English: Squalodon atlanticus (Leidy, 1856) - fossil shark-toothed porpoise tooth from the Miocene of Maryland, USA.

Mammals are the dominant group of terrestrial vertebrates on Earth today. The group is defined based on a combination of features: endothermic (= warm-blooded), air-breathing, body hair, mother's milk, four-chambered heart, large brain-to-body mass ratio, two teeth generations, differentiated dentition, and a single lower jawbone. Almost all modern mammals have live birth - exceptions are the duck-billed platypus and the echidna, both of which lay eggs.

Mammals first appear in the Triassic fossil record - they evolved from the therapsids (mammal-like reptiles). Mammals were mostly small and a minor component of terrestrial ecosystems during the Mesozoic. After the Cretaceous-Tertiary mass extinction at 65 million years ago, the mammals underwent a significant adaptive radiation - most modern mammal groups first appeared during this radiation in the early Cenozoic (Paleocene and Eocene).

Three groups of mammals exist in the Holocene - placentals, marsupials, and monotremes. Other groups, now extinct, were present during the Mesozoic.

Whales, dolphins, and porpoises are members of Order Cetacea. Cetaceans have intermediate- to very large-sized bodies that are streamlined (cigar-shaped) and have a thick blubber layer for heat insulation purposes. They are evolutionarily derived from terrestrial mammals that had four legs. The former front legs are now flippers. The hind legs are highly reduced and non-functional in whales. The skull is elongated, with one or two blowholes atop the head. The tail is horizontally-oriented, unlike the vertically-oriented caudal fin ("tail") of a fish. Vertical movement of a whale's tail provides propulsion. Whale bodies have a soft outer skin layer with almost no hair - this improves water flow around the body.

Whales are famous for being deep and long divers. Sperm whales can dive to over 9,200 feet deep. Northern bottlenose whales can hold their breath for over two hours. Unlike humans, whales have evolved mechanisms for coping with diving diseases such as nitrogen narcosis and decompression sickness.

Cetaceans are subdivided into two groups - the mysticete and the odontocete whales. Mysticete whales are the baleen whales - they include the blue whale, finback whale, humpback whale, gray whale, right whale, minke whale, sei whale, etc. The odontocetes are the toothed whales and include the sperm whale, killer whale, dolphins, porpoises, and belugas. Toothed whales have smaller bodies than the baleen whales. They have one blowhole atop the head and have prominent teeth - they feed on fish and squid and other large animals.

Classification: Animalia, Chordata, Vertebrata, Mammalia, Artiodactyla, Cetacea, Odontoceti, Squalodontidae

Locality: Randle Cliff, Calvert County, Maryland, USA

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See info. at:

en.wikipedia.org/wiki/Squalodon