Megalocephalus pachycephalus
Megalocephalus was a genus of early tetrapod from the Late Carboniferous. Its type species is M. pachycephalus. Its second species is M. lineolatus. The known specimens of M. pachycephalus were found in multiple sites in England, Ireland, and Scotland; while M. lineolatus was found in a quarry in Ohio, U.S. Both species were discovered in the late 1800s and had much material referred to and from other genera quite frequently for several decades.
The genus name Megalocephalus means "big head," while pachycephalus means "thick head."
There was a third species named M. brevicornis in 1947 based on some original fossils referred to as "Pteroplax" brevicornis, however, the original holotype was poorly described and destroyed in a museum fire in 1909.
Megalocephalus is considered a member of Baphetidae, a small family of early tetrapod amphibians. It is not entirely clear how closely related the members of this family are due to some striking differences between them, but M. pachycephalus is closely related to sister taxon Kyrinion, more so than it is to M. lineolatus. Both species were large, estimated to reach upwards of 1.5m from head to tail. Like other baphetids, the skull had an elongation of the eye socket, which is thought to have been either a location for a salt gland or extra jaw muscle attachments. Its long, sharp teeth indicate it hunted fish.
Original paper: M. pachycephalus original description; M. lineolatus original description (referred to as Leptophractus)
Wikipedia article: https://en.wikipedia.org/wiki/Megalocephalus