Synapsida

Synapsida is a group of animals that includes mammals and their extinct relatives. They are characterized by having a single opening in the skull behind the eyes, called a temporal fenestra, which allows for increased jaw muscle attachment and strength. This group first appeared over 300 million years ago and eventually led to the development of mammals. Synapsids include ancient ancestors like pelycosaurs and therapsids, as well as modern mammals. They are distinguished from other vertebrates by features related to skull structure and evolutionary lineage, reflecting their common ancestry and evolutionary adaptations over millions of years.