The skull of the foetal narwhal, Monodon monoceros L

Abstract

Understanding of the evolution of the mammalian orders and families, and of the relationships between members of an order, is often illuminated by a study of the development of the skull in the foetus. This account is the first description of the foetal narwhal. The material is extremely rare and the author is indebted to Professor L. R. Wager for the collection of the specimens when he was in Greenland as a member of the Courtauld Expedition in 1935-6. The chondrocranium and osteocranium of two early narwhal foetuses were investigated by dissection and by the preparation of models constructed from transverse sections of the head. The narwhal is a highly specialized Cetacean. In the adult it differs from all other mammals in the complete absence of hairs, and from other Odontoceti in that instead of numerous similar teeth on both jaws, it has one very long straight tusk in the upper jaw in the male, and a pair of short tusks or none in the female, neither sex having teeth in the lower jaw. Sections of the younger foetus, however, exhibit hairs on each side of the mandible, indicating that the narwhal is a typical mammal in this respect. Continuous dental laminae in upper and lower jaws, with papillae for six pairs of teeth in the maxillae, and for two pairs in the mandibles, show that the specialized dentition of the adult is a modification of the more generalized foetal dentition. The study leads to the conclusion that the narwhal is more closely related to the Delphinidae than to the Phocaenidae, and that it approaches the Delphinidae through Globiocephala .