Which extinct amphibian had a distinctive boomerang-shaped skull?

The extinct amphibian with a distinctive boomerang-shaped skull was Diplocaulus.

Diplocaulus was a lepospondyl amphibian that lived from the Late Carboniferous to the Late Permian, roughly 300 to 250 million years ago. Its most striking feature was a pair of long, rearward-projecting skull bones that made the head look like a boomerang. The best-known species, Diplocaulus magnicornis, is represented by many fossils from the Permian red beds of Texas and Oklahoma.

The animal had a stocky, salamander-like body and could reach about one metre in length. Paleontologists have proposed several explanations for the unusual skull: it may have helped with swimming, supported skin flaps, assisted burrowing, or discouraged predators from swallowing the animal. A hydrofoil-like function has also been proposed, but the exact purpose remains uncertain because soft-tissue evidence is scarce.

Diplocaulus is sometimes mistaken for a reptile because of its bizarre outline, but it was an amphibian. It is also distinct from Diploceraspis, a close relative with a similarly expanded skull. The skull’s shape is the key identification clue and makes Diplocaulus one of the most recognizable Paleozoic amphibians.