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.

Evolution and development of the mammalian jaw joint: Making a novel structure

“…In brasilodontids and trieledontids, the dentary possessed a lateral ridge that contacted the squamosal but without forming a well-developed articulation (Luo, 2011). Recently, a late Triassic brasilodontid (Brasilodon quadrangularis) has been identified as having a diphyodont mammalian dentition (Cabreira et al, 2022), confirming that acquisition of the mammalian dentition predates the jaw joint. Crucially, in brasilodontids and trieledontids, a dentarysquamosal interaction appears independently in separate lineages, implying it evolved multiple times (Barghusen & Hopson, 1970).…”

On taxonomic issues, ontogenetic series and tooth replacement. Comments on Diphyodont tooth replacement of Brasilodon—A late Triassic eucynodont that challenges the time of origin of mammals by Cabreira et al.

“…Cabreira et al (2022) made the criticism to our research in their Introduction (section 1.1; p. 1425) and in section 4.6 Debate about the dental anatomy of the mammaliamorph Thrinaxodon in the Discussion (p. 1437).…”

“…Recently, Cabreira et al (2022) published a contribution analysing the dental replacement of Brasilodon , a cynodont from the Brazilian Norian, concluding that it had a mammalian (diphyodont) type of replacement. In this note, we will only deal tangentially with their main conclusion and instead will concentrate on criticism posed by Cabreira et al (2022) to the contributions of Abdala et al (2013) and Norton et al (2020).…”

“…Numbers in square brackets have been added to denote our responses to the different points raised by Cabreira et al (2022). Text in italics is a direct quote from the paper of Cabreira et al (2022).…”

“…Contrary to what is presented in that publication, the dental pattern presented by Thrinaxodon is in fact, typical of modern eutherians and diphyodont animals . [8] The exclusion or unavailability of very small and very large specimens of Thrinaxodon for their studies precludes information on neonatal and fully adult forms (Cabreira et al, 2022, p. 2).…”