Refinement of the Brazilian Hyperodapedon Assemblage Zone (Late Triassic) and its biostratigraphic correlation with the Argentine biozones of the Ischigualasto Formation

Abstract

The Santa Maria Supersequence (SMS; Middle–Late Triassic, Brazil) crops out as fossil-rich, dispersed, and discontinuous exposures with homogeneous lithofacies. Its faunal successions are understood primarily through tetrapod-based biostratigraphic correlation. The Candelária Sequence is one of the third-order sequences of the SMS, at the base of which lies the Hyperodapedon Assemblage Zone (AZ). This AZ is biostratigraphically correlated with the Hyperodapedon-Exaeretodon-Herrerasaurus Biozone of the Ischigualasto Formation (Carnian–Norian) in Argentina. Isotopic dating on the lower levels of the Candelária Sequence and the Ischigualasto Formation also indicates a time correlation between these beds. However, debate persists whether the Hyperodapedon AZ represents a single faunal association or if it should be divided in two subzones: an older Hyperodapedon Acme Zone, characterized by the abundance of the rhynchosaur “Hyperodapedon” spp., and a younger assemblage dominated by the cynodont Exaeretodon riograndensis. Another open question is whether a second rhynchosaur taxon, Teyumbaita sulcognathus, is biostratigraphically coeval with or younger than “Hyperodapedon” spp. This study provides new insights towards the biostratigraphy of the Hyperodapedon AZ through the systematization of previously collected data, targeted collection of new specimens, and stratigraphic section measurements from three key outcrops from the Vale do Sol area. Our results support the previously proposed subdivision of the Hyperodapedon AZ. Moreover, Teyumbaita is recorded for the first time below the stratigraphic level where “Hyperodapedon” predominates, indicating the co-occurrence of these two hyperodapedontine rhynchosaurs within the Hyperodapedon Acme Zone. Regular sampling of outcrops is essential to minimize quantitative biases and enhance the accuracy of future biostratigraphic correlations.