Selmasaurus

''Selmasaurus'' is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Plioplatecarpinae subfamily alongside genera like ''Angolasaurus'' and ''Platecarpus''. Two species are known, ''S. russelli'' and ''S. johnsoni''; both are exclusively known from Santonian deposits in the United States.

''Selmasaurus'' is unique among the mosasaurs in that its skull is unusually akinetic, meaning that it is incapable of widening to swallow larger prey. Most mosasaurs have skulls which possess "coupled kinesis" (mesokinesis and streptostyly), that is, parts of the jaw can open widely to accommodate large prey.

History of 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. 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 species was named in honor of paleontologist Dale A. Russell, for his extensive work on mosasaurs. The holotype and only known specimen of ''S. russelli'' was collected from an unknown location in western Alabama, and for decades, uncertainty surrounded the precise stratigraphic horizon from which the specimen had been recovered. Then in 1998, Caitlín R. Kiernan extracted chalky matrix from the basilar canal of the basioccipital and identified calcareous nanoplankton that indicated GSATC 221 had originated from basal Campanian beds within the lower unnamed member of the Mooreville Chalk Formation (Selma Group). In her study of Alabama mosasaur biostratigraphy, Kiernan placed ''S. russelli'' within the ''Clidastes'' Acme Zone, though it was the rarest element in the fauna, accounting for only 0.3% of the biozone's assemblage (one specimen).

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, Kansas, 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.

Description

''Selmasaurus'' was a small predatory mosasaur at approximately 3–5 meters in length. It possesses a relatively low number of teeth for a mosasaur, the lowest of any known species at the time of its discovery. Originally classified as a plioplatecarpine mosasaur, it differs from all other plioplatecarpine mosasaurs in several respects, listed below as stated by Polcyn and Everhart (2008):Polcyn, M. J., and Everhart, M. J., 2008, Description and phylogenetic analysis of a new species of ''Selmasaurus'' (Mosasauridae: Plioplatecarpinae) from the Niobrara Chalk of western Kansas: In: Proceedings of the Second Mosasaur Meeting, edited by Everhart, M. J, Fort Hays Studies, Special Issue number 3, p. 13-28.

Classification

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''.

The cladogram below follows the most resolved topology from a 2011 phylogenetic analysis of the Plioplatecarpinae by paleontologists Takuya Konishi and Michael W. Caldwell.

In their 2024 description of the Moroccan plioplatecarpine ''Khinjaria'', Longrich ''et al.'' used phylogenetic analyses to recover it within a monophyletic clade of non-plioplatecarpin plioplatecarpine mosasaurids. They named this clade—also containing '' Gavialimimus'', ''Goronyosaurus'', and ''Selmasaurus''—the Selmasaurini. ''Selmasaurus johnsoni'', which is older and may be more primitive, was found in a more basal position than ''S. russelli''. The results of their analyses are shown in the cladogram below:

Palaeoecology

According to stable isotope analysis, ''S. johnsoni'' was one of the few plioplatecarpines that was not restricted to nearshore marine environments.

References

{{Taxonbar, from=Q7211269

Fossil taxa described in 1988
Mosasaurs of North America
Mooreville Chalk
Mosasaurids