The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus

ABSTRACT

The validity of the parvipelvian ichthyosaur genus Protoichthyosaurus was evaluated by examining the type material and recently recognized specimens. Protoichthyosaurus has a wide forefin with at least five primary digits, an anterior digital bifurcation in the forefin, a humerus that is nearly equal in width distally and proximally, and a coracoid with well-developed anterior and posterior notches, features it shares with the more common genus Ichthyosaurus. Previously, Protoichthyosaurus was synonymized with Ichthyosaurus because of an incomplete understanding of forefin variation in the latter taxon. However, Protoichthyosaurus has several features in the forefin that distinguishes it from Ichthyosaurus, including the presence of three elements in the distal carpal row, a contact between distal carpal 3 and the ulnare, a bifurcation of distal carpal 2 to form two elements in the metacarpal row, the proximal element of the bifurcation nearly separating distal carpal 2 from distal carpal 3, distal carpal 3 separating the intermedium from contact with distal carpal 4, and the absence of digit V. A phylogenetic analysis confirms that Protoichthyosaurus is the sister taxon of Ichthyosaurus and is nested in Ichthyosauridae. We recognize two species, P. prostaxalis and P. applebyi, sp. nov., but not P. prosostealis. Protoichthyosaurus is unequivocally from the Hettangian but might be present in the Rhaetian to lower Sinemurian. The genus is geographically widespread across the U.K.

http://www.zoobank.org/urn:lsid:zoobank.org:pub:23CA73CE-BB9A-4EAD-81BA-A6D27ABC82CB

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

Citation for this article: Lomax, D. R., J. A. Massare, and R. T. Mistry. 2017. The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1361433.

ACKNOWLEDGMENTS

We would like to thank C. Howells (NMW), M. Williams (BRLSI), M. Evans (LEICT), S. Chapman and L. Steel (NHMUK), J. Clatworthy and K. Riddington (BU, BIRUG), M. Riley (CAMSM), D. Large, W. Qi, T. Hartman, and I. Sutton (UON), C. Pickersgill (UNM), J. Radley and K. McCoy (WARMS), H. Ketchum and E. Howlett (OUMNH), N. Monaghan (NMING), I. Jenkins and B. Neuman (UORCMZ), E. Daeschler and E. Gilmore (ANSP), C. Berry (AGC), N. Clark and R. Smith (GLAHM), K. Seymour (ROM), A. Smith (NOTNH), and P. Joy (UOD) for access to and assistance with specimens at their institutions. We also thank R. Clark and the Alfred Gillett Trust, Street, for access to the Alfred Gillett collection; B. Gibson for photographs of specimens at UOD; A. Smith and W. Wahl for discussions about the UNM specimen; and P. Upchurch for advice on the phylogenetic analysis. Finally, we thank P. Druckenmiller, J. Lawrence Wujek, and an anonymous reviewer for their suggestions and comments. D.R.L.’s research was covered in part by a PGR, Dean's Doctoral Scholarship Award from the University of Manchester. J.A.M.’s travel was funded in part by a Faculty Research Grant from SUNY Brockport. Donations from alumni and friends through the Cascade Grants Programme, University of Nottingham, funded the conservation of UNM.G.2017.1 for display. We further acknowledge the Willi Hennig Society for making the TNT program available.

Handling editor: Patrick Druckenmiller.