Biogenic fossil concentration in fluvial settings: An example of a cynodont taphocoenosis from the Middle Triassic of southern Brazil

Abstract

Nearby the city of Santa Cruz do Sul (central region of Rio Grande do Sul State in Brazil), an outcrop of the Triassic Santa Maria Formation contains a rather unusual fossil occurrence consisting of a concentration of disarticulated therapsid skeletons dominated by herbivorous and carnivorous cynodonts. Non- cynodont remains are restricted to a single specimen of Rhadinosuchidae (Archosauriformes: Proterochampsia). The taphocoenosis is characterized by the chaotic accumulation of tens of isolated fossil bones within a block of mudrock 10 m wide and 20 m long, and approximately 5 m thick. The site lies within an approximately 30 m thick succession of red mudstone displaying horizons of rhizoliths and carbonate nodules which are interpreted as palaeopedogenic carbonate precipitation in alluvium on a semi arid floodplain. Preservation of the fossil bones at this site is unusually good. Microstructure of the bones shows external and internal precipitation of calcite and quartz in the vessel cavities and pores in the bone matrix indicating that they were initially filled by calcite, and than partially replaced by quartz. These features combined with the presence of calcareous rhizoliths and carbonate nodules, suggest that the bones were accumulated on a playa lake margin with the position of the shoreline being controlled by the groundwater table movements between dry and wet seasons. The taphocoenosis shows a predominance of disarticulated and disassociated skulls and mandibles. Commonly large and very small bones occur side-by-side with no preferred orientation. This taphonomic style suggests a biogenic agent of concentration of the bones by selective predation.