Reassessment of the Styxosaurus snowii (Williston, 1890) holotype specimen and its implications for elasmosaurid plesiosaurian interrelationships

Abstract

The holotype (KUVP 1301) of Styxosaurus snowii—one of the earliest described elasmosaurid plesiosaurians—consists of a well-preserved cranium, mandible and articulated sequence of anterior–mid-series cervical vertebrae found in the lowermost Campanian strata of the Smoky Hill Chalk Member in the Niobrara Formation of Kansas, USA. This particular specimen has proven important for recent phylogenies of Elasmosauridae, and is integral to resulting definitions of the subfamily-level clade, Styxosaurinae. Despite this, KUVP 1301 has not been redescribed or figured in detail since its original taxonomic establishment. We, therefore, re-evaluated KUVP 1301 and assessed its phylogenetic implications. Several notable character states are pertinent for diagnosing S. snowii at genus and species level: (1) an anisodont functional dentition comprising enlarged premaxillary and dentary teeth with a pair of maxillary ‘fangs’, and elongate posterior-most dentary teeth that overlap the upper tooth row; (2) a prominent dorsomedian crest extending from the tip of the premaxillary rostrum, and expanding into a low ‘mound-like’ boss between the external bony nasal openings and orbits; (3) a pronounced convex projection on the posterolateral edge of the squamosals; and (4) platycoelous post-axial cervical vertebral centra that are substantially longer than high, and bear both lateral longitudinal ridges and ventral notches. Character state comparisons with the congeneric subfamily specifier Styxosaurus browni suggest that taxonomic distinction is possible, but equivocal. We, therefore, restrict our definition of Styxosaurus to morphologies observable in KUVP 1301. Furthermore, phylogenetic analysis of our first-hand data returns inconsistent elasmosaurid intra-clade relationships, especially with regard to Styxosaurinae. Consequently, we posit that a more targeted reassessment of Elasmosauridae is necessary to resolve both species-level topologies and higher taxonomy within the group.

Sven Sachs* [[email protected]] Naturkunde-Museum Bielefeld, Abteilung Geowissenschaften, Adenauerplatz 2, 33602 Bielefeld, Germany; Johan Lindgren [[email protected]], Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden; Benjamin P. Kear [[email protected]] Museum of Evolution, Uppsala University, Norbyvägen 16, Uppsala SE-752 36, Sweden. *Also affiliated with: Im Hof 9, 51766 Engelskirchen, Germany. Received 26.5.2016; revised 13.7.2018; accepted 24.7.2018.

Key words:

• Plesiosauria
• Elasmosauridae
• Styxosaurinae
• phylogeny
• intra-clade relationships

Acknowledgements

We thank David Burnham (University of Kansas) for providing access to KUVP 1301, and for his hospitality during the collections visit by SS. Mike Everhart (Hays) contributed valuable discussions. Valentin Fischer (University of Oxford) and a second anonymous reviewer provided constructive comments. This research was financed by a Swedish Research Council Distinguished Young Researchers Grant (642-2014-3773) awarded to JL.

Supplemental material

The supplemental material for this paper is available online at https://doi.org/10.1080/03115518.2018.1508613.