~225 million years ago (Late Triassic)Reputable sourceWell documented

The First Mammals

On the timeline · around ~225 million years ago (Late Triassic) · The Triassic

What happened

Tiny, shrew-like mammals and their close relatives appeared alongside the earliest dinosaurs. Brasilodon quadrangularis, from what is now Brazil, shows mammal-like features around 225 million years ago, with Morganucodon following by about 205 million years ago.

Why it matters

Mammals — our own lineage — did not arise after the dinosaurs but at the same time, living for over 150 million years as small, mostly nocturnal creatures in a dinosaur-dominated world.

Sources

Natural History Museum. The oldest known animal with mammalian-like teeth unearthed in Brazil · Reputable source

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