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

BITTENCOURT, J.; ARCUCCI, ANDREA; MARSICANO, CLAUDIA; LANGER, MAX

JOURNAL OF SYSTEMATIC PALAEONTOLOGY

CAMBRIDGE UNIV PRESS

Lugar: Cambridge; Año: 2015 vol. 13 p. 189 - 219

1477-2019

Lewisuchus admixtus is an enigmatic early dinosauriform from the Cha~nares Formation, Ladinian of Argentina, which hasbeen recently considered a member of Silesauridae. Yet, it differs markedly from Late Triassic silesaurids in dental andvertebral anatomy. Indeed, a detailed redescription of its holotype allowed the identification of several features of theskeleton previously unrecognized amongst silesaurids. These include pterygoid teeth, a dorsomedial posttemporal openingon the otoccipital, foramina associated with cranial nerves X?XII on the caudal region of the prootic?otoccipital, andpostaxial neck/trunk vertebrae with craniocaudally expanded neural spines. The presence of a single row of presacralscutes was also confirmed. Some elements previously referred to, or found associated with, the holotype, includinga lower jaw, pedal elements and an astragalus, more probably correspond to proterochampsid remains. Theanatomical information available for the holotype of L. admixtus was rescored into a new phylogenetic dataset fordinosauromorphs, mostly based on previous works. Lewisuchus admixtus and Pseudolagosuchus major are treated asdistinct OTUs because their preserved skeletons mostly lack overlapping parts. The parsimony analysis supports the basalposition 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 previouslysuggested.