Cranial morphology and phylogenetic relationships of the Late Triassic temnospondyl Hyperokynodon keuperinus

The late Carnian temnospondyl taxon Hyperokynodon keuperinus is based on two fragmentary skulls from the Schilfsandstein (Stuttgart Formation) of Jagerhaus quarry at Heilbronn, southern Germany. Re-examination of the material reveals a relatively wide anterior palate with a long, countersunk area between the vomers and keeled, laterally compressed tusks. The frontal is very slender, the postorbital, jugal and squamosal are extremely narrow, the supratemporal is as long as the postfrontal, and the margin of the skull widens posterolaterally. H. keuperinus shares with Tertrema acuta the absence of a parachoanal tooth row on the vomer and a raised area between the anterior palatal openings. These two taxa are found to form a clade at the base of Lonchorhynchinae and crownwards of the trematosaurines. H. keuperinus was a large aquatic predator in the streams and deltas of the late Carnian, and its occurrence in a tide-influenced deposit suggests tolerance of brackish conditions.