ISSN 00310301, Paleontological Journal, 2013, Vol. 47, No. 11, pp. 1270–1281. © Pleiades Publishing, Ltd., 2013. The Phylogenetic Position of Ambiortus: Comparison with Other Mesozoic Birds from Asia1 J. K. O’Connora and N. V. Zelenkovb aKey Laboratory of Evolution and Systematics, Institute of Vertebrate Paleontology and Paleoanthropology, 142 Xizhimenwai Dajie, Beijing China 10044 bBorissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia email: [email protected], [email protected] Received August 6, 2012 Abstract—Since the last description of the ornithurine bird Ambiortus dementjevi from Mongolia, a wealth of Early Cretaceous birds have been discovered in China. Here we provide a detailed comparison of the anatomy of Ambiortus relative to other known Early Cretaceous ornithuromorphs from the Chinese Jehol Group and Xiagou Formation. We include new information on Ambiortus from a previously undescribed slab preserving part of the sternum. Ambiortus is superficially similar to Gansus yumenensis from the Aptian Xiagou Forma tion but shares more morphological features with Yixianornis grabaui (Ornithuromorpha: Songlingorni thidae) from the Jiufotang Formation of the Jehol Group. In general, the mosaic pattern of character distri bution among early ornithuromorph taxa does not reveal obvious relationships between taxa. Ambiortus was placed in a large phylogenetic analysis of Mesozoic birds, which confirms morphological observations and places Ambiortus in a polytomy with Yixianornis and Gansus. Keywords: Ornithuromorpha, Ambiortus, osteology, phylogeny, Early Cretaceous, Mongolia DOI: 10.1134/S0031030113110063 1 INTRODUCTION and articulated partial skeleton, preserving several cervi cal and thoracic vertebrae, and parts of the left thoracic Ambiortus dementjevi Kurochkin, 1982 was one of girdle and wing (specimen PIN, nos. 3790/271273; the first described Early Cretaceous ornithuromorph Figs. 1–3). The specimen preserves morphologies that birds. It comes from the Khurilt Ulaan Bulag locality were at first indicative of a fairly derived ornithuro of the Andaikhudag Formation (Hauterverian—Bar morph bird (e.g. curved scapula, bent acromion pro remian) in the Central Mongolian Altai (Kurochkin, cess, welldeveloped procoracoid process); the taxon 1982, 1985, 1999, 2000). Although dwarfed by the cur was originally considered a paleognathous bird, how rent Mesozoic bird collections in China, several other ever its position within Aves has shifted with the dis isolated Mesozoic bird taxa are also known from Mon covery of new taxa from the Jehol Biota. Most recently golia, including Apsaravis ukhaana Norell et Clarke, in a large cladistic analysis aimed towards Mesozoic 2001, Hollanda luceria Bell et al., 2010, Nanantius val birds as a whole, Ambiortus is resolved as a basal orni ifanovi Kurochkin, 1996, Elsornis keni Chiappe et al., thuromorph (O’Connor et al., 2011a). 2006, and several specimens of Gobipteryx minuta When Ambiortus was first described (Kurochkin, Elzanowski, 1974 (Elzanowski, 1974; Kurochkin, 1982), the Mesozoic record of fossil birds was 1996; Norell and Clarke, 2001; Chiappe et al., 2006; extremely sparse (Kurochkin, 1999). Gansus yumen Bell et al., 2010); these taxa span the ornithothoracine ensis Hou et Liu, 1984 was the first Mesozoic bird clade, making Mongolia one of the richest places for described from China (Hou and Liu, 1984). The holo Mesozoic birds in the world. Only enantiornithines type specimen was collected from the Early Creta (Nanantius, Gurilynia nessovi Kurochkin, 1999 and ceous Aptian Xiagou Formation in Gansu Province, Elsornis) and ornithuromorphs (Apsaravis, Hollanda, northwestern China (Hou and Liu, 1984); known Ambiortus) are known and, except for Ambiortus, these from only a single foot, this taxon was never compared taxa are all from the Late Cretaceous; no primitive with Ambiortus because of the lack of overlapping longtailed birds or basal pygostylians (e.g. Sapeorni material. Even a decade later, only a small handful of thiformes, Confuciusornithiformes), which form Early Cretaceous birds were known: primarily Con major components of the rich Early Cretaceous Jehol fuciusornis Hou et al., 1995 and a handful of fragmen avifauna, are known. The holotype and only known tary enantiornithines (Hou, 1997). Two ornithurines, specimen of Ambiortus dementjevi is a wellpreserved Chaoyangia beishanensis Hou et Zhang 1993 and Lia oningornis longidigitrus Hou, 1996, had also been 1 The article is published in the original. described (Hou et al., 1996), but the former lacked 1270 THE PHYLOGENETIC POSITION OF AMBIORTUS 1271 overlapping material with Ambiortus and the unusual scientific understanding of Mesozoic birds has morphology of the latter has recently proven to be changed rapidly over the past three decades, reexami enantiornithine (O’Connor, 2012). In the last major nation of incomplete materials in light of recent infor study to address Ambiortus, the taxon was compared to mation has the potential to reveal important new the Inner Mongolian bird Otogornis genghisi Dong, information (O’Connor et al., 2011b; O’Connor and 1993 from the Lower Cretaceous Yijinghouluo For Zhou, 2013). The holotype of Ambiortus dementjevi is mation (Kurochkin, 1999). This taxon, known from a one of the most complete Cretaceous ornithuromorph partial thoracic girdle and pair of wings, was originally specimens known from outside of China, yet it has not identified as an enantiornithine (Dong, 1993), been reexamined in light of the current taxonomic although when fully described was unassigned to a spe and morphological diversity. Here we redescribe cific clade (Hou, 1994). The comparative study, how Ambiortus, including previously unpublished data, and ever, concluded that both taxa were paleognathous compare it to recently described taxa, revealing new birds (Kurochkin, 1999). Although when comparative information regarding the phylogenetic placement of material was limited Ambiortus and Otogornis may have this taxon and the distribution of ornithuromorph lin born some superficial similarity (Kurochkin, 1999), a eages in Asia as a whole. relationship between these two taxa and their place Terminology in this paper primarily follows ment within Paleognathae is no longer supported; this Baumel and Witmer (1993); Latin is retained for mus is primarily due to lack of evidence and a greater cles (Baumel et al., 1993), while English equivalents understanding of Early Cretaceous ornithuromorph are used for osteological features. morphology. For example, Kurochkin (2000) noted Institutional abbreviations: CAGS, Chinese Acad that the presence of a Ushaped furcula and convex emy of Geological Sciences, Beijing, China; FRDC, scapular cotyla on the coracoid indicated that Ambior Fossil Research and Development Center, Third tus was a neornithine, features now known to be Geology and Mineral Resources Exploration Acad present in most basal ornithuromorphs. In light of emy, Gansu Provincial Bureau of Geo–Exploration more recent finds and a greater understanding and Mineral Development, Lanzhou, China; IVPP, of Enantiornithes, Otogornis is considered a member Institute of Vertebrate Paleontology and Paleoanthro of this large clade (Zhou et al., 2008), although the pology, Beijing, China; PIN, Borissiak Paleontologi taxon has been difficult to place in a phylogenetic con cal Institute of Russian Academy of Sciences, Mos text and its taxonomic position within Enantiornithes cow, Russia. is unstable (O’Connor, 2009). The short and robust shape of the humerus, absence of a procoracoid pro cess on the coracoid and straight scapula, all support SYSTEMATIC PALEONTOLOGY the enantiornithine affinity of this taxon (Zhou and Hou, 2002; O’Connor, 2009). Morphologies that sug AVES gested this taxon may be more advanced and in fact a ORNITHUROMORPHA paleognath bird (Kurochkin, 1999) are uncorrobo Genus Ambiortus Kurochkin, 1982 rated by the specimen; furthermore, the clearest char acters that define Paleognathae are from the skull, D i a g n o s i s. An ornithuromorph bird the size of which is preserved in neither Ambiortus nor Otogornis. extant teal Anas crecca, with the unique combination of the following features: a wide procoracoid process Record of the early radiation of Ornithuromorpha, perpendicular to the shaft; scapular blade long and the clade that includes modern birds (Neomithes), has thin; scapular acromion hooked; lateral trabecula of grown rapidly during the last decade (Chiappe, 2007; the sternum mediolaterally wide, with a concave O’Connor et al., 2010; Brocklehurst et al., 2012). medial margin and convex lateral margin so that the Where previously the global record was dominated by distal end of the process appears to curve medially; largely fragmentary specimens (e.g. Vorona berivotren ventral edge of the proximal end of the humerus sis Forster et al., 1996, Vegavis iaai Clarke et al., 2005) strongly developed and with a distinct tubercle on its and even in China known taxa were based on incom cranial surface; transverse groove is short, fossalike, plete and poorly preserved specimens (e.g. Chaoyan and runs dorsoventrally; pneumotricipital fossa of the gia, Gansus, Songlingornis linghensis Hou, 1997), humerus not developed; deltopectoral crest projected recent discoveries from the Early Cretaceous Jehol dorsally; bicipital crest distally abruptly ending Group and Xiagou Formation are largely complete (revised from Kurochkin, 2000). and wellpreserved (e.g. Yanornis