Skeletal morphology of an early ichthyopterygian, cf. Utatsusaurus hataii, from Japan with an emphasis on CT scan data of the skull

The Early Triassic Utatsusaurus hataii is one of the earliest ichthyopterygians and shows numerous plesiomorphic features. An ichthyopterygian specimen, NSM-PV-20028, from the Osawa Formation in Miyagi Prefecture, Japan, has long been referred to U. hataii, but it remained undescribed in detail. In the present study, the specimen was re-examined and its referral to this species is tentatively sustained, thus being treated as cf. U. hataii. The present study conducted the first digital segmentation of the skull bones of a specimen referred to this species, based on X-ray computed tomographic (CT) image data; and provided detailed information on the entire skeleton of the specimen, including exceptionally well-preserved pelvic girdle and interclavicle. In addition, the two-dimensional retrodeformation method was applied to the specimen to recover the original skeletal proportion. In the result of phylogenetic analysis, NSM-PV-20028 was recovered in a polytomy with U. hataii, which does not contradict our referral of the specimen. The postcranial skeleton of the studied specimen includes both plesiomorphic characters and derived characters, suggesting a transitional stage to a fully aquatic life, emphasizing the importance of this specimen when discussing aquatic adaptation of ichthyopterygians.