Eretmosaurus

Eretmosaurus is an extinct genus of plesiosaur from the Early Jurassic Blue Lias of England. Only the type species is known, which is E. rugosus.

History
The holotype consists of several vertebrae that were discovered in the Blue Lias in Gloucestershire. Owen (1840) was the first to describe the specimen, which was assigned to Plesiosaurus rugosus.

Later, Owen (1865) described a headless skeleton discovered in the Ammonites stellaris zone of the Blue Lias at Granby, Nottinghamshire (NHMUK 14435) that he assigned to P. rugosus, and Seeley used NHMUK 14435 as the basis for describing and naming Eretmosaurus rugosus in 1874.

A petition was filed with the ICZN over the holotype by Brown and Bardet (1994), and NHMUK 14435 was allocated as the official neotype in 1996.

Benton and Spencer (1995) mentioned a second species of Eretmosaurus: E. macropterus; they rectified this mistake within the same paper by mentioning on page 116 that E. macropterus actually belongs to Microcleidus.

Classification
Eretmosaurus was classified into Rhomaleosauridae by Persson (1963), then into Pliosauridae by Brown (1981), then into Elasmosauridae by Bardet (1995) and Bardet et al. (1999), and most recently into Microcleididae by Benson et al. (2012).