Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chanares Formation, Argentina), its inclusivity, and relationships amongst early dinosauromorphs

摘要

Lewisuchus admixtus is an enigmatic early dinosauriform from the Chanares Formation, Ladinian of Argentina, which has been recently considered a member of Silesauridae. Yet, it differs markedly from Late Triassic silesaurids in dental and vertebral anatomy. Indeed, a detailed redescription of its holotype allowed the identification of several features of the skeleton previously unrecognized amongst silesaurids. These include pterygoid teeth, a dorsomedial posttemporal opening on the otoccipital, foramina associated with cranial nerves Ⅹ-Ⅻ on the caudal region of the prootic-otoccipital, and postaxial neck/trunk vertebrae with craniocaudally expanded neural spines. The presence of a single row of presacral scutes was also confirmed. Some elements previously referred to, or found associated with, the holotype, including a lower jaw, pedal elements and an astragalus, more probably correspond to proterochampsid remains. The anatomical information available for the holotype of L. admixtus was rescored into a new phylogenetic dataset for dinosauromorphs, mostly based on previous works. Lewisuchus admixtus and Pseudolagosuchus major are treated as distinct OTUs because their preserved skeletons mostly lack overlapping parts. The parsimony analysis supports the basal position of L. admixtus within dinosauriforms, prior to the silesaurid-dinosaur split, rather than at the base of Silesauridae. This suggests that a higher number of early dinosauriform clades branched in the Middle and Late Triassic than previously suggested.