The skull base in Cingulata (Xenarthra, Mammalia): early ossification, homologies, and comparisons across mammals

Abstract

The mammalian skull base ossifies from chondral elements into successive bones described as the basicranial stem or axis. The axis includes the basioccipital, basisphenoid, presphenoid, and mesethmoid bones. Accurately tracing the ontogenetic origin of this area is essential, as endochondral development plays a crucial role in understanding the homology of these axial bones. This area contributes to building the ethmo-sphenoidal area and contains phylogenetic signal. In our study, we examined the ontogeny of the neurocranial base and allied bones (i.e., alisphenoid and orbitosphenoid) in representatives of two living armadillo clades: Dasypus hybridus (n = 18) (Dasypodidae), Chaetophractus vellerosus (n = 9) and Chaetophractus villosus (n = 11) (Chlamyphoridae). The presphenoid is not ossified in D. hybridus, C. villosus, or C. vellerosus; moreover, there is no clear evidence that this bone forms a separate ossification in other cingulates. In addition, there is evidence for the existence of postnatal mesethmoid ossification in armadillos. There are different ontogenetic trajectories in armadillos, precocial for Dasypus and altricial for Chaetophractus. The presence of the mesethmoid is likely a plesiomorphic condition for Placentalia.