Endocranial cast anatomy of the Early Miocene glyptodont Propalaehoplophorus australis (Mammalia, Xenarthra, Cingulata) and its evolutionary implications

Abstract

Propalaehoplophorus is an Early Miocene genus of glyptodonts, a group of extinct armored mammals closely related to armadillos and endemic to South America. Here, we present the first digital reconstruction of the endocranial cavity of the glyptodont Propalaehoplophorus australis and compare it to endocasts of Late Miocene and Pleistocene glyptodonts, pampatheres, and extant armadillos. Propalaehoplophorus australis shares exclusively with other glyptodonts the neocortical sulcation pattern and cranial nerve (CN) V3 pathway. It also shares with both other glyptodonts and pampatheres the rhinal fissure trajectory, small piriform lobe, marked dorsal expansion of neocortical fronto-parietal region, conspicuous thickness of superior longitudinal sinus, and presence of a well-marked lateral sulcus and medial shape of petrosal bone; this last trait is also observable in Chlamyphorus. The olfactory bulbs of Pr. australis, Holmesina, and Pampatherium are anteriorly elongated and partially laterally divergent as in the glyptodont Pseudoplohophorus absolutus. Other features, like the globular proximal shape of olfactory peduncles, topological arrangement of CNs IX-XII, differentiated petrosal lobule of paraflocculus, and orientation of spinal cord are shared among Pr. australis, Ps. absolutus, pampatheres, and extant armadillos. The similarities between Pr. australis, remaining glyptodonts, and pampatheres could be synapomorphies of pampatheres + glyptodonts. By contrast, Pr. australis, pampatheres, and all the analyzed armadillos share the same configuration of the pathway of CNs IX-XII, a feature that could support the basal position of Pr. australis among glyptodonts for which cranial remains are known. In this context, the brain cavity seems to be a promising source of information for revealing the evolutionary history of this mammalian clade.