Sperm Whales, Evolution

Abstract

Sperm whales include the cetacean families Physeteridae and Kogiidae. Modern sperm whales are represented by two genera only, but sperm whales were much more diverse in past times. Phylogenetically, sperm whales are usually thought to be close to the root of the odontocetes, and they retain many characters that are primitive for odontocetes. Physeterids and kogiids may be subsequent branches of the basal odontocete phylogenetic tree, or both may be derived from a common sperm whale ancestor, such as Ferecetotherium. Physeterids are highly specialized for teutophagy at great depths, and many parts of sperm whale anatomy show adaptations for this behavior. Macroevolutionary changes also occurred in the mandible. In modern sperm whales, the maxilla (upper jaw) is much wider than the mandible as a result of the widening of the rostrum and a narrowing of the mandible. These give the lower jaw a peculiar undersized image. This mismatch evolved gradually in physeterids. In physeterid evolution, the length of mandibular symphysis increased from one-twelfth of the length of the mandible in the Oligocene to one-fourth in the Miocene and one-half in Physeter. The mandible of Ferecetotherium has 27 teeth, whereas that of Diaphorocetus has 14. The forelimb of physeterids is very different from that of Eocene cetaceans but similar to that of other odontocetes. The humerus is shortened and the elbow is immobile. One genus with two modern species constitutes the Kogiidae. Kogia is similar to Physeter but the body of Kogia is much smaller and the head is smaller. The spermaceti organ is smaller than in Physeter, the blowhole more posterior, and the rostrum shorter. Kogiids are poorly represented in the fossil record, and most specimens are incomplete, although some localities have yielded good material. The genus Kogiopsis is known from a single mandible, and fragmentary skulls are known for Miocene Scaphokogia and Pliocene Praekogia.