evolved from synapsids (also called mammal-like reptiles, but
are better referred to as stem-mammals, and are not reptiles).
is a synapsid example from the Permian. Mammal evolution was a
gradual, extended process that spanned some approximately 70 million
years, from about the middle Permian to the Middle Jurassic. By
the middle Triassic, many animals had appeared that looked like
mammals. Hadrocodium wui was a basal mammal species that lived
during the early Jurassic; it was discovered in the famous Lufeng
Basin in Yunnan Province, southwestern China. While Hadrocodium
did not have all mammal characteristics, it did have a separate
jawbone, large brain, and a sophisticated hearing. It weighed
a miniscule two grams. Whether Hadrocodium was warm- or cold-blooded
remains in dispute. It co-existed with several other primitive
mammals with much larger body size.