Hippopotamoidea is a superfamily of cetartiodactyls that are nowadays limited to two extant species: Hippopotamus amphibius, the common hippopotamus, and Choeropsis liberiensis, the Liberian hippopotamus. These two mammals are endemic to Africa and inhabit ecosystems closely linked to water. They are the only extant members of a specialized ecological guild called the large semi-aquatic herbivores. The majority of the diversity of this superfamily was composed by the paraphyletic anthracotheres, a geographically, temporally, and ecologically more diverse group from which hippopotamids likely originated. Historically, the phylogenetical relationships of these taxa were debated, especially since the establishment of the clade Cetancodonta which comprises cetaceans and hippopotamids. Research in this area has sought to address these issues using cranial, intracranial morphology, and dental anatomy. Their postcranial anatomy has until now been mostly unexploited data, sometimes due to poor preservation. At Toros-Menalla (TM), a Late Miocene fossiliferous area in Chad, the last African anthracothere Libycosaurus bahri has been found coexisting with the large hippopotamid Hexaprotodon garyam. Their coexistence in humid environments suggests some form of niche-partitioning. The locomotor apparatus is a significant means by which animals interact with their environment. It is a valuable resource for clarifying the phylogenetic issues previously cited, as well as for discussing functional and ecological considerations mentioned in previous literature. This study proposes an anatomical comparison between these two coexisting hippopotamoids and their closest extant ecomorph, the common hippopotamus. We have established a framework for the identification and differentiation of the postcranial skeleton of hippopotamoids by observing characters on a sample of approximately 650 specimens. We also highlight the importance of including the postcranial skeleton in future phylogenetical analyses. Additionally, we discuss the postcranial anatomy of those taxa in the context of the African Miocene environments, allowing new functional and ecological interpretations for the interaction between hippopotamoids and changes in their wet environments, which remain a major driver in the evolutionary history of these large artiodactyls.
Scribano et al. (Thu,) studied this question.