Synapsids are one of the two major groups of vertebrates which evolved from basal amniotes.
'Synapsids' means 'fused arch', and refers to their skulls. The group includes mammals and earlier groups related to mammals. They were originally called "mammal-like reptiles", which is a bit confusing.
Synapsids are one of the two major groups of amniotes. Both groups evolved from early amniotes about 345 million years ago during the early or mid Carboniferous period. The other is the sauropsids, the group which includes reptiles and birds.