Whales are artiodactyls that traded legs for flukes
Cetaceans—whales, dolphins, porpoises—are secondarily aquatic mammals nested among even-toed ungulates. Roughly ninety living species split into about seventy-five toothed odontocetes and the filter-feeding baleen whales. They must surface to breathe, swim with horizontal flukes, and hunt only meat—some with echolocation.
Hind limbs reduced to pelvic vestiges; bodies streamlined for nektonic life mostly in open seas, though some enter rivers. Industrial whaling chased blubber and oil until the International Whaling Commission moved to halt commercial hunts, yet hunting continues under exceptions and disputes.
Echolocating dolphins and deep-diving sperm whales show how far a land ancestor can remake itself without quitting air.
From Basilosaurus fossils to modern pods, the clade keeps proving that returning to the ocean never meant becoming fish.
The name traces through Latin cetus to the Greek kētos, a huge fish or sea monster. Baleen and toothed whales are thought to have split around thirty-four million years ago, and about ninety living species are recognised, 75 of them toothed. Baleen whales strain food through keratin bristles; gray whales scoop molluscs from the seabed, and rorquals balloon their throat pleats to engulf small prey. Toothed whales, from sperm whales to porpoises, swallow fish and squid whole and find them by echolocation, using a fatty organ in the forehead called the melon. The blowhole sits atop the head, single in toothed whales and paired in baleen whales, so the animal can breathe while almost fully submerged. Spines hold anywhere from 40 to 93 vertebrae, the seven neck bones are shortened or fused for stability, the flippers carry extra finger bones, and the tail fluke lies flat and beats up and down, unlike a fish's upright tail.
Source: Cetacean
