Sauropod dinosaurs from the Lower Cretaceous of eastern Asia: taxonomic and biogeographical implications

Abstract

ABSTRACT. Sauropod dinosaurs are poorly represented in the Lower Cretaceous of eastern Asia. Here, we describe a number of isolated sauropod teeth from the Kuwajima Formation (?Berriasian±?Hauterivian) of Shiramine, Japan. The mosaic of shared derived characters and symplesiomorphies displayed by the teeth indicate that they are referable to a basal member of the titanosauriform radiation. A taxonomic review of previously described sauropod specimens from eastern and south-eastern Asia reveals that a diversity of sauropods (including a titanosaurian, a basal titanosauriform and a ?euhelopodid, as well as several forms of indeterminate systematic position) was present in this region in the Early Cretaceous. This diversity con¯icts with previous suggestions that eastern Asia was biogeographically isolated from the rest of Laurasia until the late Early Cretaceous and that the sauropod fauna was limited to the endemic East Asian clade Euhelopodidae. The presence of titanosauriform sauropods in the basal Cretaceous of Japan and Thailand indicate that the proposed faunal isolation of eastern Asia ended approximately 20 myr earlier than usually suggested.

Titanosauriform teeth from the Cretaceous of Japan

“…Labial grooves: shallow grooves running parallel to the distal and mesial margins of the crown on its labial surface (Barrett et al 2002); Lingual ridge: mesiodistally broad ridge running parallel to the long axis (apicobasal axis) of the crown on its lingual surface (Barrett et al 2002); Slenderness index (SI): the ratio of crown height to maximum mesiodistal crown width (Upchurch 1998, Barrett et al 2002.…”

“…Barrett et al 2002, Takakuwa et al 2008. On the other hand, among the sauropod teeth with high SI value, such asymmetry is often lost, and it is difficult to identify the tooth orientation.…”

“…later (Tanimoto and Mizutani 1999b). However, Barrett et al (2002) denied both identifications and identified it as a member of the Titanosauriforms.…”

“…These teeth from the Kuwajima Formation are called the Kuwajima sauropod teeth hereafter. These teeth consist of 9 teeth, which were described in detail by Barrett et al (2002), but some notes on the facet and SI are given below. The wear facet of the Kuwajima sauropod teeth is all type 1, except for one specimen (SBEI 583), which shows type 4.…”

“…Sauropod fossils have so far been reported from 8 localities in Japan (Hasegawa et al 1991, Tanimoto and Suzuki 1997, Azuma and Tomida 1998, Tomida et al 2001, Barrett et al 2002, Saegusa et al 2008, Azuma and Shibata 2010. Except for a badly preserved Proceedings of the Third Gondwanan Dinosaur Symposium Correspondence to: Haruo Saegusa E-mail: saegusa@hitohaku.jp humerus from the Upper Cretaceous Miyako Group at Moshi, Iwaizumi Town, Iwate Pref.…”