Re-examination of the elasmosaurid Styxosaurus snowii

Styxosaurus snowii holotype skull

An iconic Late Cretaceous plesiosaur

by Sven Sachs & Benjamin P. Kear

Styxosaurus is one of the most famous members of the Elasmosauridae, plesiosaurs which were characterised by hyperelongate necks comprising up to 75 vertebrae (Kubo et al. 2012, Sachs et al. 2013). The holotype specimen of Styxosaurus snowii, KUVP 1301, is housed at the University of Kansas Natural History Museum in Lawrence (Kansas, U.S.A.), and incorporates a complete but compressed skull with an associate series of 29-30 neck vertebrae.

Styxosaurus snowii holotype skull
Styxosaurus snowii holotype skull in left lateral view (click to enlarge)

This specimen was the first elasmosaurid with a intact skull that was ever described. The famous Elasmosaurus platyurus (described by Cope, 1868), also from Kansas, only preserves parts of the skull such as the anterior part of the snout and mandible (see Everhart, 2005, Sachs, 2005). Other historically documented elasmosaurids like the holotype of Mauisaurus haasti (described by Hector, 1874) from New Zealand had no skull preserved.

Styxosaurus snowii dentiton
Close-up of the dentition of the Styxosaurus snowii holotype

KUVP 1301 was found in the upper section of the Smoky Hill Chalk Member of the lowermost Campanian (Late Cretaceous) Niobrara Formation (Williston 1906, Sachs et al. 2018). The fossil was recovered by Elias Putnam West (1820-1892) near Hell Creek in Logan County (Kansas, USA) in the summer of 1890. West was an active fossil collector, but also worked as archaeologist, judge and lawyer. He discovered a number of vertebrate fossils including remains of the mosasaurids Tylosaurus and Placecarpus, as well as the teleost fish Xiphactinus (for more information see Everhart, 2015). West donated KUVP 1301 to the University of Kansas at Lawrence, where Samuel W. Williston, then Professor of Geology at the University of Kansas, immediately commenced research on it. Williston subsequently published a short note in the journal Science (Williston, 1890a), in which where named it as a new species of the genus ‘Cimoliosaurus’ [sic] (see Kear 2002) — C. snowii (Williston, 1890a). The species name honoured Francis Snow, who was chancellor of the University of Kansas at the time.

Styxosaurus snowii drawings
Drawings of Styxosaurus snowii type material from Williston (1890b) (click to enlarge)

Williston (1890b) later updated his description, provided some line drawings of the skull and a neck vertebra and changed the taxonomic designation to ‘Cimoliosaurus’ (Elasmosaurus?) snowii. Then, in 1906, he published on additional material including vertebrae, limb girdle and limb bones from the same locality, which are today housed in the Yale Peabody Museum of Natural History in New Haven, U.S.A. Williston (1906) formally referred both KUVP 1301 and the additional skeletal elements to the genus Elasmosaurus.

The generic name Styxosaurus was established by Samuel P. Welles (from the University of California, Berkeley) in 1943, who revised the North American elasmosaurids in his landmark monograph “Elasmosaurid plesiosaurs with description of new material from California and Colorado“, and erected the now well known taxa Hydrotherosaurus alexandrae, Thalassomedon haningtoni, Morenosaurus stocki, Aphrosaurus furlongi, Fresnosaurus drescheri, and Styxosaurus snowii.

Alzadasaurus pembertoni holotype
Alzadasaurus pembertoni holotype (from Sachs & Kear, 2015). Photo courtesy Mike Everhart.

Nine years later Welles named a second species of Styxosaurus, Styxosaurus browni, based on a partial skeleton comprising a skull with 75 associated vertebrae, the pectoral girdle and limb elements (Welles, 1952). This specimen (AMNH 5835, in the American Museum of Natural History in New York) derives from the lower Campanian Sharon Springs Formation, and was found in Niobrara County Wyoming (sensu Carpenter 1999, following Barnum Brown, unpublished notes). AMNH 5835 has also alternatively been synonymised with Hydralmosaurus serpentinus (Carpenter, 1999), but is currently considered a valid species (Otero 2016).

Welles (1962) similarly considered Styxosaurus snowii to be a nomen vanium (a taxon lacking  sufficient diagnostic characters), and reassigned it to the genus Elasmosaurus, but retained the generic name Styxosaurus for S. browni.

Storrs (1999) re-examined all the Late Cretaceous plesiosaurs from Kansas and listed a number of diagnostic traits in KUVP 1301. He also restricted the species to the holotype specimen.

Carpenter (1999) further attributed a number of previously described specimens to Styosaurus snowii, including the type material of Alzadasaurus pembertoni (Welles & Bump, 1949). which is a virtually complete skeleton (SDSMT 451) housed in the South Dakota School of Mines in Rapid City, South Dakota. Carpenter (1999) thus expanded the diagnosis of Styxosaurus snowii based upon these remains, and in particular noted the presence of 62 cervical vertebrae, an elongate atlas-axis centrum, and the lack of a midline bar on both the pectoral and pelvic girdles. This has been accepted by most subsequent studies, but the holotype specimen KUVP 1301 has never since been researched first hand.

Neck of the Styxosaurus snowii holotype
Neck of the Styxosaurus snowii holotype. The skull was removed for examination (click to enlarge)

Studies by Sven Sachs, Johan Lindgren and Benjamin P. Kear

We examined the Styxosaurus snowii type material in November 2015. It is presently on display, but we were permitted to remove the skull for a detailed inspection. The preservation is excellent and revealed hitherto unobserved details of the crushed left side of the skull where the medial aspect of the mandible and part of the palate are exposed. The vertebral column is embedded in resin and could not be manipulated. Interestingly, however, the vertebral count varied from previous publications: Willison (1890a) listing 26 vertebra; Williston (1890b, 1903) and Welles (1943, 1952, 1962) mentioning 28; and Storrs (1999) 28-30. We found 29-30 neck vertebrae to be present, with the posterior-most centrum being reassembled from fragments including what appears to be non-plesiosaurian bone.

Styxosaurus snowii skull close-up
Close-up showing the orbita and the midline boss

The diagnostic cranial features also did not concur with Carpenter (1999), especially the external nasal opening, which was situated at the level of the third and fourth maxillary tooth positions instead of the sixth and seventh maxillary teeth. KUVP 1301 otherwise bears an anisodont (irregular) dentition with five teeth in the premaxilla that interlock with the largest dentary teeth; the first dentary tooth crowns also directly abutting each other. The 4-5 maxillary teeth form prominent fangs, and the last dentary teeth are likewise enlarged.

The mandibular symphysis is produced into a ridge, and a prominent midline crest extends along the top of the snout to form a boss between the external nasal opening and the orbits. This midline boss is present in a few other elasmosaurids, including Styxosaurus browni and Futabasaurus suzukii. The squamosals have a convex expansion along their posterior edge, which likewise occurs in Nakonanectes bradti and Libonectes morgani. The neck vertebrae of KUVP 1301 are all platycoelous (flat articular faces) and and almost twice as long as high. This is a key feature of the subfamily Styxosaurinae that was coined by Otero (2016).

Symphyseal portion of the mandible of KUVP 1301 in ventral view (click to enlarge)

Close-up of the symphyseal portion of the mandible in ventral view (click to enlarge)

Styxosaurus snowii cervical vertebrae

Anterior cervical vertebrae in lateral view

We published our study in a special volume of the Australasian palaeontology journal Alcheringa (Sachs et al., 2018). We conclude that the diagnostic traits used to distinguish S. snowii from other elasmosaurs have relied upon extrapolations or other referred specimens, and they are therefore inapplicable for the holotype. The definition of Styxosaurus snowii is therefore be restricted to character states observable only in KUVP 1301. Finally, the severe diagenetic compaction of the Styxosaurus browni skull makes intra-generic comparisons equivocal (e.g., Storrs 1999, Otero 2016).

Styxosaurus snowii skull in ventral view
Styxosaurus snowii skull in ventral view showing the transverse compression (click to enlarge)
Styxosaurus depicted by Joschua Knüppe
Styxosaurus depicted by Joschua Knüppe (click to enlarge)



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