A new rauisuchid (Archosauria, Pseudosuchia) from the Upper Triassic (Norian) of New Mexico increases the diversity and temporal range of the clade.

Emily J Lessner,Nathan D Smith,Randall B Irmis,Sterling J Nesbitt,Michelle R Stocker,Alan H Turner

doi:10.7717/peerj.2336

Abstract

Rauisuchids are large (2–6 m in length), carnivorous, and quadrupedal pseudosuchian archosaurs closely related to crocodylomorphs. Though geographically widespread, fossils of this clade are relatively rare in Late Triassic assemblages. The middle Norian (∼212 Ma) Hayden Quarry of northern New Mexico, USA, in the Petrified Forest Member of the Chinle Formation, has yielded isolated postcranial elements and associated skull elements of a new species of rauisuchid. Vivaron haydeni gen. et. sp. nov. is diagnosed by the presence of two posteriorly directed prongs at the posterior end of the maxilla for articulation with the jugal. The holotype maxilla and referred elements are similar to those of the rauisuchid Postosuchus kirkpatricki from the southwestern United States, but V. haydeni shares several maxillary apomorphies (e.g., a distinct dropoff to the antorbital fossa that is not a ridge, a straight ventral margin, and a well defined dental groove) with the rauisuchid Teratosaurus suevicus from the Norian of Germany. Despite their geographic separation, this morphological evidence implies a close phylogenetic relationship between V. haydeni and T. suevicus. The morphology preserved in the new Hayden Quarry rauisuchid V. haydeni supports previously proposed and new synapomorphies for nodes within Rauisuchidae. The discovery of Vivaron haydeni reveals an increased range of morphological disparity for rauisuchids from the low-paleolatitude Chinle Formation and a clear biogeographic connection with high paleolatitude Pangea.

Highlights

There is much confusion in the phylogeny and taxonomy of the Triassic ‘rauisuchians.’ That group typically references large-bodied, mostly carnivorous pseudosuchian archosaurs that clearly are not aetosaurs, phytosaurs, ornithosuchids, or crocodylomorphs (Gower, 2000; Nesbitt et al, 2013), but establishing the relationships of this assortment of large, quadrupedal, Triassic predators has been challenging
The strict consensus recovered all members of Rauisuchidae in a polytomy (R. tiradentes, Pol. silesiacus, Pos. kirkpatricki, Pos. alisonae, T. suevicus, and V. haydeni); this clade was the sister taxon to Crocodylomorpha
Among those characters that can be scored for the taxon, Vivaron haydeni is differentiated from all other members of Rauisuchidae by the presence of five premaxillary teeth (6:2)

Summary

Introduction

There is much confusion in the phylogeny and taxonomy of the Triassic ‘rauisuchians.’ That group typically references large-bodied, mostly carnivorous pseudosuchian archosaurs that clearly are not aetosaurs, phytosaurs, ornithosuchids, or crocodylomorphs (Gower, 2000; Nesbitt et al, 2013), but establishing the relationships of this assortment of large, quadrupedal, Triassic predators has been challenging. Excavations from 2004 to 2015 at the Hayden Quarry at Ghost Ranch, New Mexico, USA have recovered a number of new rauisuchid skeletal elements. All Late Triassic rauisuchid fossils from Texas, Arizona, and New Mexico were referred to Postosuchus kirkpatricki, including those from the nearby Canjilon Quarry at Ghost Ranch (Long & Murry, 1995). The discovery of a rauisuchid clearly distinguishable from Postosuchus provides reason to reevaluate all previously referred rauisuchid material from the southwestern United States. We describe these skeletal elements from the Hayden Quarry and erect a new taxon Vivaron haydeni gen. Our analyses reveal that rauisuchids occupied a large biogeographic range with a wide latitudinal distribution over Pangea during the Carnian and Norian (Benton, 1986; Nesbitt et al, 2013)

Methods

Results

Conclusion