Naming an Innominate: Pelvis and Hindlimbs of Miocene Whales Give an Insight into Evolution and Homology of Cetacean Pelvic Girdle

Abstract

Cetaceans have highly transformed pelvic and hindlimb bones as a secondary adaptation to an aquatic lifestyle. Paleontological records of these bones are very scarce, which hampers interpretations of their homology and evolution. Newly found innominates, femora and tibia of Miocene baleen whales of family Cetotheriidae have primitive structure, which makes it possible to compare them with archaeocetes. As a result, a traditional view of a cetacean innominate as containing vestiges of all three pelvic bones and acetabulum is corroborated by new data and interpretations. A part of ischium (possibly, the superior ramus) is reduced in modern whales, but its vestige can sometimes be observed. A developmental mechanism underlying this pattern can hypothetically involve alterations in Pbx1 and Pbx2, Prrx1 and Prrx2, Pit1 or BMP7 expression: the observed anatomy is similar to reported cases of BMP7 mutations, which are combined with previously reported alterations in Shh regulation. Thus, pelvic and hindlimb reduction in modern cetaceans can in some aspects be compared to ‘Mermaid Syndrome’.