Evolution and Development of the Interparietal in Dolphins and Other Toothed Whales (Odontocetes).

Mammal skulls of diverse shapes and sizes exhibit a skull roof composed of the same bones with a similar topological arrangement: bilateral frontal and parietal bones meet at the midline, and in many cases, an interparietal bone lies posterior to the parietals. Despite this conserved skull roof arrangement, the interparietal has been a puzzling bone in comparative embryology and evolutionary morphology due to its mixed developmental origins and its variable patterns of fusion with neighboring bones, resulting in mixed reports of presence and absence in different taxa. The skull roof of dolphins, in contrast to other mammals, features a prominent interparietal that separates the parietal bones so that the parietals do not meet at the midline. While this unusual configuration has been described in individual specimens, the evolutionary developmental patterns that led to the dolphin arrangement are largely unknown. To understand how this unique arrangement arose, we surveyed presence, absence, morphology, position, and developmental origins among early odontocete fossils and extant odontocete fetuses. Among extant odontocetes, delphinoids all have multi-element interparietal bones: one comprising the traditional "interparietal" between the paired parietal bones, and two that fuse posteriorly to the supraoccipital. However, extant physeteroids lack an identifiable interparietal, possibly resulting from the fusion of interparietal elements to the supraoccipital during early development. Thus, in physeteroids, the parietals meet at the midline as in other mammals. Olympicetus, an Oligocene stem odontocete, exhibits several well-preserved multi-element interparietals, including one underneath and at the midline of its paired parietal bones. However, other Oligocene stem odontocetes from South Carolina revealed no identifiable interparietal at multiple growth stages, which may have resulted from fusion of interparietal elements to other skull bones. These new data suggest that fusion patterns of the multiple elements of the interparietal are highly variable among odontocetes and support the hypothesis that the presence of a large interparietal separating the parietals is a uniquely delphinoid feature.