The Challenge of Whales for Evolution
Nearly all of today’s mammals are tetrapods, having both a front pair and a hind pair of limbs. They generally have a mouth full of hard teeth, and have nostrils near the front of their snout. Fossil specimens of extinct mammals also have these features.
Whales are clearly mammals: they nourish their young in utero through a placenta, give birth to live young, feed newborns with milk, and possess a neocortex and three middle ear bones. However, whales are animals that are thoroughly adapted to living in the water. Modern whales and dolphins typically display no hind legs, their nostrils are located near the top of their heads, and they have various features in their head and jaws that facilitate hearing underwater. Also, one major class of whales lacks regular teeth: the whales of order Mysticeti use baleen plates to filter out food from the water.
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