The Brasilodon is an early mammal/mammaliaform threshold at 225 million years ago. The more conservative textbook marker is the Morganucodon at 205 million years ago. The Natural History Museum notes that Brasilodon dates to 225 million years ago and predates Morganucodon, which is usually considered the first mammal, by about 20 million years.
First Mammaliaforms
~215 Million years ago (+/- 10 million)
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