Sirenian pachyostosis revisited

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.