Figure 1. A simplified phylogeny of the Cetacea. Living Cetacea – the Odontoceti and Mysticeti – evolved from Archaeoceti (Eocene) at about the end of Eocene times. In odontocetes (e.g. Tursiops, bottlenosed dolphin), the tooth-bearing maxilla (stippled) in the skull shows features linked to echolocation. Most fossil mysticetes and all living mysticetes lack teeth, and the maxilla carries baleen plates used in filter-feeding (e.g. Balaena, right whale). Archaeocetes (e.g. Zygorhiza) show no evidence of structures that might have functioned in echolocation or filter-feeding. (Modified from an illustration by Fordyce, 1989.)

Figure 2. The evolutionary history of Cetacea. Known ranges for families of living and extinct Cetacea, plotted against geological time. Inferred relationships between families are indicated by fine lines.