Which period mammal first appeared with jaws

The Devonian Period of the Paleozoic Era lasted from 417 million years ago to 354 million years ago

Recently, paleontologists discovered conclusive fossil evidence for the first important split in the mammal family tree, the one between placental andmarsupial mammals. Technically, the first, marsupial-like mammals of the late Triassic period are known as metatherians.