Mammal-like reptile

The basal amniotes (reptiliomorphs) from which synapsids evolved were historically simply called "reptiles". Therefore, non-mammalian synapsids were then described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as pelycosaurs or pelycosaur-grade synapsids. These paraphyletic terms have fallen out of favor and are only used informally (if at all) in modern literature, as it is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyleticsister taxon of sauropsids, rather than a part of the sauropsid lineage. Modern alternatives sometimes used include stem mammals, proto-mammals, paramammals or pan-mammals. Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontian pelycosaurs, at the end of the Early Permian, a transition known as Olson's Gap/Extinction.