Paleoenvironmental and Paleoecological reconstruction of Santa Maria Supersequence units (Middle to Late Triassic, Brazil) based on stable isotope data: Influence of the Carnian Pluvial Episode

Abstract

Bones and teeth incorporate stable isotopes of C and O from an animal's food and water sources that can provide paleoecological and paleoenvironmental information. In this study, thirty-six samples of fossil bones and teeth were collected from two Triassic allostratigraphic units of the Rio Grande do Sul State, Brazil, and analyzed for δ13C and δ18O values. Specific strata include Pinheiros-Chiniquá Sequence (Late Ladinian/Early Carnian) which includes the Dinodontosaurus Assemblage Zone (15 specimens analyzed) and the basal portion of the Candelária Sequence (Late Carnian) in which the Hyperodapedon Assemblage Zone occurs (21 specimens analyzed). The latter assemblage zone was deposited during the Carnian Pluvial Episode, generally characterized by warmer and wetter conditions. Specimens of tooth enamel from the Dinodontosaurus Assemblage Zone have δ13C values ranging from −10.8 to −7.0 ‰ (VPDB) and δ18O values from 16.3 to 23.5 ‰ (VSMOW). In contrast, specimens from the Hyperodapedon Assemblage Zone have overlapping but generally higher δ13C values ranging from −9.0 to −4.8 ‰ and overlapping but higher δ18O values from 19.7 to 27.4 ‰. Evidently, the Carnian Pluvial Episode coincided with higher δ13C and δ18O values, which could represent warmer (higher δ18O) conditions in this region. Although higher δ13C values can indicate greater aridity, atmospheric δ13C compositions were dramatically (2–3 ‰) higher during the Carnian than the earlier Middle Triassic epoch. Within uncertainties, the 2 ‰ increases to fossil δ13C values could be consistent with either constant or increased precipitation. Climate change during the Late Triassic could have helped drive profound faunal turnover during this episode, including dinosaur evolution and diversification.