Plesiosaur or pliosaur?
Pliosaur remains are more common in the older Jurassic Oxford and Kimmeridge Clays than Portland Group rocks. Indeed, pliosaurs are no longer considered to be found in the Portland Limestone, so this is almost certainly a plesiosaur vertebra. The Portland Limestone has preserved some of the fine structural details of the vertebra.
Find out more on the reptiles pages.