Only one species is known, B. lucasi, initially described by Samuel Wendell Williston in 1903 from a partial fossil skeleton discovered in a quarry in Kansas, United States.

Thus, the circumstances surrounding the discovery of USNM 4989 remained unclear until 2007, when letters from Sternberg obtained by Kenneth Carpenter were formalized in an article written by his colleague Michael J.

[10] In 1860, Richard Owen documented a partial skull having been discovered around an unspecified date by George Cubitt in Cretaceous sediments from the town of Dorking, in the county of Surrey, England.

Based on teeth characteristics, Owen referred this specimen to Polyptychodon interruptus, a species of pliosaurid that he himself had already named in 1841 from fossils dating from the same period which were discovered in the counties of Sussex and Kent.

[a] In a phylogenetic study published in 2013, Roger B. J. Benson and colleagues recovered it as being close to several other specimens assigned to Brachauchenius, and therefore suggested that it would be better to refer it to that genus as well, although without specific affiliation.

[14] In his revision of Polyptychodon published in 2016, Daniel Madzia designates this genus as a nomen dubium due to the lack of accessibility and diagnostics regarding the holotype tooth of the taxon, the latter having possibly even been lost.

However, he considers it likely that DOKDM G/1-2 could be designated as the neotype of this genus in order to maintain its validity, but he refrains from doing so at this time since the descriptions provided by Owen are obsolete.

The Jenrich brothers, aided by George F. Sternberg and a local cow rancher named Jim Rouse, exhumed the skull later that year.

The specimen is then removed from its mount, and is officially described in 2013 by Bruce A. Schumacher, Carpenter and Everhart as the holotype of a new genus and species which they name Megacephalosaurus eulerti.

Although its locality of origin is unknown, matrix analysis extracted from the fossil identifies nannofossil assemblages that are associated with limestone deposits of the Greenhorn Formation.

[17] In 2000, a relatively complete pliosaurid skeleton, since cataloged as VL17052004-1, was discovered in the town of Villa de Leyva in Boyacá, Colombia, before being exhumed during 2004-2005.

The skeleton was first described in 2005 by Oliver Hampe as belonging to Brachauchenius,[21] before being designated as the holotype of a new genus and species under the name Stenorhynchosaurus munozi by María Eurídice Páramo and colleagues in 2016.