Selmasaurus

Selmasaurus Temporal range: Late Cretaceous
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Family:	†Mosasauridae
Parafamily:	†Russellosaurina
Subfamily:	†Plioplatecarpinae
Genus:	†Selmasaurus Wright & Shannon, 1988
Species
†S. russelli Wright & Shannon, 1988 (type) †S. johnsoni Polcyn & Everhart, 2008[1]

Selmasaurus is an extinct genus of medium-sized plioplatecarpine mosasaur from the Late Cretaceous of the United States.

Discovery

First recognized by geologist Samuel Wayne Shannon in his 1975 Master's thesis, "Selected Alabama Mosasaurs," the taxon remained a nomen nudum until it was officially described in 1988 in an article coauthored by Wright.^[2] The type specimen, formerly reposited at the Geological Survey of Alabama and cataloged as GSATC 221, was transferred in 2005 to the Alabama Museum of Natural History (Tuscaloosa). The holotype of this genus consists of a very well preserved but incomplete and disarticulated skull, the left atlantal neural arch, atlas centrum, and a single neural arch from a cervical vertebra. Preserved skull elements include the frontal, parietal, left ectopterygoid, left jugal, supratemporals, basioccipital and basisphenoid, and quadrates. The holotype and only known specimen of S. russelli was found from the Mooreville Chalk Formation (Campanian stage) of western Alabama. This species was named in honor of paleontologist Dale A. Russell, for his extensive work on mosasaurs.

A remarkably well preserved and nearly complete Selmasaurus skull and partial postcranial skeleton was discovered by Steve Johnson and family in 1996, from the Santonian or Campanian marine horizon in the Niobrara Formation of Niobrara Chalk, western Kansas. Recovered in 1997 and donated to the Sternberg Museum of Natural History in Hays, KS in 2001, the remains were determined to be a new species of Selmasaurus in 2008 after over a decade of study by Polcyn and Everhart. Named S. johnsoni after its discoverer, the skull is one of the most complete mosasaur skulls recovered and thus provides new anatomical information for Selmasaurus, a better understanding of plioplatecarpine ingroup relationships, extends the geographic and temporal range of the genus, and documents further diversity within Plioplatecarpinae. The holotype and the only known specimen is housed at the Sternberg Museum of Natural History under catalog number FHSM VP-13910.

Relationship to other mosasauroids

Wright and Shannon classified Selmasaurus as a member of the mosasaur subfamily Plioplatecarpinae, which also includes the genera Platecarpus, Plioplatecarpus, and Ectenosaurus, largely on the "basis of the mode of circulation through the basicarnium." The genus may be most closely related to Ectenosaurus, though it possesses a much shorter, stouter skull. Additional specimens would greatly expand our understanding of Selmasaurus russelli.

Cranial kinesis

The mosasaur Selmasaurus is notable in that its skull is unusually akinetic. Most mosasurus have skulls which possess "coupled kinesis" (mesokinesis and streptostyly), that is, parts of the jaw can open widely to accommodate large prey.

References

• Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Peabody Museum of Natural History, Yale University, Bulletin 23.