Whales hold a place of honor among paleontologists as a lineage whose ancestry and evolution once had seemed forever mysterious, but is now well understood. Comparative anatomy, fossils and genetics have demonstrated that these masters of the sea are closely related to even-toed hoofed mammals, the artiodactyls like pigs, camels, deer and cattle. In fact, not only are they related, whales are part of the artiodactyl lineage and are the closest living relatives of hippos.
They rapidly emerged, with the earliest so-called walking whales living along the north shore of the Indian Ocean, just 15 million years after the extinction of the non-avian dinosaurs and marine reptiles (among many other animals). Fully aquatic whales emerged after another five million years and the two modern lineages diverged just ten million years later; their earliest fossils coming from 36-million-year-old rocks. The early archaic whales, archaeocetes like Basilosaurus, resembled extinct mosasaurs and living orcas and hunted in similar ways.
Of the two living groups, one of them, the toothed whales, or odontocetes, retains teeth. This group includes beaked whales, sperm whales, dolphins, and their kin; essentially any living whale with teeth as adults. The other living lineage, the baleen whales, or mysticetes, don't have teeth as adults but instead have a stiff, bristly tissue known as baleen growing from the roof of the mouth and used for filtering food from the water.
Baleen is one of evolution’s unique answers to the question, “How can I get the highest number of little animals out of a lot of water with minimal effort?”
