Sinapsid

Synapsida is a major group of vertebrates that includes all living mammals and their extinct relatives, and it's distinguished by a unique skull feature—a single opening behind each eye socket that separates synapsids from other major amniote groups like reptiles and birds. Understanding synapsids matters because it helps us trace the evolutionary history of mammals and reveals how a group of ancient tetrapods developed the distinctive anatomical features that would eventually lead to modern mammals.

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