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When did the first true mammals appear?

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Triassic paleogeography — Triassic Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the early Triassic Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The first true mammals appeared during the Late Triassic Period. These early, shrewlike animals, such as the genus Morganucodon, are represented in the fossil record by remains found in Rhaetian Stage deposits, which date to the end of the Triassic.213

It is important to note that the evolutionary transition from therapsid reptiles to mammals at the close of the Triassic is not clearly demonstrated by a continuous series of well-preserved fossils. Furthermore, there are two competing definitions for what constitutes a mammal. Under a broader definition based on the origin of the mammalian jaw joint, Morganucodon is considered one of the oldest mammals. Under a more restrictive definition—limited to the most recent common ancestor of living mammals and its descendants—these early forms are classified as Mammaliaformes, a clade that includes mammals and their closest extinct relatives.23