Intervertebral and Epiphyseal Fusion in the Postnatal Ontogeny of Cetaceans and Terrestrial Mammals

In this paper we studied three related aspects of the ontogeny of the vertebral centrum of cetaceans and terrestrial mammals in an evolutionary context. We determined patterns of ontogenetic fusion of the vertebral epiphyses in bowhead whale ( Balaena mysticetus) and beluga whale ( Delphinapterus leucas) , comparing those to terrestrial mammals and Eocene cetaceans. We found that epiphyseal fusion is initiated in the neck and the sacral region of terrestrial mammals, while in recent aquatic mammals epiphyseal fusion is initiated in the neck and caudal regions, suggesting locomotor pattern and environment affect fusion pattern. We also studied bony fusion of the sacrum and evaluated criteria used to homologize cetacean vertebrae with the fused sacrum of terrestrial mammals. We found that the initial ossification of the vertebral pedicles in the fetus may be a reliable indicator of sacral homology in modern cetaceans. Finally, we also studied fusion of the centra of cervical vertebrae in B. mysticetus and found that it is not completed until after sexual maturity, and after 20 years of age.