Redescription and reassessment of the phylogenetic affinities of euhelopus zdanskyi (dinosauria: Sauropoda) from the early cretaceous of China

Abstract

SYNOPSIS Euhelopus zdanskyi was the first dinosaur described from China. Both traditional and modern cladistic assessments have found support for an endemic clade of Chinese sauropods (Euhelopodidae) that originated during an interval of geographic isolation, but the monophyly of this clade has remained controversial. The phylogenetic affinity of the eponymous genus Euhelopus is central to this controversy, yet its anatomy has not been completely restudied since the original German-language monograph in 1929. We jointly re-examined the cranial and postcranial anatomy of the holotypic and referred materials of Euhelopus to provide a new diagnosis for the genus and to explore its phylogenetic affinities. Diagnostic features of Euhelopus include: postaxial cervical vertebrae that have variably developed epipophyses and more subtle pre-epipopophyses below the prezygapophyses; cervical neural arches with an epipophysealprezygapophyseal lamina separating two pneumatocoels; anterior cervical vertebrae with three costal spurs on the tuberculum and capitulum; divided middle presacral neural spines, which in anterior dorsal vertebrae bear a median tubercle that is as large or larger than the metapophyses; middle and posterior dorsal parapophyseal and diapophyseal laminae arranged in a K configuration; and presacral pneumaticity that extends into the ilium. Following this morphological study, we rescored Euhelopus for the two most comprehensive sauropod data matrices (Wilson 2002; Upchurch et al. 2004a), which previously yielded vastly different hypotheses for its relationships. Both matrices decisively demonstrate that Euhelopus is closely related to Titanosauria; traditional and cladistic claims that Euhelopus, Omeisaurus, Mamenchisaurus and Shunosaurus formed a monophyletic Euhelopodidae endemic to East Asia are not supported. These results suggest that there were at least two clades of very long-necked sauropods in East Asia, occurring in the Middle Jurassic (i.e. Omeisaurus + Mamenchisaurus) and Early Cretaceous (e.g. Euhelopus, Erketu), with the latter group perhaps also occurring in Europe (Canudo et al. 2002). It is probable that the Euhelopus + Erketu lineage invaded East Asia from another part of Pangaea when isolation ended in the Early Cretaceous. The large number of basal titanosauriforms from East Asia has been interpreted to mean that this area may represent their centre of origin (You et al. 2003), but the titanosaur fossil record and phylogenetic studies indicate that the group probably originated prior to the Middle Jurassic and acquired a virtually global distribution before Pangaean fragmentation. © The Natural History Museum 2009.