Paleobiological Perspectives on Mesonychia, Archaeoceti, and the Origin of Whales

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Organisms living today are grouped together taxonomically because they are similar to each other and different from others. How similar organisms are within a group and how different the group is from other groups depends on the broader context of similarities and differences uniting and distinguishing groups. The rank to which a group is assigned depends in part on similarities and differences, but also on what we know about evolutionary history. Extant whales (order Cetacea) have long been known to be mammals because they share with other mammals such basic distinguishing characteristics as endothermy, lactation, large brains, and a high level of activity. Living cetaceans share, in addition, a suite of special characteristics related to life in water that distinguish them from land mammals: These include large body size, a reduced and simplified dentition, an audition-dominated sensory and communication system, a hydrodynamically streamlined body form with a muscular propulsive tail, and of course many ancillary anatomical, behavioral, and physiological differences.