Paleontological and developmental evidence resolve the homology and dual embryonic origin of a mammalian skull bone, the interparietal.

The homologies of mammalian skull elements are now fairly well established, except for the controversial interparietal bone. A previous experimental study reported an intriguing mixed origin of the interparietal: the medial portion being derived from the neural crest cells, whereas the lateral portion from the mesoderm. The evolutionary history of such mixed origin remains unresolved, and contradictory reports on the presence or absence and developmental patterns of the interparietal among mammals have complicated the question of its homology. Here we provide an alternative perspective on the evolutionary identity of the interparietal, based on a comprehensive study across more than 300 extinct and extant taxa, integrating embryological and paleontological data. Although the interparietal has been regarded as being lost in various lineages, our investigation on embryos demonstrates its presence in all extant mammalian "orders." The generally accepted paradigm has regarded the interparietal as consisting of two elements that are homologized to the postparietals of basal amniotes. The tabular bones have been postulated as being lost during the rise of modern mammals. However, our results demonstrate that the interparietal consists not of two but of four elements. We propose that the tabulars of basal amniotes are conserved as the lateral interparietal elements, which quickly fuse to the medial elements at the embryonic stage, and that the postparietals are homologous to the medial elements. Hence, the dual developmental origin of the mammalian interparietal can be explained as the evolutionary consequence of the fusion between the crest-derived "postparietals" and the mesoderm-derived "tabulars."