Wikipedia
The Eupelycosauria originally referred to a suborder of ' pelycosaurs' (Reisz 1987), but has been redefined (Laurin and Reisz 1997) to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals. They first appear during the Early Pennsylvanian epoch (i.e.: Archaeothyris, and perhaps an even earlier genus, Protoclepsydrops), and represent just one of the many stages in the acquiring of mammal-like characteristics (Kemp 1982), in contrast to their earlier amniote ancestors. The defining characteristics which separate these animals from the Caseasauria (also pelycosaurs) are based on details of proportion of certain bones of the skull. These include a long, narrow supratemporal bone (in contrast to caseasaurs where this bone is almost as wide as it is long), and a frontal bone with a wider connection to the upper margin of the orbit (Laurin and Reisz 1997).