NEW INFORMATION ON TITANICHTHYS (PLACODERMI: ARTHRODIRA) FROM THE CLEVELAND SHALE MEMBER OF THE OHIO SHALE FORMATION (FAMENNIAN) OF OHIO, USA

The Cleveland Shale member of the Ohio Shale Formation contains one of the richest Late Devonian fish assemblages in the world. This fauna is dominated by sharks and large-bodied arthrodires including the well known Dunkleosteus and the closely related, but poorly described Titanichthys. The genus Titanichthys was established over a century ago, but due to the fragmentary nature of most specimens its relationship to other arthrodires is poorly understood. A recently discovered specimen referred to Titanichthys from the Cleveland Shale Member of the Rocky River area of Ohio has provided new information allowing for a revised description of the genus and a more complete phylogenetic analysis of these arthrodires.

The new, mostly complete specimen is articulated and includes several cranial plates previously undescribed in American Titanichthys specimens; these include the rostral, postmarginal, posterior superognathal, postsuborbital and submarginal plates, with a possible fragment of a sclerotic ring. The anterior border of median dorsal plate preserves a narrow, deep embayment in addition to the normal emargination seen in this taxon and other derived arthrodires. This deep embayment has not been previously observed in Titanichthys or other related taxa, and thus may represent an autapomorphy that can only be confirmed by the discovery of new specimens.

The new phylogenetic analysis indicates a close relationship of Titanichthys with Heintzichthys, also found in the Cleveland Shale, and has confirmed Titanichthys position within the aspinothoracid arthrodires.