A complete skull of a stem mammal from the Late Triassic of Brazil illuminates the early evolution of prozostrodontian cynodonts

Abstract

Triassic cynodonts from South America are key taxa in the investigation of the emergence of mammalian characters. One of the most iconic species from the Carnian is Prozostrodon brasiliensis, found in Late Triassic strata from southern Brazil. This non-mammaliaform cynodont represents the earliest-diverging member of Prozostrodontia, a clade that encompasses Mammalia and their closest relatives. Previous descriptions of the skull of Pr. brasiliensis were based on specimens that did not preserve the posterior region, obscuring essential details of the basicranium. Here, we describe a new, complete, and exceptionally well-preserved skull of Pr. brasiliensis found in the same block as the holotype skeleton of the early predatory dinosaur Gnathovorax cabreirai and rhynchosaur specimens. Anatomical data from this specimen provide novel insights into the initial radiation of prozostrodontian cynodonts and reveal a new endemic clade of South American cynodonts – Prozostrodontidae – on the stem lineage of mammals.