CONICET | Buscador de Institutos y Recursos Humanos

Los Colorados Formation is a worldwide recognized continental unit deposited in the Ischigualasto-Villa Unión basin in San Juan and La Rioja Provinces, Northwestern Argentina. It is mainly composed of lenticular sandstone beds and laterally extensive laminated to massive mudstone units, recording sedimentation in different fluvial settings. These rocks represent a case study for the abundance and diversification events that affected Upper Triassic tetrapods. Although the tetrapod fauna coming from this unit is relatively well-known, their taphonomic modes are scarcely known. In this sense, we present a preliminary petrographic analysis of both epiphyses and diaphysis of bone remains collected in these deposits, and X-ray diffraction, which allowed us to interpret the damaging process that acted on the bone tissue. The most destructive taphonomical process affecting the bones is caused by chemical action of dissolved material acting within fossil-diagenesis. Thin section of the epiphysis shows preserved trabecular bone with cracks perpendicular to the fibers, obtured cavities filled with calcite set in two cycles: microgranular on the periphery and polycristaline euhedral on the centre. Sub-periosteal bone, however, is relatively well-preserved. Diaphyses present a highly altered trabecular bone, obtured cavities with calcite in three cycles: two microgranular peripherical rows and large euhedral crystal in the internal lumen. The sub-periosteal bone is mostly altered. X-ray diffraction shows predominant calcite in the epiphysis, whereas gypsum is mostly present in the diaphysis. Summing up, diagenetic cracks allow the income of circulating solutions (mostly calcite and gypsum) in the bone, expanding and destroying it.

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