Convergence or parallelism in the evolution of cetaceans

The Order Cetacea consists of three suborders: the Mysticeti and Odontoceti (living) and the Archaeoceti (extinct). The similarity between the living suborders is the result of convergence, and the Archaeoceti are not ancestral to the other two. Comparison of the principal characters distinguishing the Order Cetacea shows that they are characteristic also of the Sirenia and to some extent of the Pinnipedia. Common structural features of the three cetacean suborders, such as body form, position of the nostril at the apex of the head, lack of vocal cords, and similar shape of lungs, differ in detail, suggesting convergence rather than parallelism., The two living suborders differ in the structure of the cutaneous integument, biochemical characteristics of the skin fat, structure of the forelimbs, and histology of the spongy structure of the bones. Other characters distinguishing the Odontoceti from the Mysticeti are listed. The possibility is suggested that the present suborders of Cetacea should be promoted to the rank of orders. — F. C. Whitmore, Jr.