First occurrence of a herrerasaurid dinosaur in a traversodontid cynodont-dominated Late Triassic fauna of southern Brazil.

Abstract

Herrerasauridae comprises the oldest large-bodied predatory dinosaurs and is currently known exclusively from the Carnian (Late Triassic) of South America. In both Argentina and Brazil, herrerasaurid occurrences have previously been restricted to rhynchosaur-dominated assemblages, with no records from younger strata characterized by the abundance of the traversodontid cynodont Exaeretodon. Here, I describe CAPPA/UFSM 0539, a partial left pubis recovered from the Exaeretodon-dominated strata of the Hyperodapedon Assemblage Zone (AZ) of southern Brazil. The specimen preserves a combination of pubic features that is currently unique to herrerasaurids, including a well-developed pubic boot with anterior and posterior distal expansions. A phylogenetic analysis further supports its referral to an indeterminate herrerasaurid. This specimen represents the first record of Herrerasauridae from the Exaeretodon-dominated strata, demonstrating that members of the clade persisted despite the faunal turnover associated with the decline of rhynchosaurs and the increasing abundance of Exaeretodon. The occurrence expands the known diversity of large predators in these ecosystems and suggests that herrerasaurids retained the ecological plasticity to occupy apex predatory roles under changing community compositions. If the Brazilian Exaeretodon-dominated strata are younger than the Hyperodapedon Acme Zone, CAPPA/UFSM 0539 may also represent the youngest known record of Herrerasauridae worldwide.