New research into the evolution of cetacean dentition has revealed why, unlike other mammals, dolphins and whales do not have precise occlusion and distinct tooth classes.1

A comparison of mammalian dental patterns showing the difference in regionalisation of tooth morphology. (A) mouse, (B) pig (picture is of an immature pig with an unerupted M3) and (C) dolphin

Armfield et al. studied fossil records to trace when whales evolved their simple, peg-like teeth, discovering that 48 million years ago, before they had fully adapted to their new aquatic existence, whales still possessed the four usual mammalian tooth classes: incisors, canines, premolars and molars. By studying pig embryos (close relatives of whales) researchers could explore how teeth shaped during development, monitoring gene expression to understand how occlusal patterns formed. Bone morphogenetic protein (Bmp4) and fibroblast growth factor (Fgf8) were found to regulate signalling pathways that define whether a tooth will develop into an incisor or molar respectively. In dolphin embryos, however, researchers noted an overlapping presence of Bmp4, causing teeth in these new areas to form as ectopic, simple-crowned teeth.

As whales evolved from land mammals the developmental changes in dentition are most likely the result of a major shift in diet and food processing during the cetacean evolution, as cetaceans use their teeth to grab and hold prey, but not to chew their food. As precise occlusion was lost, genetic expression changed, resulting in the modern, simplified dentition that persists today.