What are Synapsids?
Synapsids (“fused arch”) are one of two groups of amniotes, or non-amphibian terrestrial animals, which includes mammals and their extinct relatives, the therapsids. The other group, the sauropsids, includes reptiles, birds, and extinct relatives. Together, synapsids, sauropsids, and amphibians make up all tetrapods, both extant and extinct, except for a few stem-group tetrapods that existed before these groups branched off from each other approximately 320 million years ago, in the late Carboniferous. The only living synapsids are mammals. Since the word “mammal” is widely understood, and “synapsid” is not, sometimes the word synapsid is used to refer specifically to the extinct grouping, which was formerly called mammal-like reptiles. More recently, it was realized that these animals in fact lacked certain features universal among reptiles, and so were given their own group. Sometimes they are referred to as “stem-mammals.” One characteristic common among synapsids is the temporal fe
