Occurrence of Gastroliths in Baurusuchus (Baurusuchidae, Mesoeucrocodylia) from Adamantina Formation, Bauru Basin

Abstract

AbstractThe fossil record of Baurusuchidae Mesoeucrocodylia in the General Salgado county and its surroundings is one of the best regarding preservation, completeness and articulation of skull and skeleton remains, including specimens that shown even the most delicate bone structures and cartilaginous tissues. One of them, UFRJ DG 288-R, has preserved its skull and about 80% of its skeleton. During the removal of the rock matrix, on what should be the specimen’s abdominal region, together with fragments of gastralia were observed some small well-polished weathered angulous to subrounded clasts, presumed to be gastroliths. These xenoliths occur in a restrict cluster, where at least four peeble-sized stones are visible in the surface of the specimen. The texture and fabrics of these rock fragments differs from the surrounding matrix, presenting darker purplish tones. In thin section, one of the fragments revealed isotropic texture, with opaque minerals in abundance, of euhedric to subhedric habits. The mineralogical composition presents mainly clay minerals as weathering products; biotite and chlorite are common, being the last the result of hidratation of the first. Although highly altered, the low quartz content, small-sized and well-formed crystals suggests maphic composition in volcanic context, possibly representing a basalt fragment. In extant Crocodyliformes, the occurrence of gastroliths is commonly associated with food processing in the stomach, diving ballast, hunger stress and/or supplementary mineral ingestion. As baurusuchids are characterized as medium to large size fully terrestrial predator/scavengers, based on skeletal data, the ballast function of gasthroliths is excluded in this case. As mentioned above, extant crocodyliforms such as Caiman and Crocodylus ingest stones when under stressful conditions of lack of food, water or when in high population density. Osteoderms are one of the primary sources for calcium and a reservoir for the homeostasis. The hunger stress can be observed in the osteoderms histological cuts as an abnormal concentration of osteoclasts at its inner region overwhelming the presence of osteoblasts and consuming the osteocytes. The preservation of osteoderms associated with gasthroliths may reveal if the stone ingestion in baurusuchids is a normal or driven by hunger behavior. The paleoenvironmental conditions dominating the Adamantina Formation during the Late Cretaceous are considered mainly arid, marked with strong seasonality, alternating long droughts and short rainy periods, associated with flashflood events. The stressful condition created during the dry season would cause famine and mass mortality, thus forcing animals to endure or escape. The baurusuchids underwent the dry seasons through behavorial responses of self burial and probably stone ingestion, although natural bahavior cannot be eliminated as a cause. Financial support provided by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant no 305780/2006–9), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Instituto Virtual de Paleontologia/ Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (IVP/FAPERJ, grant no E–26/152.541/2006).