Comparative anatomy and phylogenetic contribution of intracranial osseous canals and cavities in armadillos and glyptodonts (Xenarthra, Cingulata)

Abstract

The evolutionary history of the Cingulata, as for many groups, remains a highly de- bated topic to this day, particularly for one of their most emblematic representatives: the glyptodonts. There is no consensus among morphological and molecular phylog- enies regarding their position within Cingulata. As demonstrated by recent works, the study of the internal anatomy constitutes a promising path for enriching morpho- logical matrices for the phylogenetic study of armadillos. However, internal cranial anatomy remains understudied in the Cingulata. Here we explored and compared the anatomy of intracranial osseous canals and cavities in a diverse sample of ex- tant and extinct cingulates, including the earliest well-preserved glyptodont crania. The virtual 3D reconstruction (using X-ray microtomography) of selected canals, that is, the nasolacrimal canal, the palatine canal, the sphenopalatine canal, the canal for the frontal diploic vein, the transverse canal, the orbitotemporal canal, the canal for the capsuloparietal emissary vein and the posttemporal canal, and alveolar cavities related to cranial vascularization, innervation or tooth insertion allowed us to com- pare the locations, trajectories, and shape of these structures and to discuss their potential interest for cingulate systematics. We tentatively reconstructed evolution- ary scenarios for eight selected traits related to these structures in which glyptodonts often showed a close resemblance to pampatheres, to the genus Proeutatus, and/or to chlamyphorines. This latter pattern was partly congruent with recent molecular hypotheses, but more research is needed on these resemblances and on the potential effects of development and allometry on the observed variations. Overall, these com- parisons have enabled us to highlight new anatomical variation that may be of great interest to further explore the evolutionary history of cingulates and the origins of glyptodonts on a morphological basis.