Nicolas Ferdinand Wagner

Proboscideans exhibit limb morphologies specialised for supporting extreme body masses under predominantly compressive loading. These graviportal adaptations include repositioning of the limb bones closer to the trunk, resulting in a columnar stance. Among proboscideans, two humeral morphotypes have been proposed (slender and robust), yet their functional significance under compressive loading remains poorly understood. Finite element analysis (FEA) is applied to evaluate biomechanical performances of proboscidean humeri subject to axial compression, simulating conditions associated with a columnar stance and deviations from it. Our results reveal clear differences between morphotypes, with slender humeri performing worse than the robust morphotype and functioning optimally under near‐axial loading. Robust humeri consistently exhibit lower peak stress under both neutral and off‐axis loading and show considerable resistance to mediolateral stresses. Across all taxa, stress magnitudes generally increase when deviating from a neutral orientation, especially during posterior loading orientations, highlighting the mechanical importance of maintaining columnar limb posture. These findings support the hypothesis that robust morphologies are better adapted to resist multidirectional stresses, while slender morphologies favour energetic and locomotor efficiency. This study demonstrates the applicability of FEA in assessing compressive performance in postcranial elements and provides a novel framework for investigating biomechanical adaptations in graviportal taxa.

Pachyostosis, the increase in volume of bones, has been documented in a myriad of tetrapods that secondarily adapted to the aquatic environment. However, this convergent trait has hitherto only been quantified in complete ribs and long bones. Here we present a methodology designed to quantify the degree of pachyostosis in ribs and vertebrae, both for complete and fragmentary skeletal elements. Using surface models, we measured volumes of costal and vertebral regions and standardized them using size proxies to devise a pachyostosis index. Sirenians are used as a case study, given the disparity of pachyostosis degrees that have already been documented in the clade, especially in their ribs. The new methodology can detect this disparity and further demonstrates that pachyostosis also affects their vertebrae. Milder cases of pachyostosis, such as in the extant Dugong dugon, are likely secondary specializations to a specific lifestyle. The craniocaudal distribution of the pachyostosis index values is also potentially helpful to further distinguish finer lifestyle differences. This new method has the potential to clarify the causes of the disparity in pachyostosis degrees observed in the fossil record of sirenians, and it may also prove valuable in broader, non-mammalian contexts.