Revision of the early crocodylomorph Trialestes romeri (Archosauria, Suchia) from the Lower Upper Triassic Ischigualasto Formation of Argentina: one of the oldest-known crocodylomorphs

LECUONA, A.; EZCURRA, M.D.; IRMIS, R.B.

Papers in palaeontology

The Palaeontological Association

Año: 2016 vol. 2 p. 585 - 622

Trialestes romeri (Reig) is an earlycrocodylomorph from the Ischigualasto Formation (late Carnian ? early Norian;Ischigualasto ? Villa Union Basin, Argentina) and one of the oldest-knownmembers of this clade. Two specimens of this species are known, the holotype(PVL 2561) and a referred specimen (PVL 3889), both consisting of associatedcranial and postcranial remains. These specimens are incomplete and poorly preservedthus leading previous authors to propose different phylogenetic hypotheses forthe species. Trialestes romeri wasoriginally interpreted as a basal dinosaur, subsequently considered to be acrocodylomorph with some dinosaurian characters (e.g. mesotarsal ankle joint,functionally tridactyl pes), and even proposed as a chimera representing twodifferent genera. Some recent workers have mentioned and discussed Trialestes, but none have described itin detail or included it in a quantitative phylogenetic analysis. Here, wedescribe in detail all the material assignable to the species and test itsphylogenetic relationships using a comprehensive data matrix focused on earlyarchosaurs. We support the referral of PVL 3889 to Trialestes and reject the presence of a mesotarsal ankle joint inthis specimen. We recovered Trialesteswithin Crocodylomorpha, closer to Crocodyliformes than Pseudhesperosuchus, Hesperosuchus,Dromicosuchus and Sphenosuchus. Several stratigraphicalunits worldwide approximately coeval with the Ischigualasto Formation, havealso yielded early crocodylomorph remains, such as the upper Santa Mar ıa(Brazil), Pekin (USA), and lower Maleri (India) formations. However, thecrocodylomorph specimens recovered from those units are more incomplete,possess uncertain phylogenetic affinities, or are part of chimaerae. Therefore,Trialestes represents the mostcompletely known of the earliest non-crocodyliform crocodylomorph taxa known todate.