Rhachiocephalus (Paperback)

High Quality Content by WIKIPEDIA articles! Rhachiocephalus is an extinct genus of non-mammalian synapsid. Synapsids ('fused arch') are a class of animals that includes mammals and everything more closely related to mammals than to other living amniotes. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals" under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other major group being the sauropsids (reptiles and birds). They are distinguished from other amniotes by having a single opening (temporal fenestra) in their skull behind each eye, which developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous Period.