Early to Middle Triassic Ichthyopterygians From the Sulphur Mountain Formation of East-Central British Columbia, Canada: Phylogenetic and Evolutionary Implications.

摘要

Ichthyopterygia is a diverse clade of Mesozoic marine reptiles that first appears in the Early Triassic, already exhibiting a highly-derived skeleton modified for an aquatic lifestyle. However, fossil evidence for informative early forms is incomplete and only a few locations worldwide have produced material. As a result, we have a poor understanding of the early evolution of this interesting clade.;Recent exploration of Sulphur Mountain Formation (Early–Middle Triassic) outcrop in the alpine of east-central British Columbia has resulted in an abundance of new ichthyopterygian fossil material, increasing our knowledge of early group members. Based on cranial and postcranial data, TMP 89.127.3 is identified as a new taxon retrieved within Grippidia, a clade formerly restricted to Lower Triassic deposits in Norway and China. The second specimen (PRPRC 2007.08.81) comprises a complete skull roof preserved in dorsal aspect and is referred to Utatsusaurus, representing the second occurrence of this genus from the Sulphur Mountain Formation. The third examined specimen (PRPRC 2004.05.08), referred to Mixosauria, is represented by a partial skull bearing a premaxilla with a pointed posterior margin and an enlarged supratemporal terrace.;Due to material being collected from talus debris, a major challenge concerning the study of Sulphur Mountain Formation material is the loss of stratigraphic context. In order to identify the original strata from which specimens were weathered, geologic details were described for each specimen matrix sample and matched to the most similar sedimentary features that were identified for the Vega-Phroso Siltstone, Whistler, and Llama Members of the Sulphur Mountain Formation. The resulting hypothesis of stratigraphic placement indicates that taxa separate out stratigraphically, with Utatsusaurus, Chaohusaurus, and Grippia cf. longirostris (Gulosaurus) restricted to the Vega-Phroso Siltstone Member, and Phalarodon to the Llama Member.