The evolution and biological role of the neomorphic ossification of the nasal cartilage in Neosclerocalyptini glyptodonts

Cranial neomorphic ossifications are extremely rare and their biological roles are largely unexplored. In mammals, the few known examples concern almost exclusively the anterior part of the snout, in both extant and extinct taxa. Among these atypical anatomies, Neosclerocalyptini glyptodonts have one of the most extraordinary neomorphicossifications of nasal cartilage. Previous work hypothesized that this ossification corresponds to an optimization of the hydric and thermal regulation from the nasal cavity, coinciding with the global cooling of the climate from the Pliocene onwards. Until now, only the endocranial anatomy of their youngest representative has been explored, preventing an evolutionary appraisal of this climato-anatomical hypothesis. Using X-ray microtomography and an exceptionally well-preserved specimen representing the oldest Pleistocene species, we reveal that in Neosclerocalyptus pseudornatus this neomorphic ossification restricts the narial opening and is not open to the nasal cavity, unlike the youngest species, weakening the hypothesis of an early physiological function. We show also that paranasal sinuses, ethmoturbinates, and septoturbinates appear to be similar within the genus. We argue that the most likely functional origin corresponds to structural support associated with the overall cranial organization and expansion of the cephalic shield. In younger species, the enhancement of hydric and thermal regulation capacities would then be an exaptation.