma), lated to gekkotans (contra Hoffstetter 1964; Estes 1983a). Jianshangou Bed (c. 124—125 Ma), Dawangzhangzi Bed Rather it appears to be a basal squamate. (c. 123-122 Ma), Jingangshan Bed (c. 121 Ma), top of the Jiufotang Formation (110 Ma) (Wang & Zhou 2003). Thus, under current understanding, the rocks of the Jehol Group span a period from the Valanginian to the Aptian. Underly- ing the Yixian Formation (but unconformably) is the Jurassic GEOLOGICAL BACKGROUND Tuchengzi Formation, dated at 139 Ma. The age and geology of the Jehol Group have been described The specimens of Yabeinosaurus described here were in some detail in recent years (e.g. Wang & Zhou 2003; Zhou recovered from the Jianshangou Bed (at Sihetun locality, et al. 2003) and these works should be consulted for a com- Liaoning: IVPP V13285, Figs 1 & 2), the Dawangzhangzi prehensive discussion. The current consensus limits the Jehol Bed (at Dawangzhangzi locality, Liaoning: IVPP V13284; EARLY CRETACEOUS LIZARD GENUS YABEINOSAURUS 321 Figure 2 Yabeinosaurus tenuis. IVPPV13285, adult skull, mainly in ventral view, with skull roof bones displaced and rotated to the left of the skull, and left manus. Abbreviations: l.j, left jugal; l.mx, left maxilla; l.n, left nasal; l.q, left quadrate; l.sq, left squamosal; ms, manus; par, parietal; pmx, premaxilla; r.j, right jugal; r.prf, right prefrontal; r.q, right quadrate; r.sq, right squamosal; sm, septomaxilla; ?, unknown. Fig. 3), the Jingangshan Bed (at Jingangshan locality, Liaon- ORIGINAL HOLOTYPE. Central National Museum, Man- ing: IVPP V12641; Fig. 1) and the Jiufotang Formation churia, CNMM 3735 (now lost). (at Dapingfang locality, Liaoning: IVPP V13272). Endo & Shikama's (1942) holotype (CNMM 3735) is now lost, TYPE LOCALITY AND HORIZON. Zaocishan, Liaoning. but was recorded as coming from the Tsaotzushan Form- Jingangshan Bed, Yixian Formation. ation, Tsaotzushan (Zaocishan), 21 km southwest of Ihsien NEOTYPE. YFM R002, Yizhou Fossil Museum, Yixian (= Yixian), Chinchou Province (= Jinzhou City region, Je- County, Liaoning. hol Province). This horizon is now considered to be equi- valent to the Jingangshan Bed (Wang X. L., pers. comm., NEOTYPE LOCALITY. Jingangshan locality, Jingangshan Downloaded by [National Science Library] at 02:17 22 March 2016 2004). The neotype specimen described by Ji et al. (2001) Bed, Yixian Formation. also came from the Jingangshan Bed, Jingangshan locality. REFERRED MATERIAL. Institute of Vertebrate Paleontology Thus Yabeinosaurus persisted through the depositional his- and Paleoanthropology IVPP V12641 (juvenile skeleton); tory of the Jehol Group, but has yet to be recorded from the IVPP V13272A,B (alarge immature skeleton); IVPP V13284 earliest level. The indeterminate holotype of'Yabeinosaurus' (a small immature skeleton); IVPP V13285 (adult or subadult youngi (IVPP
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