Amongst Vertebrata Mammalia is an extensively adapted and diverse class. Hair and mammary glands are two defining features of mammals that set them apart from other vertebrates. At the end of the Cretaceous period, when dinosaurs went extinct, mammals were given the opportunity to rapidly evolve and become very prosperous. Within Mammalia one order in particular, Cetacea, shows widespread adaptation and diversification. Cetacea includes all whales, dolphins, and porpoises. These once terrestrial species have made adaptations that allow for their current lifestyle. They have become highly modified for living in aquatic niches.
Several key adaptations are conducive to Cetacea. Specifically, the forelimbs have modified into flippers, the pectoral girdle is reduced and externally absent, flukes and dorsal fins adapted from connective tissue, and they are streamlined and well insulated. Approximately fifty million years ago this taxonomically diverse clade arose from artiodactyl ancestors. When ancestors of cetacean phylogeny switched to piscivory it caused rapid change (Thewissen et al. 2007). Low competition and plenty of resources are two crucial factors of adaptive radiation, both of which were met at the time Cetacea evolved. According to Steeman et al. toothed and baleen whales are excellent examples of adaptive radiation. Filter-feeding systems and toothed whales spread rapidly because it allowed for the transition into unoccupied ecological niches. Baleen whales have adapted mandibular arches that allow for engulfment of krill.
Within Cetacea many species have evolved to specific aquatic niches, including both marine and freshwater. River dolphins are another example of adaptive radiation amongst Cetacea. River dolphins differ from marine dolphins because they are morphologically and phylogenetically distinct
