Ophiacodon

Here's number two in the ancient synapsid collection: Ophiacodon!  This was closely related to Archaeothyris (willemsvdmerwe.deviantart.com/…), both being members of the Ophiacodontidae.  Indeed Ophiacodon dates back about as far, being recorded from about 306 million years ago in the late Carboniferous, but it had a much longer 'career', making it into the Permian, about 280 million years ago.  It was much larger than Archaeothyris, to an overall length of 3 m, and with a skull up to 50 cm long!  That's almost as big as a hippo's.  Like a hippo, this animal has been considered to be amphibious, able to live on land and also in water.  This is indeed possible, and the many recurved teeth in Ophiacodon's jaws would have been fine for catching fish.  But it likely could walk well on land, and might have been entirely terrestrial.  If so, it was the largest land predator of its time.  Specimens of Ophiacodon show a kind of bone called fibrolamellar bone, otherwise known from young, rapidly growing mammals, from birds, and from some dinosaurs.  This kind of bone is associated with rapid growth and, likely, warm-bloodedness.  Thus, the warm-bloodedness of mammals might already have existed in this ancient forebear of theirs!   I reconstructed Ophiacodon with belly plates, which are known from Archaeothyris - although I couldn't find a photo or drawing of just how those looked, so the above is conjectural.  The rest of its body might have had bone plates, or scales, or bare, glandular skin - we just don't know yet.