Bone microstructures and fossilization in Pleistocene vertebrates from Rio Madeira Formation, Southwestern Amazon, Brazil

Abstract

Bone microstructures and their relationships with the fossilization were studied in skeletal elements of Megatheriidae and Toxodontidae individuals (Pleistocene megafauna of Araras, Rondonia) collected from the 1970s to the 1990s in fluvial deposits from Rio Madeira Formation. The use of microscopy techniques (conventional optics with polarized light and scanning electron) and chemical determination (energy dispersive spectrometry) revealed the fossilization process with predominance of carbon acting in the studied specimens. It is the first record for Pleistocene vertebrates fossils from fluvial deposits of Rio Madeira Formation. This process is characterized by the presence of the black amorphous matrix in bone microstructures. Initially, the process of fossilization associated with carbon acts over microstructures of compact bone portions and, then, degrading the spongy bones. This performance generated different stages of microstructural degradation, being possible in some cases detailed histological identifications and, in others, of total degradation, the dissemination of only the black amorphous matrix. The black amorphous matrix has carbon as the predominant chemical constituent with contents up to 90.19%. In addition, diagenetic events generated microcracks in the black amorphous matrix of the Havers and Volkmann canals (post-fossilization), as well in osteons (post-burial) and radial vascular canals (postmortem).