The genus Eretomosaurus was erected for ‘Plesiosaurus’ rugosus by Seeley (1874). Eretmosaurus is a rogue taxon in cladistic analyses and researchers have been in disagreement about its taxonomic affinity. Eretmosaurus has been included in several different families: the Rhomaleosauridae based on the anatomy of its girdles (Persson, 1963); the Pliosauridae (Brown, 1981), the Elasmosauridae (Bardet, 1995; Bardet et al. 1999). The latter authors give no specific reason for their classification, but they were using a broad definition of Elasmosauridae that included Lower Jurassic taxa. Benson et al. (2012) included Eretmosaurus in the Microcleididae, a family they erected for several Jurassic long-necked plesiosaurs, some of which were previously classified as elasmosaurids (e.g. Microcleidus). Eretmosaurus was last described by Owen (1865) and is currently classified as a microcleidid pending revision of the taxon and/or the discovery of more complete material.
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 our understanding of plesiosaur postcranial anatomy is unsatisfactory.
Confusion over the holotype material for this species led to a formal petition being 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).
Ammonites stellaris zone, Sinemurian, Lower Jurassic
NHMUK 14435, an almost complete skeleton including an almost complete vertebral column and associated ribs, complete pectoral girdle poorly exposed and complete pelvic girdle with all four limbs complete and in articulation. Skull missing.
To compile. Much material has been allocated to this species based on the rough rugose appearance of the vertebrae, but this is now considered an ontogenetic chatacteristic of no systematic significance.