When Did Mammals First Appear On Earth?

Were you to take a clock time machine to theTriassic Period(252 - 201 million old age ago ) , not only would you meetancient dinosaur speciesbut some of the former knownmammalsto roam ( or , should we say , scuttle ) across the major planet .

There areover5,000speciesof mammals alive today inhabiting all realm of the Earth – frompink river dolphinsin the River Amazon to thearctic foxin the alpine tundra . While vastly different in appearance and behavior , mammalian share a unwashed band of characteristics that tell them from other class of animal , such asbirdsandreptiles . This includes beingwarm - full-blooded , raise milk , and , with some notable exception ( think : theplatypusand theechidna ) , giving birth to know young . The lineage of all these species can be line back to early mammals that successfully coexisted with the likes of theTyrannosaurus rexandDilophosaurus .

In fact , theoldest known mammalian fossilsdate back an telling 225 million year . This was a period of vivid change , evolutionarily verbalize , which follow the Permian - Triassic extermination outcome ( otherwise cognise as theGreat Dying ) , which decimated life on Earth and killed off as many as 90 percent of species living at the time . harmonise to the fossil record , the mammalian that live back then lean to be little , nocturnal , and rodent - like in appearance .

Indeed , the earliest mammalian we are presently aware of is theBrasilodon quadrangularis – a diminutive critter discover as small and “ shrew - like ” . It is believed to have been just 20 centimeter ( 7.9 inches ) long and is call back to have spent most of its life in underground tunnel , much like the shrew alert today .

Prior to the discovery of theBrasilodon , theMorganucodonwas the oldest known mammal and , like theBrasilodon , would have most closely resembled a innovative - day shrew . Fossils of theMorganucodondate back some 205 million geezerhood ago andresearchsuggests these creatures would have feed on crunchy insects such as beetles .

Both theBrasilodonand theMorganucodon – and indeed all mammals awake today – are suppose to have evolved from a group of pre - mammalian vertebrates calledcynodonts(meaning “ wiener teeth ” ) , which uprise during the later Permian Period and survived the “ Great Dying ” .