Diplocaulus was an usual genus of boomerang-headed amphibian from the Permian Period. The use for its boomerang-shaped head is a mystery. Maybe it used it to glide through the water like a hydrofoil, or maybe it used it to dig up food, maybe for fighting for a mate, or maybe something else. Very young Diplocaulus did not have the boomerang-shaped head, but older Diplocaulus eventually grew it.
Many fossils of Diplocaulus show that a predator must have been eating it, because they were torn apart and have fossilized teeth in them. The teeth belonged to Dimetrodon. It's not obvious how any predator would have been able to eat Diplocaulus without being choked by swallowing its head. But if something could eat it piece by piece, it would be able to avoid swallowing the head. So synapsids like Dimetrodon would not have trouble eating it.
Diplocaulus was about three feet long. Its closest relative was Diploceraspis, which also had a boomerang-shaped head and resembled Diplocaulus in many ways.