Craniodental reinterpretations and new specimens of Protuberum cabralense, a bizarre traversodontid cynodont from the earliest Late Triassic of Brazil

Abstract

Traversodontid cynodonts form one of the most abundant tetrapod clades in continental Triassic beds of southern Brazil, with more than eleven species described. Within this clade, Protuberum cabralense is one of its most bizarre members, characterized by a robustly built skull and the presence of rounded protuberances in the dorsal surface of the ribs, ilium and neural spines in presacral vertebrae. The holotype and paratypes were collected in the 1970's but described only in 2009, being positioned in the clade Gomphodontosuchinae. Here we reanalyzed the skull of the holotype specimen and reinterpreted some anatomical traits, such as the number of upper incisors, position of the paracanine fossa, number of postcanines, morphology of the pterygo-paraoccipital foramen, among others. Through CT scan images the endocranial cavities are described and compared with Santagnathus mariensis (whose endocranial cavities are firstly described here) and other traversodontids. Also, two new occurrences of the species are presented, increasing its geographical distribution within the Dinodontosaurus Assemblage Zone (Pinheiros-Chiniquá Sequence, latest Ladinian-earliest Carnian) and its relevance to biostratigraphy. The reinterpretation of several traits resulted in a novel phylogenetic placement for Protuberum cabralense, nested outside the Gomphodontosuchinae clade, even as an earlier divergent than massetognathine forms.