Biogenic control on the origin of a vertebrate monotypic accumulation from the Late Triassic of southern Brazil

Abstract

This study is a taphonomic investigation of a new Late Triassic monotypic association from the Hyperodapedon Assemblage Zone (Santa Maria Formation, southern Brazil). All skeletal elements belong to the traversodontid cynodont Exaeretodon, representing a minimum of four individuals from different ontogenetic stages. Biogenic traces are identifiable on some elements, such as invertebrate scavenging traces, vertebrate bite marks, and evidence of trampling which was probably responsible for accelerating the disarticulation of upper postcanine teeth and for fracturing a skull and a lower jaw. We classify this accumulation as generated by extrinsic biogenic action (predation/necrophagy and possibly trampling) due to the following reasons: (i) random spatial orientation of the elements enclosed into the matrix; (ii) absence of hydraulic equivalence among the specimens and presence of mudstone lenses in close association with the fossils; (iii) presence of nearly all Voorhies’ groups; (iv) association of a large number of cranial elements presenting different stages of disarticulation; and (v) presence of biogenic traces. Both disarticulation patterns and invertebrate scavenging traces indicate that the bones remained exposed for some time before burial. During this time of exposure, carnivore vertebrates also scavenged on some nutritive and transportable postcranial elements. We suggest an ecteniniid cynodont as a potential modifier agent (predation/necrophagy) of this particular monotypic accumulation of synapsids.