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The Taxonomic Status of the Late Cretaceous Dromaeosaurid

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The Taxonomic Status of the Late Cretaceous Dromaeosaurid 第53卷 第1期 古 脊 椎 动 物 学 报 pp. 29-62 2015年1月 VERTEBRATA PALASIATICA fi gs. 1-4 The taxonomic status of the Late Cretaceous dromaeosaurid Linheraptor exquisitus and its implications for dromaeosaurid systematics XU Xing1 Michael PITTMAN2 Corwin SULLIVAN1 Jonah N. CHOINIERE3 TAN Qing-Wei4 James M. CLARK5 Mark A. NORELL6 WANG Shuo1,7 (1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences Beijing 100044, China [email protected]) (2 Vertebrate Palaeontology Laboratory, Life and Planetary Evolution Research Group, Department of Earth Sciences, The University of Hong Kong Pokfulam, Hong Kong, China) (3 Evolutionary Studies Institute, Centre of Excellence: Palaeontology, University of the Witwatersrand Johannesburg WITS 2050, South Africa) (4 Long Hao Institute of Geology and Paleontology Hohhot 010010, China) (5 Department of Biological Sciences, George Washington University Washington DC 20052, USA) (6 Division of Paleontology, American Museum of Natural History New York 10024, USA) (7 University of Chinese Academy of Sciences Beijing 100049, China) Abstract The dromaeosaurid Linheraptor exquisitus was named in 2010 based on a nearly complete skeleton recovered from the Upper Cretaceous Wulansuhai Formation at the Gate Locality, in Bayan Mandahu, western Nei Mongol, China. However, three recent studies regarded L. exquisitus as a subjective junior synonym of Tsaagan mangas, a dromaeosaurid from the Upper Cretaceous Djadokhta Formation of the Ukhaa Tolgod locality, Mongolia. Here we refute this synonymy based on 61 morphological features that distinguish L. exquisitus from T. mangas. Many of these features are based on new observations from previously unprepared areas of the L. exquisitus holotype, most notably from the left lateral side of the skull. These observations underscore and strengthen our original taxonomic separation of L. exquisitus and T. mangas. Evidence from L. exquisitus points to an unexpectedly complex distribution of derived osteological features amongst dromaeosaurids, because this species possesses features that were previously identifi ed as autapomorphies of T. mangas or indeed of various other dromaeosaurids. Our review demonstrates that the proposed synonymy between L. exquisitus and T. mangas ignores many subtle morphological variations. Increased taxonomic sampling breaks down seemingly obvious diagnostic differences into more subtle morphological variations, which are potentially of great importance for fine-scale phylogenetic analyses. Rigorous quantitative methods using continuous data represent a promising way to exploit this type of information in future systematic studies. Key words Nei Mongol, Late Cretaceous, Dromaeosauridae, Linheraptor, Tsaagan, taxonomy, systematics 国家自然科学基金重大国际合作项目(批准号：41120124002)和国家重点基础研究发展计划项目(编号： 2012CB821900)资助。 收稿日期：2014-03-18 30 古 脊 椎 动 物 学 报 53卷 1 Introduction The dromaeosaurid Linheraptor exquisitus was established based on a well-preserved skeleton recovered from the Upper Cretaceous Wulansuhai Formation at the Gate Locality, in Bayan Mandahu, western Nei Mongol, China (Xu et al., 2010). The initial description of L. exquisitus suggested that this taxon was closely related to Tsaagan mangas, a dromaeosaurid reported from the Upper Cretaceous Djadokhta Formation of the Ukhaa Tolgod locality, Mongolia (Norell et al., 2006), and that the two taxa might represent an endemic Asian clade (Xu et al., 2010). However, Evans et al. (2013), Senter et al. (2012) and Turner et al. (2012) considered L. exquisitus a junior synonym of T. mangas, although Evans et al. (2013) simply supported the taxonomic assessment of Turner et al. (2012) whilst Senter et al. (2012) presented no evidence to support their claim. Whether L. exquisitus is a valid taxon is important for several reasons. Firstly, this issue will affect, albeit in a minor way, compilations of data on Late Cretaceous dinosaur diversity. Secondly, it is important for understanding the Bayan Mandahu fauna (Jerzykiewicz et al., 1993). The rocks of the Djadokhta and Wulansuhai formations have similar geology (red alluvial sandstones deposited in extensional basins under arid conditions) and faunas (including dinosaurs, lizards, mammals and birds), and have been thought to belong to the same contemporaneous depositional environment (Jerzykiewicz et al., 1993). Previous studies have referred many Bayan Mandahu fossils to taxa known from the Djadokhta Formation (Currie and Peng, 1994; Dong and Currie, 1996; Jerzykiewicz et al., 1993). Similarly, Turner et al. (2012) considered the Bayan Mandahu and Djadokhta faunas to come from “equivalent” formations, implying contemporaneity and potential close faunal similarity. However, several recent studies have suggested that some previous referrals of Bayan Mandahu specimens to Djadokhta taxa were incorrect (Longrich et al., 2010; Xu et al., 2012) and that the Bayan Mandahu fauna is distinct from the Djadokhta fauna (Makovicky, 2008; Longrich et al., 2010; Xu et al., 2012, 2013). Two of the studies considered the Bayan Mandahu fauna older than the Djadokhta fauna (Makovicky, 2008; Xu et al., 2013). There are no absolute dates for the sediments of the Djadokhta and Wulansuhai formations, although magnetostratigraphy constrains the Bayn Dzak and Tugrugyin members of the former formation to ~75-71 Ma in the Campanian Stage of the Upper Cretaceous (Dashzeveg et al., 2005). Different age ranges for the Bayn Dzak member spanning the Cenomanian to early Maastrichtian (~100.5- 72.1 Ma (Gradstein et al., 2012)) have been proposed: early Late Cretaceous (Berkey and Morris, 1927), ages between the Coniacian and Santonian (89.8~83.6 Ma (Gradstein et al., 2012)) (Gradzinski et al., 1969; Kielan-Jaworowska, 1969, 1970, 1974; Kielan-Jaworowska and Dovchin, 1969; Lefeld, 1965, 1971), Cenomanian (~93.9-100.5 Ma (Gradstein et al., 2012)) (McKenna, 1969) and late Santonian/early Campanian (~83.6-86.3 Ma/~83.6-72.1 Ma (Gradstein et al., 2012)) (Rozhdestvensky, 1971). More recent studies have tended to favour a Campanian age assignment based on faunal comparisons between Mongolian, American, and 1期 Xu et al.: The taxonomic status of Linheraptor exquisitus and its implications 31 Canadian dinosaurs, which are dated by ammonite and palynological biozonations (Fox, 1978). Another study (Lillegraven and McKenna, 1986) considered the member to be Campanian- early Maastrichtian age (~83.6-72.1 Ma (Gradstein et al., 2012)). A Campanian age was assigned to the Djadokhta Formation using faunal data from both Bayn Dzak, Mongolia and Bayan Mandahu, Nei Mongol (Eberth, 1993; Jerzykiewicz et al., 1993), whilst existing faunal (Averianov, 1997; Lillegraven and McKenna, 1986) and magnetostratigraphic data (Dashzeveg et al., 2005) were used to justify the adoption of the same age for the Ukhaa Tolgod drainage basin in Mongolia (Dingus et al., 2008). Comparisons between mammals from the Darbasa Formation of Kazakhstan with those from the Djadokhta Formation were used to cautiously assign a Campanian age to the latter (Averianov, 1997), this supported an earlier study based on Djadokhta Formation mammals (Gradzinski et al., 1977). The poor age control for the Djadokhta and Wulansuhai formations – the Campanian alone covers ~11.5 million years (83.6-72.1 Ma (Gradstein et al., 2012)) – warrants further dating work, particularly using high- precision techniques such as 40Ar-39Ar dating. This work would determine whether subtle differences between L. exquistus and T. mangas arose because they lived at separate times during the Cretaceous, or favour alternative explanations if they lived together at the same time. Longrich et al. (2010) suggested the former scenario to explain observed differences in other Djadokhta and Wulansuhai taxa. High-resolution biostratigraphy and geochronology has been used successfully to recognise the significance of subtle morphological variations in the Late Cretaceous dinosaurs of North American faunas (Eberth et al., 2013; Mallon et al., 2012; Ryan and Evans, 2005; Sampson et al., 2010, 2013a, b) so this provides a useful framework to apply to contemporaneous Asian faunas. Finally and most importantly, the validity of L. exquisitus has implications for systematic work on dromaeosaurids. Acceptance of the synonymy between L. exquisitus and T. mangas would imply the existence of a new combination of morphological features in T. mangas, a derived velociraptorine taxon, and would have a signifi cant impact on dromaeosaurid systematic studies. It is therefore imperative to clarify this issue – differing interpretations percolate through the literature, causing needless confusion. We fi nd several problems with Turner et al.’s (2012) arguments. Xu et al. (2010) listed two autapomorphies for L. exquisitus, and an additional 11 osteological features to further distinguish L. exquisitus from T. mangas. Firstly, Turner et al. (2012) did not comment on the two autapomorphies of L. exquisitus, which were the most important evidence used to establish the taxon. Secondly, Turner et al. (2012) regarded some osteological differences identifi ed by Xu et al. (2010), such as the contrasting postorbital morphologies of L. exquisitus and T. mangas (see paragraph on the postorbital and text herein), as minor ones lacking any taxonomic signal. However, examination of the holotypes of both taxa shows these differences to be perfectly real, and they contribute individually to a cumulative case against
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