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Reassessment of the Styxosaurus snowii (Williston, 1890) holotype specimen and its implications for elasmosaurid plesiosaurian interrelationships
Nat Kunde Museum Bielefeld, Abt Geowissensch, Adenauerpl 2, D-33602 Bielefeld, Germany;Hof 9, D-51766 Engelskirchen, Germany.
Lund Univ, Dept Geol, Solvegatan 12, SE-22362 Lund, Sweden.
Uppsala University, Music and Museums, Museum of Evolution.ORCID iD: 0000-0002-3128-3141
2018 (English)In: Alcheringa, ISSN 0311-5518, E-ISSN 1752-0754, Vol. 42, no 4, p. 560-574Article in journal (Refereed) Published
Abstract [en]

The holotype (KUVP 1301) of Styxosaurus snowiione of the earliest described elasmosaurid plesiosauriansconsists 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.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2018. Vol. 42, no 4, p. 560-574
Keywords [en]
Plesiosauria, Elasmosauridae, Styxosaurinae, phylogeny, intra-clade relationships
National Category
Geology
Identifiers
URN: urn:nbn:se:uu:diva-380513DOI: 10.1080/03115518.2018.1508613ISI: 000460705100009OAI: oai:DiVA.org:uu-380513DiVA, id: diva2:1301371
Funder
Swedish Research Council, 642-2014-3773Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved

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Kear, Benjamin P.

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