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[dinosaur] Styxosaurus (Elasmosauridae) redescribed + Cretaceous Texas ichthyosaurians




Ben Creisler
bcreisler@gmail.com


New marine reptile papers:


Sven Sachs, Johan Lindgren & Benjamin P. Kear (2018)
Reassessment of the Styxosaurus snowii (Williston, 1890) holotype specimen and its implications for elasmosaurid plesiosaurian interrelationships.
Alcheringa (advance online publication)
doi:Â https://doi.org/10.1080/03115518.2018.1508613Â Â
https://www.tandfonline.com/doi/full/10.1080/03115518.2018.1508613



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.

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Nathan E. Van Vranken (2018)
An overview of ichthyosaurian remains from the Cretaceous of Texas, USA.
Alcheringa (advance online publication)
doi:Â https://doi.org/10.1080/03115518.2018.1523462Â Â
https://www.tandfonline.com/doi/full/10.1080/03115518.2018.1523462
Â

The clade nominal Ophthalmosauridae encompasses all currently recognized ichthyosaurian taxa from the mid-Cretaceous (AlbianâCenomanian) time interval in North America. These would have inhabited what is today known as the Western Interior Seaway. The remains represent mid to large-sized (up to around 5 m in maximum body length) ichthyosaurians that are mainly based upon isolated teeth and vertebrae, although some partial skeletons have been recovered from both the USA and Canada. Most of these fossils have taxonomic affinities with platypterygiines, such as the ubiquitous 'form genus' Platypterygius. This taxon has historically been the default identification for most North American Cretaceous ichthyosaur specimens. However, in comparison with remains from elsewhere, the ichthyosaurian fossils from Texas have been relatively understudied. Here, I review the existing records and present a concise list of all Cretaceous ichthyosaurian material currently held in public museum collections across the state. I have recognized 11 Cretaceous ichthyosaurian occurrences from Texas, including an indeterminate tooth, numerous vertebral centra, and some fragmentary skeletal remains attributable to Platypterygius. Ichthyosaurians therefore demonstrably occupied the Cretaceous epeiric seas of Texas from at least the Albian to middle Cenomanian, a distribution that traverses both the western and eastern coastlines of the Western Interior Seaway during its early phase of southern epicontinental incursion.

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