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Cranial Osteology of Beipiaosaurus Inexpectus

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Cranial Osteology of Beipiaosaurus Inexpectus 第57卷 第2期 古 脊 椎 动 物 学 报 pp. 117–132 figs. 1–3 2019年4月 VERTEBRATA PALASIATICA DOI: 10.19615/j.cnki.1000-3118.190115 Cranial osteology of Beipiaosaurus inexpectus (Theropoda: Therizinosauria) LIAO Chun-Chi1,2,3 XU Xing1,2* (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 * Corresponding author: [email protected]) (2 CAS Center for Excellence in Life and Paleoenvironment Beijing 100044) (3 University of Chinese Academy of Sciences Beijing 100049) Abstract Beipiaosaurus inexpectus, a key taxon for understanding the early evolution of therizinosaurians, has not been fully described since it was briefly reported on by Xu, Tang and Wang in 1999. Here we present a detailed description of the cranial anatomy of the holotype of this theropod dinosaur. B. inexpectus is unique in some of its cranial features such as the postorbital process of the frontal is large and its abrupt transition from the orbital rim, a long and sharp anterior process of the parietal, the elongate ventral ramus of the squamosal process of parietal, and external mandibular fenestra deep dorsoventrally and extremely posteriorly located. A number of plesiomorphic cranial features (such as relatively large dentary and less downturned degree of dentary symphysis) suggest that B. inexpectus is an early-branching Therizinosaurian, as proposed by previous studies. New information derived from our study is not only important for our understanding of the cranial anatomy of B. inexpectus but also significant to the study of the evolution of Therizinosauria. Key words Beipiao, Liaoning, Early Cretaceous, Theropoda, Therizinosauria, cranial osteology Citation Liao C C, Xu X, 2019. Cranial osteology of Beipiaosaurus inexpectus (Theropoda: Therizinosauria). Vertebrata PalAsiatica, 57(2): 117–132 Therizinosauria is a maniraptoran theropod clade, which is defined here as the most inclusive clade including Therizinosaurus, but not Ornithomimus velox or Oviraptor philoceratops (Zanno, 2010a). The late-branching members of this clade, i.e., the Therizinosauridae (defined as the least inclusive clade including Therizinosaurus and Erlianosaurus), have a distinctive combination of skeletal features, such as rostrally edentulous snout, dentary with a lateral shelf, small tightly packed lanceolate teeth, elongate neck, extremely broad and opisthopubic pelvis, shortened tibia, and tetradactyl pes (Clark et al., 1994, 2004; Zanno, 2006, 2010b). To date, this clade is known from the Cretaceous period of Asia and North America (Barsbold, 1976; Perle, 1979; Russell and Dong, 1993; Xu et al., 1999; Kirkland et al., 2005), although Eshanosaurus deguchiianus from the Lufeng Formation, 国家自然科学基金(批准号：41688103, 91514302)和中国科学院战略性先导科技专项(B类) (编号： XDB18030504)资助。 收稿日期：2018-06-05 118 Vertebrata PalAsiatica, Vol. 57, No. 2 Yunnan, China might represent an Early Jurassic taxon (Xu et al., 2001; Barrett, 2010). Beipiaosaurus inexpectus from the Lower Cretaceous Yixian Formation in Liaoning, China is a key taxon for understanding therizinosaurian evolution because its discovery provides direct evidence for the presence of filamentous feathers in this clade and demonstrates the independent evolution of sauropodomorphan-like features such as the tetradactyl pes in late-branching members of the clade (Xu et al., 1999). Despite the importance of Beipiaosaurus, only three brief papers have been published on the skeletal and integumentary features of this taxon (Xu et al., 1999, 2003, 2009). A detailed osteological description of the holotype specimen (IVPP V 11559) is needed in order to fully understand this important taxon. Here we present such a description of the cranial osteology, which will certainly shed new light on the cranial evolution within Therizinosauria. Institutional abbreviations IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; UMNH VP, Utah Museum of Natural History, Salt Lake City, Utah. Anatomical abbreviations a.f, frontal articulation; amp, anteromedial process; an, angular; ap.fp, accessory process of frontal process; a.sup, supraoccipital articulation; dt, denticles; emf, external mandibular fenestra; en, external naris; ff, floccular fossa; fp, frontal process; f.pp, postorbital process of frontal; jp, jugal process; lf, lateral foramina; l.p, left parietal; ls, lateral shelf; mc, mandibular condyle; mg, Mecklian groove; ob, orbital; pld, posterolateral depression; po.sqp, squamosal process of postorbital; p.pp, postorbital process of parietal; pqf, paraquadrate foramen; p.sqp, squamosal process of parietal; ptf, pterygoid flange; r.f, right frontal; r.ls, right laterosphenoid; rp, replacement pits; sa, surangular; stf, supratemporal fenestra; tc, tooth crown; tr, tooth root; vo, vomer; V2, foramen for maxillary branch of the trigeminal nerve; VII, foramen for the facial nerve; VIII, foramen for the vestibulocochlear nerve. 1 Systematic paleontology Dinosauria Owen, 1842 Theropoda Marsh, 1881 Coelurosauria sensu Gauthier, 1986 Therizinosauria Russell & Dong, 1993 Therizinosauroidea Maleev, 1954 Beipiaosaurus inexpectus Xu et al., 1999 (Figs. 1–3) Holotype1) IVPP V 11559, a partial, semi-articulated skeleton including some cranial 1) After Beipiaosaurus inexpectus was named in 1999, additional material of the holotype (IVPP V 11559) was collected from the type locality (Xu et al., 2003). The holotype thus comprises more skeletal elements than listed in the original naming paper. Liao & Xu - Cranial osteology of Beipiaosaurus inexpectus 119 elements and most of the postcranial elements. Type locality and horizon Sihetun locality near Beipiao, Liaoning, China. Lower Cretaceous (Aptian) Yixian Formation (Swisher et al., 2001). Revised diagnosis (from cranium only) A small therizinosaurian distinguishable from other therizinosaurians in having the following cranial autapomorphies: postorbital process of frontal large and abruptly transits from orbital rim, parietal with a long and sharp anterior process, ventral ramus of parietal squamosal process extremely long, and external mandibular fenestra deep dorsoventrally and extremely posteriorly located. 2 Description and comparisons The orbit is large and probably more anteriorly than laterally positioned as indicated by the triangular frontal with a strongly laterally extended postorbital process (Fig. 1A). The supratemporal fenestra is relatively small as indicated by transversely wide parietals, and it is sub-trapezoidal in outline, with the medial border being much shorter than the lateral border (Fig. 1B). The external mandibular fenestra is large, dorsoventrally deep, and located close to the posterior end of the mandible (Fig. 1C, D). This is mostly probably apomorphic for Beipiaosaurus inexpectus. 2.1 Skull Nasal One thin bone is tentatively identified as a nasal, but it is unknown which side of the snout it comes from (Fig. 2A). It is a flat bone with a slender anterior portion, which represents the premaxillary process, and a wide posterior portion, which represents the main body. The posteriormost portion of the bone is missing. The bone lacks a transverse arching that is typical of the theropod nasal, but this might be due to a postmortem distortion. As in Erlikosaurus (Clark et al., 1994; Lautenschlager et al., 2014) and Jianchangosaurus (Pu et al., 2013), the nasal is an elongate bone to join the formation of a long rostrum, unlike the robust and subtriangular outline of Falcarius (Zanno, 2010b). The premaxillary process is extremely slender, about one-third of the main body in width, and this ratio is smaller than in Erlikosaurus where it nearly reaches 50% (Clark et al., 1994). It tapers anteriorly with a straight ventral margin to form the posterodorsal border of the external naris as in Erlikosaurus (Clark et al., 1994) and Jianchangosaurus (Pu et al., 2013). The elongated premaxillary process indicates the presence of a long external naris as in other therizinosaurians such as Jianchangosaurus and Erlikosaurus. Also similar to other therizinosaurians, no distinct maxillary process (subnarial process) is present. The main body is flat and few informative features are available. Frontal The posterior half of the right frontal is preserved in dorsal view (Fig. 1A). The frontal is inferred to be sub-triangular in outline as in other therizinosaurians, including Falcarius (Zanno, 2010b), Jianchangosaurus (Pu et al., 2013), and Erlikosaurus (Clark et al., 1994), but its posterior portion is transversely broader relative to the anterior portion 120 Vertebrata PalAsiatica, Vol. 57, No. 2 Fig. 1 Close-up photographs of cranial elements of Beipiaosaurus inexpectus (IVPP V 11559) A. right frontal, left parietal, and right laterosphenoid in dorsal view; B. right parietal in dorsal view; C. left surangular and left angular in lateral view; D. vomer and right dentary in medial view; E. right postorbital in lateral view Scale bars for A–C and E equal 1 cm; for D equal 2 cm Liao & Xu - Cranial osteology of Beipiaosaurus inexpectus 121 Fig. 2 Close-up photographs of cranial elements of Beipiaosaurus inexpectus (IVPP V 11559) A. partial nasal; B. right quadrate in posterolateral view; C. left prootic in medial view. Scale bars equal 1 cm (three times as wide as the latter), which is in contrast to that in most other therizinosaurians (typically twice as wide). The frontal is flattened dorsally as in Falcarius (Zanno, 2010b) and Jianchangosaurus
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