Unravelling eutherian and metatherian divergence through dental evolution

The emergence of therian mammals stands as one of the most significant transitions in vertebrate evolution, marked by profound evolutionary innovations, notably the development of tribosphenic molar structures in the conserved dental formulae of postcanines in eutherians (placentals and kin) and metatherians (marsupials and kin). However, elucidating evolutionary paths of these dental transformations has posed enduring challenges. Here, we report detailed morphology of the early eutherian Acristatherium and reconstruct ancestral dental formulae at key nodes of Theria, Eutheria and Metatheria. Our results reveal a distinction among three dental formulae at key nodes of Eutheria, Metatheria and Theria. Contrary to the prevalent hypothesis of serial homology in postcanines, we refine the premolar-molar independence model, initially proposed by Dashzeveg and Kielan-Jaworowska, and emphasize independent development in premolar and molar classes to interpret dental transformations in early therians, based on a synthesis of palaeontological and developmental criteria. It reveals that eutherians lost the distal molar, while metatherians underwent premolar loss when diverging from the therian common ancestor characterized by five premolars and four molars. The pattern of dental transformations suggests distinct evolutionary experimentation over deep time and conserved genetic and developmental controlling mechanisms, ultimately shaping phenotypic diversity.