THE MIXOSAURID ICHTHYOSAUR PHALARODON CF. P. FRAASI FROM THE MIDDLE TRIASSIC OF GUIZHOU PROVINCE, CHINA

Abstract

The family Mixosauridae Baur, 1887 is a dominant group of Middle Triassic ichthyosaurs. Its generic composition has been controversial, but recent findings from southern China enabled Jiang et al. (2006) to recognize two monophyletic taxa within the clade, suggesting the presence of two genera within the family, namely Mixosaurus Baur, 1887 and Phalarodon Merriam, 1910. The latter genus, which was invalidated at one point (Nicholls et al., 1999; McGowan and Motani, 2003), was recently resurrected by Schmitz (2005) by validating its type species. Mixosaurus is Tethyan in distribution, whereas Phalarodon had been known mostly from North America and Spitsbergen, apart from a possible juvenile from Switzerland (Brinkmann, 1997, 1998). More recently, Jiang et al. (2003) reported a largely complete, yet poorly preserved skeleton as the first record of the genus Phalarodon from Asia and referred it to Phalarodon sp. However, important synapomorphies were not clearly identified, and evidence has since emerged that the specimen had been tampered with by farmers after it was collected. In the light of the cladistic analysis by Jiang et al. (2006), the referral of the specimen to the genus Phalarodon is questionable. A new specimen was excavated in Panxian County, near the western border of Guizhou Province, by the Geological Museum and Department of Geology of Peking University. The new material, the authenticity of which is unquestionable, is from the same stratigraphic horizon as the specimen of Jiang et al. (2003). It contains a well-preserved skull and some postcranial bones, and for the first time firmly establishes the presence of the genus Phalarodon in the western Pacific. Also, the skull is preserved in a very unusual condition: it had been split near the sagittal plane, enabling examination of the poorly known interior suture pattern. This illustrates that mixosaurids had extensive …