"BATTLE BETWEEN ICHTHYOSAURS AND ELASMOSAURS.The icthyosaurus which was similar to a dolphin in appearance, was one of the most feared marine reptiles; it had undoubtedly adopted the most favourable shape for moving in the water. The icthyosaurs, which were about 8 meters long and the owners of 200 very sharp teeth, could attack even the elasmosaurs successfully."
One of the larger elasmosaurid plesiosaurs, Hydrotherosaurus reached lengths of around 13m (42ft 7in). And, like the unrelated Tanystropheus, about half of that was neck — although Hydrotherosaurus had 60 cervical vertebrae compared to Tanystropheus' 10-12.
Although often depicted with swan-like or snake-like highly flexible necks, elasmosaurs weren’t actually capable of such extreme postures. Their necks seem to have been relatively stiff, and may have functioned as rudders — turning the head towards something would cause the whole body to move in that direction.
The lifestyle of these animals is still a little unclear. They’ve been proposed as “spear fishers”, sticking their long necks and heads into schools of fish to snatch up prey before the main bulk of their bodies became visible. Stomach contents of seafloor crustaceans and molluscs, along with trace fossils of odd gutter-like gouge marks, however, raise the possibility of elasmosaurs instead scooping up mouthfuls of prey-rich sediment.