Diplocaulus was a roughly three-four foot long salamander-like amphibian with a strange boomerang shaped head.  It lived during the Permian and spent a large majority fo its life in the water.  It was likely preyed upon by various predators such as Dimetrodon, Ophiacodon and Ctenospondylus as well as other aquatic animals.  Its head is a bit of a mystery as to what it was for.  Theories suggest use as camouflage, helping it to hide at the muddy bottom, or perhaps it was packed with sensitive nerves to help sense food under the mud.  Another suggestion is that its wide head made it difficult to swallow whole by chasing fish.