Romer and Price (1940) did a bang-up piece on pelycosaurs back in the day. From their tome I pulled two images, one of ophiacodonts known then (Fig. 1) and of sphenacodonts (we’ll do that tomorrow).
Romer and Price were kind enough to put mm lengths under each picture. I thought it might be nice to see how they look when scaled (Figs. 3,4). Ophiacodon retroversus gets pretty impressive, especially when it develops two lateral temporal fenestrae!
According to the large reptile tree, Elliotsmithia was the most primitive. Eothyris and Tetraceratops were not ophiacodonts, but caseasaurs and limnoscelids, respectively. Varanops and Varanosaurus skulls we looked at earlier here. They don’t look quite the same in bone as they do here.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.