Diagenetic pattern of vertebrate fossils from the Traversodontidae Biozone, Santa Maria Formation (Triassic), southern Brazil

Two outcrops of the Traversodontidae Biozone, located in the municipal districts of Santa Cruz do Sul and Venâncio Aires, were analyzed for their diagenetic patterns. Both outcrops share a very similar faunal content, with a predominance of non-mammalian cynodonts, and also a common biostratinomic pattern. Macroscopically, the bones are characterized by the presence of a thin, dark coloured layer composed by manganese oxides, attached to the external surface and for the absence of volumetric distortions caused by diagenesis, which often occur in the fossils of the Santa Maria Formation. On the microscopic level, a poor osteological preservation is found. X-ray diffractometry and scanning electron microscopy analyses revealed a high percentage of quartz in the samples, unlike the fossil bones found in other locations and stratigraphic levels within the Santa Maria Formation, where only minimal concentrations of silica are found. The amount of quartz filling the bones reflects different paleoenvironmental conditions and is here also correlated to the volumetric changes in the bones at the different biozones within the Santa Maria Formation. In addition to the diagenetic patterns shared by the Santa Cruz do Sul and Venâncio Aires outcrops, their similar biostratinomy and faunas corroborate the hypothesis that both represent a distinct biozone within the Santa Maria Formation.