The genus Eretomosaurus was erected for ‘Plesiosaurus’ rugosus by Seeley (1874). Eretmosaurus is a rogue taxon as workers have been in disagreement about its taxonomic affinities. Eretmosaurus was included in the Rhomaleosauridae based on the anatomy of its girdle bones (Persson, 1963); the Pliosauridae (Brown, 1981), and the Elasmosauridae (Bardet, 1995; Bardet et al. 1999). The latter authors give no specific reason for their classification.
The holotype lacks a head so the rogue nature of Eretmosaurus can be explained by missing data (Wilkinson, 1995). This suggests that postcranial characters may be unreliable for systematic purposes or, more likely, that or that our understanding of plesiosaur postcranial anatomy is unsatisfactory. Eretmosaurus was last described by Owen (1865) and is currently classified as Plesiosauria incertae sedis pending revision of the taxon and/or the discovery of more complete material.
After confusion over the holotype material of this species, a formal petition was filed with the ICZN (Brown and Bardet, 1994) to formally allocate NHMUK 14435 as the official holotype. In a case of mistaken identity, Benton and Spencer (1995) discuss ‘Eretmosaurus macropterus’ (e.g. pp.115, 120) but this is actually a species of Microcleidus, as the same authors point out on p. 116 (ibid).