Apart from the destroyed holotype and its three partial casts (that survived), a neotype and two additional individuals are currently referred to Atychodracon megacephalus, making it a relatively well represented rhomaleosaurid.

BRSMG Cb 2335 represented a complete and articulated skeleton including the skull and lower jaw measuring 4.960 meters in total body length, and was one of several plesiosaurian specimens displayed in the Bristol City Museum and Art Gallery during the first half of the twentieth century.

BRSMG Cb 2335, was collected from the Blue Lias Formation at the Street-on-the-Fosse village, located about 14 km northeast of Street, Somerset, England.

[1] BRSMG Cb 2335 was destroyed during a World War II air raid on 24 November 1940, however detailed descriptions and illustrations of the specimen as well as high quality historical photographs still exist to this day.

Each of the casts is a replica of parts of the original specimen, and comprises a representation of the skull, nine front neck vertebrae including the atlas-axis, and the right forelimb.

LEICS G221.1851, nicknamed "The Barrow Kipper", represents a complete and well preserved skeleton housed at the New Walk Museum in Leicester, and was discovered at Barrow-upon-Soar, of Leicestershire, England.

However, a 2015 revision of "P." megacephalus pointed out that according to International Code of Zoological Nomenclature article 51.1, designating a neotype in this case is not required since representations of the holotype exist, and are enough to define the species objectively.

However, based on its distinctive morphology and the results of a preliminary phylogenetic analysis, WARMS 10875 seems to represent a new unnamed rhomaleosaurid species potentially related to Atychodracon and Eurycleidus.

On the other hand, several phylogenetic analyses recovered Eurycleidus as the sister taxon of Atychodracon when both are included, which seems to imply a close kinship between the taxa, as originally suggested by Andrews (1922).

Additionally, Atychodracon shows a more stout and robust humerus, and a reverse relation in the radius to ulna lengths (the former being shorter than the letter in Eurycleidus).

[1] In a revision of many pliosauroid taxa, Andrews (1922) was the first to recognize that "P." megacephalus is morphologically more closely related to "Plesiosaurus" arcuatus than to species of the Rhomaleosaurus/Thaumatosaurus complex.

However, in 1994 Cruickshank designated a neotype for the species, and due to the Rhomaleosaurus/Thaumatosaurus issue being resolved in favor of the former (while Thaumatosaurus is a nomen dubium) he referred to it by the new combination, Rhomaleosaurus megacephalus.

[1] The following two cladograms are simplified after two recent analyses, showing only the relationships within Rhomaleosauridae, and a few basal taxa whose position within the family is highly uncertain.