[2][5] A key feature is the three facets at the distal end of the humerus; the middle is the smallest and articulates with the intermedium, which clearly separates Brachypterygius from Ophthalmosaurus, the most common Late Jurassic ichthyosaur.

[7] Owen (1840) erected Ichthyosaurus trigonus based upon a single dorsal vertebra (ANSP 10124) from the Kimmeridge Clay of Westbrook, Wiltshire, UK.

[14] McGowan and Motani (2003) considered it to be a species of Brachypterygius,[1] but a 2014 re-assessment of Cambridge Greensand ichthyosaurs found it to be a nomen dubium indeterminate beyond Ophthalmosaurinae.

[16] A large skull was discovered in the Kimmeridge Clay of Stowbridge, Norfolk, UK and named as a new genus and species, Grendelius mordax, by McGowan in 1976.

[1] However, a 2015 publication by Zverkov and colleagues instead found Grendelius mordax to be sufficiently different from Brachypterygius extremus to be considered both a separate genus and species.