Utatsusaurus: Full Description Makes Into Ichthyosaurian Hockey Puck

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

Authors:

1. Robin S. Cuthbertson (a)
2. Anthony P. Russell (a)
3. Jason S. Anderson (b)

Affiliations:
a. Department of Biological Sciences , University of Calgary , 2500 University Drive, Calgary , Alberta , Canada , T2N 1N4

b. Department of Comparative Biology and Experimental Medicine , University of Calgary, 3330 Hospital Drive , Calgary , Alberta , Canada , T2N 4N1

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.