Bone histology of Smilodon populator and Puma concolor from the Pleistocene of equatorial Brazil with comments on fossil diagenesis

Climate change over the last 50,000 years has significantly impacted large mammals. Large felids have been one of the groups most affected by these fluctuations. Palaeohistology is a tool for understanding the palaeobiological aspects of extinct species. Taphonomic analysis, in turn, allows us to understand the formation of fossil assemblages and palaeoclimatic characteristics. In this study, we sampled the femoral histology of the largest saber-toothed felid in the Americas, Smilodon populator (MCC-868V), and a fossil specimen of Puma concolor (MCC-991V), both from Quaternary deposits in Rio Grande do Norte, Northeast Brazil. The presence of EFS (External Fundamental System), generations of secondary osteons, and remodeled bone tissue indicate that both individuals had reached sexual maturity. However, MCC-868V was a young adult at the time of death, and MCC-991V was a senescent adult. Estimated mass for the S. populator specimen (157 kg) is lower than expected, even though the individual analyzed was a fully grown adult. There are no histopathological indicators that this S. populator individual underwent restrictions throughout its growth, which suggests a truly smaller individual. Smaller size may be a trait selected in a local population in Northeastern Equatorial Brazil due to the specific environmental and ecological conditions of the region. The morphometric analysis found that size did not correlate with bone thickness both among several felid taxa and within a Smilodon population. As no significant histological differences were found between P. concolor and S. populator. Fractures filled with iron oxide were found in both specimens. There is permineralization in S. populator and micritization in P. concolor.