It lived during the Permian of North America (~299-251 mya) and was one of the largest lepospondyls, reaching around 1m long (3′3″) with the largest specimens’ skulls measuring up to 40cm wide (16″). Although most reconstructions tend to depict it as a shrinkwrapped “hammerhead salamander”, fossil body imprints suggest that it actually had flaps of skin connecting the outer tips to its body – which would have given it much more of a stingray sort of shape.
The function of its weird head shape is still unknown, although there are plenty of hypotheses. It may have acted as a hydrofoil for more efficient swimming, or been used to root through muddy riverbeds for prey, or it may have simply made Diplocaulus too wide for some larger predators to easily swallow.
Or, since juveniles had more “normal” salamander-like skulls and only developed the boomerang shape as they matured, it may also have had been involved in display and sexual selection.