Vertebral osteology and swimming style in living and fossil whales (Order: Cetacea)

Abstract

Morphological modification of the cetacean vertebral column for axial locomotion was one of many changes that occurred during and after the invasion of aquatic habitats by the ancestors of living whales. These modifications resulted in a column that can be divided into neck, chest, torso, and fluke structural and functional units. Variations in these units allow subdivision of living whales into three morphological groups that reflect differences in regional flexibility of the column and of locomotor style. Both odontocete and mysticete cetaceans exhibit trends toward reduction of relative centrum length in the neck and torso, restricting the flexibility of the column with increasing adaptation to aquatic lifestyles. Mysticetes and physeterid odontocetes retain the archaeocete pattern of constant vertebral shape along the torso, which acts as an undulatory unit. The location of secondary sites of reduced centrum length in non-physeterid odontocetes subdivides them into subgroups, one with reduced posterior, and the other with reduced anterior, torso flexibility. The relationships between particular osteological patterns and swimming styles in living taxa are used to support predictions of swimming style in fossil species after adoption of a fluke in the late Eocene.