ABSTRACT
Cetacea includes 41 genera and more than 80 species (Table 3.1), representing 4% of the mammalian (placental) diversity. All Cetacea are adapted to a permanent aquatic life. From an evolutionary point of view, this means that they exhibit morphological traits that have evolved since their common ancestor that was a terrestrial placental mammal. Specialization to life in water has led to major modifications in all body parts and functions, such as loss of external hind limbs, a hydrodynamic torpedo-like body shape, disappearance of the external ear, the quasi disappearance of hair, and great modifications of the skull and head. Cetacea is thus a highly derived group, meaning that their members possess many synapomorphies (shared derived characters) that evolved after separation from their most recent common ancestor. Consequently they have probably lost most of the characters that allow us to trace their relationships with other mammalian orders. The difficulty experienced by morphologists in attempting to identify the cetacean sister taxa was well expressed by Simpson (1945, p. 213): “Their place in the sequence of cohorts and orders is open to question and is indeed quite impossible to determine in any purely objective way. There is no proper place for them in a scala naturae or in the necessarily one-dimensional sequence of a written classification.”
