relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to Plesiosaurus. None of the younger Jurassic or Cretaceous species belong to Plesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned to Plesiosaurus is P. dolichodeirus. Several other European Early Jurassic species have been assigned to new genera. P. brachypterygius, P. guilielmiimperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion, Seeleyosaurus and Occitanosaurus. The following cladogram follows an analysis by Benson et al., 2012, and shows the placement of Plesiosaurus within Plesiosauria. Stratigraphy Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset. It appears to be the most common species of plesiosaur in the Lias Group of England. Plesiosaurus is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the Charmouth Mudstone; using the Lias Group ammonite fossil zones, these rocks date to the early Sinemurian stage. Some other Plesiosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian or early Hettangian rocks. Paleoecology Plesiosaurus fed mainly on clams and snails, and is thought to have eaten belemnites, fish and other prey as well. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. Plesiosaurus gave live birth to live young in the water like sea snakes. The young might have lived in estuaries before moving out into the open ocean. It has been postulated that the long neck of Plesiosaurus would have been a hindrance when trying to speed up, any bend in the neck creating turbulences. If that is the case then Plesiosaurus would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them. See also Timeline of plesiosaur research List of plesiosaur genera Loch Ness Monster References Notes Sources Andrews, C. W. 1896. "On the structure