Selmasaurus is a genus of medium-sized plioplatecarpine mosasaur from the Upper Cretaceous Mooreville Chalk Formation (Campanian) of western Alabama (United States). 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 ("A new plioplatecarpine mosasaur (Squamata, Mosasauridae) from Alabama", Journal of Vertebrate Paleontology, Vol. 8, No. 1, 31 March 1988) in an article coauthored by Shannon and paleontologist Caitlin R. Kiernan. 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 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. To date, it is the only known specimen of Selmasaurus, which contains only a single species S. russelli (named in honor of paleontologist Dale A. Russell, for his extensive work on mosasaurs).
Shannon and Kiernan classified Selmasaurus as a member of the mosasaur family 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 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.