Cetaceans are not closely related to other aquatic vertebrates and represent a unique lineage derived from hoofed land mammals that returned to an aquatic lifestyle about 50 million years ago (Thewissen, 1998). The transition from land to water, i.e. the development from weight-bearing forelimb with five hoofed toes to tissue-encased flipper not capable to support locomotion on land, is well documented by fossils (Thewissen et al., 2001, 2009) and very different from other aquatic vertebrates. Consequently many morphological characteristics that cannot be detected in fossils have remained little documented or speculative.
Continue reading