File:Ovis dalli (dall sheep) (Denali National Park, Alaska, USA) 2.jpg

English: Ovis dalli Nelson, 1884 - dall sheep in Alaska, USA. (July 2011) (photo provided by James Cheshire)

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.

Seen here is a dall sheep in Alaska. Rams (males) have prominent curved horns. This species occurs in mountainous Alaska (especially the Alaska Range and Brooks Range areas) and northwestern Canada.

Classification: Animalia, Chordata, Vertebrata, Mammalia, Artiodactyla, Bovidae

Locality: Denali National Park, south-central Alaska, USA

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See info. at:

en.wikipedia.org/wiki/Dall_sheep