Mammals | Mammalia
Mammals are a warm-blooded vertebrate class, Mammalia, and are found on every continent and in the oceans. Mammals are distinguished by the female mammary glands used to feed their young, having hair or fur, three middle ear bones, and a hinged jaw. Mammals diverged from reptiles and birds in the late Triassic, a fact which is known from studying the bones and skulls of Mammalian ancestors to see that the jaws of prehistoric reptiles became the ear bones of mammals today. One of the most adaptable animal classes on the planet, mammals utilize every possible form of transportation (walk, crawl, burrow, swim, climb, etc.) and may live solitarily or in groups for protection depending on the species.
Mammals first appeared during the Triassic period which was about 252 million to 201 million years ago and were members of the reptilian order Therapsida. This order had a subclass, Synapsida which are sometimes referred to as mammal-like reptiles. They were part of the Carboniferous period and are considered one of the earliest reptilian groups.
The only 2 mammal species that lay eggs are the duck-billed platypus and the echidna, also known as the spiny anteater. The reasons these 2 mammals still lay eggs may be due to their distant ancestors as well as other primitive traits like their reptile-like shoulders.
The most common mammal species are humans as it is the most numerous species of mammal on Earth. As of 2022, the human population reached 7.9 billion. Human beings are the only species that are close to exceeding the number of members of the brown rat and the house mouse.
