the sister group of Synapsida within Amniota.

Primitive and advanced synapsids

The synapsids are traditionally divided for convenience, into therapsids, an advanced group of synapsids and the branch within which mammals evolved, and stem mammals, (previously known as pelycosaurs), comprising the other six more primitive families of synapsids. Stem mammals were all rather lizard-like, with sprawling gait and possibly horny scutes,

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