Bone histology of the ribs of the archaeocetes (Mammalia: Cetacea)

Histological study of the ribs in two archaeocetes, Basilosaurus cetoides and Zygorhiza kochii, reveals conspicuous pachyostosis, with hyperostosis of periosteal cortices and absence of a free medullary cavity. In addition, abundant remains of calcified cartilage matrix, associated with extensive globular ossification, exist within the medulla. Among Cetacea, these features seem to be unique to the archaeocetes. Conversely, the structural peculiarities of bone in these animals are identical to those of the Sirenia and would arise from similar causes: inhibition of the chondroclastic and endosteal osteoclastic resorptions, and prolongation of subperiosteal osteogenesis during ontogeny, exemplifying morphological adaptation to aquatic life via heterochrony. The possible hormonal mechanisms of these features, as also their supposed adaptive consequences for the buoyancy of the archaeocetes, are discussed in relation to data already available for the Sirenia.