Diphyodont tooth replacement of Brasilodon-A Late Triassic eucynodont that challenges the time of origin of mammals.

ABSTRACT

Two sets of teeth (diphyodonty) characterise extant mammals but not reptiles, as they generate many replacement sets (polyphyodonty). The transition in long-extinct species from many sets to only two has to date only been reported in Jurassic eucynodonts. Specimens of the Late Triassic brasilodontid eucynodont Brasilodon have provided anatomical and histological data from three lower jaws of different growth stages. These reveal ordered and timed replacement of deciduous by adult teeth. Therefore, this diphyodont dentition, as contemporary of the oldest known dinosaurs, shows that Brasilodon falls within a range of wide variations of typically mammalian, diphyodont dental patterns. Importantly, these three lower jaws represent distinct ontogenetic stages that reveal classic features for timed control of replacement, by the generation of only one replacement set of teeth. This data shows that the primary premolars reveal a temporal replacement pattern, importantly from directly below each tooth, by controlled regulation of tooth resorption and regeneration. The complexity of the adult prismatic enamel structure with a conspicuous intra-structural Schmelzmuster array suggests that, as in the case of extant mammals, this extinct species would have probably sustained higher metabolic rates than reptiles. Furthermore, in modern mammals, diphyodonty and prismatic enamel are inextricably linked, anatomically and physiologically, to a set of other traits including placentation, endothermy, fur, lactation and even parental care. Our analysis of the osteodental anatomy of Brasilodon pushes back the origin of diphyodonty and consequently, its related biological traits to the Norian (225.42 ± 0.37 myr), and around 25 myr after the End-Permian mass extinction event.