First mammal

According to Official Guinness Records,

In 1991, a partial skull of a mammal named Adelobasileus cromptoni was reported from 225-million-year-old rocks in New Mexico, USA. The first true mammals, as represented by odd teeth, appeared about 220 million years ago during the Late Triassic. This creature was similar in appearance to modern tree shrews of the order Scandentia.Modern mammals (therians) arose in the Mid-Cretaceous period, and the earliest representatives of modern orders, such as Purgatorius, the first primate, by the end of the Cretaceous period, 65 million years ago.

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