A reinterpretation of taphonomy and palaeoecology of the early Carnian Santacruzodon Assemblage Zone type-locality from the Santa Maria Supersequence, Brazil

Abstract

Sampling bias and repeated resampling can lead to misinterpretation of the taphonomic history of a fossiliferous deposit. This is the case of the Schoenstatt Sanctuary site (Santa Cruz do Sul, Rio Grande do Sul, Brazil), the type-locality and richest vertebrate fossil occurrence of the early Carnian Santacruzodon Assemblage Zone (Santa Cruz Sequence, Santa Maria Supersequence, Paraná Basin). This site is characterized by the abundance of traversodontid non-mammalian cynodonts, especially Santacruzodon hopsoni and Menadon besairiei (plus Massetognathus ochagaviae). The abundance of cranial remains in a 2.5 m-thick massive mudstone led earlier workers to conclude that the accumulation was biogenically controlled (e.g., selective scavenging). Here we propose a new taphonomic interpretation, based on re-investigation of >2700 fossils, collected at the site over the past 27 years. Rhizoliths and early diagenetic features (e.g., micrite crusts) indicate the occurrence of several pedogenically modified levels within the outcrop. This occurrence of multiple calcic vertisols, added to the absolute dating already obtained for the Santa Cruz Sequence, may be related to the global Carnian Pluvial Event. Re-analysis of the historical fossil collections from this site provides the basis for a biostratigraphic division of Massetognathus (lower strata), Menadon (lower and middle strata) and Santacruzodon (upper strata), leading to a three-fold sub-zonation of the assemblage zone (AZ). The taphonomic mode of some Menadon skeletons suggest they were scattered by floodwaters, while burrowing and co-habitation behaviours are indicated for Santacruzodon. The revision led also to identification of at least 12 new non-cynodont morphotypes, including kannemeyeriiform dicynodont (already known for the AZ), archosauriform, rhynchosaurian and procolophoid taxa, increasing the faunal diversity of outcrop and Santacruzodon AZ.