Return to the sea. The life and evolutionary. Times of marine mammals / Возвращение в море. Жизнь и эволюция. Времена морских млекопитающих

Mammals, like nearly all other tetrapods (or four- legged animals), evolved on land. Marine mammals are a diverse assemblage of at least seven distinct evolutionary lineages of mammals that independently returned to the sea and include whales, dolphins, and porpoises (Cetartiodactylans); seals, sea lions, and walruses (Pinnipedia); sea cows (Sirenia); extinct sea cow relatives (Desmostylians); polar bears; sea and marine otters; and extinct aquatic sloths. The secondary adaptation of mammals to life in water required various morphological specializations, including for some lineages dramatic changes in body size and shape compared to their terrestrial relatives. Marine mammals are relatively large, with streamlined bodies and reduced appendages (for example, small or no external ears) and thick fur or fat layers for insulation. Other modifi cations for swimming and diving include the transformation of limbs into fl ippers and/or use of the tail for propulsion in water. <...>