Eretmosaurus Seeley, 1874
The genus Eretomosaurus was erected for 'Plesiosaurus' rugosus by Seeley (1874). Eretmosaurus is something of a rogue taxon as workers have been in disagreement about its taxonomic affinities. Eretmosaurus and was included in the Rhomaleosauridae based on the girdle bones (Persson, 1963); the Pliosauridae (Brown, 1981), and the Elasmosauridae (Bardet, 1995; Bardet et al. 1999). The latter authors gave 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) and suggests that postcranial characters may be unreliable, or that our understanding of postcranial anatomy in plesiosaurs 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 exact holotype material of this species, a formal petition was filed with the ICZN (Brown and Bardet, 1994) so formally allocate BMNH 14435 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).
E. rugosus Owen (1865) (Type and only species)
Type material: BMNH 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.
Referred material: 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.
Age and Location
Ammonites stellaris zone, Sinemurian, Lower Jurassic, Granby, Leicestershire.
Photograph of BMNH 14435 on display in the NHM, London. Because this specimen in mounted high on the wall and covered in glass, it is unfortunately very difficult to see. Photo by Chris Crump, used with kind permission.
Lithograph of BMNH 14435, the neotype of Eretmosaurus rugosus. The lithograph is reversed compared with the fossil material.