Reinterpretation of the cranial morphology of Utatsusaurus hataii (Ichthyopterygia) (Osawa Formation, Lower Triassic, Miyagi, Japan) and its systematic implications

ABSTRACT

Utatsusaurus hataii is an early member of Ichthyopterygia from the Osawa Formation (Lower Triassic) of Miyagi Prefecture, Japan. Although the holotype of the species lacks a well-preserved cranium, UHR 30691 preserves a near-complete skull and may be referred to Utatsusaurus based on a similar geologic occurrence and, most importantly, the following shared features with the holotype: subthecodont tooth implantation; the presence of distomesially compressed tooth crowns; a humerus that is longer than wide; and an ulnar facet on the humerus that is as wide as the radial facet. The first complete description of the skull of UHR 30691 and the incorporation of data derived from it into a phylogenetic analysis retrieves a monophyletic Ichthyopterygia that is supported by 18 unambiguous synapomorphies. The updated analysis recovers Utatsusaurus in a basal clade with Parvinatator (united by one unambiguous feature), which collectively form the sister group to all other ichthyopterygians.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline/UJVP

ACKNOWLEDGMENTS

Much appreciation is extended to K. Chiba, K. Tanaka, Y. Ishida, S. Hayashi, and Y. Kobayashi for their hospitality during the first author's visit to Hokkaido University, Sapporo, Japan. Special thanks go to Y. Kobayashi (Hokkaido University Museum, Sapporo, Japan) for facilitating the study of UHR 30691 and to K. Chiba for hand carrying the specimen to and from the senior author. M. Manabe (National Science Museum, Tokyo, Japan) and J. Nemoto (Tohoku Museum, Sendai, Japan) kindly permitted access to Utatsusaurus material in their care. Many thanks are due to R. Motani for numerous insightful conversations on the morphology of basal ichthyopterygians, and P. Thorne and M. Benton who provided a digital copy of their cited data matrix. Lastly, special acknowledgment goes to R. O’Keefe, E. Maxwell, and M. Zammit whose suggestions greatly added to the manuscript. This research was supported by a Duerksen Memorial Scholarship (University of Calgary) and a University of Calgary–Faculty of Veterinary Medicine research travel grant awarded to the senior author. Support was also provided by a NSERC Discovery Grant and University of Calgary starter grant awarded to J. Anderson, and a NSERC Discovery Grant awarded to A. Russell.

Handling editor: Robin O’Keefe.