The Late Triassic record of cynodonts: Time of innovations in the mammalian lineage

Abstract

The Triassic period witnessed a great diversification of lineages, recovering from one of the worst extinction events known in Earth’s history. Therapsids, the lineage that includes mammals as the only living members, enjoyed remarkable success during the Triassic. This clade includes the Late Permian to Early Cretaceous non-mammaliaform cynodonts, represented by a paraphyletic array of taxa successively more closely related to mammaliaforms (considered as basal mammals by several palaeontologists). In the Middle Triassic, cynodonts are represented by numerous taxa that thrived mostly in Gondwana, whereas only one taxon, Nanogomphodon, has been registered in Laurasia. Cynodont diversity during this time interval is mainly composed of gomphodonts, featuring bucco-lingually expanded postcanines, whereas the members of their sister-group, the mostly sectorial-toothed probainognathians, are very scarce. On the contrary, Early Jurassic non-mammaliaform cynodonts are most abundant in Laurasia (although also present in Gondwana) and only represented by probainognathians, particularly the sectorial-toothed tritheledontids and the ubiquitous herbivorous tritylodontids. The Late Triassic thus constitutes a pivotal time lapse, marked by an expansion of the geographical distribution and diversification of cynodonts. During this time, cynodont assemblages include representatives of old and new lineages and the first mammaliaforms are documented. This contribution presents a review of the diversity and geographic distribution of Late Triassic to Early Jurassic cynodonts, and summarizes the main morphologies represented in the lineage, including Mammaliaformes, a key group in our understanding of the early evolution of mammals.