Most mosasaurs have skulls which possess "coupled kinesis" (mesokinesis and streptostyly), that is, parts of the jaw can open widely to accommodate large prey.

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.

Then in 1998, Caitlín R. Kiernan extracted chalky matrix from the basilar canal of the basiocciptal 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).

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.

[4] 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):[3] Suprastapedial process descends to about half the quadrate height, and is in contact with but not fused to the peg-like infrastapedial process; elongate medially constricted, subrectangular parietal table, broadening anteriorally and constricting posteriorly forming narrow parasagittal ridges that diverge at their terminus; posteroventral median process of parietal meets supraoccipital in narrow elongate contact; anteromedial process of supratemporal clasped by medial excavations of the parietal rami for nearly entire length of rami; canals for the basilar arteries enter the basioccipital as two small foramina separated by a median septum, diverge within the basioccipital and cross the basisphenoid suture in a lateral and deeply ventral position.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."