Its neck was so long that when it was discovered, it was thought to be the tail, leading American Palaeontologist Edward Drinker Cope to humiliate himself when he published a reconstruction of the great marine reptile with the neck being the tail and the head being on back-to-front on the end of the tail - which his archenemy Othneil Charles Marsh picked up on when he got to handle the specimens himself and realised (much to his amusement at the expense of his great rival of the 'Bone Wars' no doubt)
Thanks to Othneil Charles Marsh, we have an accurate reconstruction of the huge 14 metre long Elasmosaurus - last and largest of the Plesiosauroid Plesiosaurs. There is a similar species of almost the same size as Elasmosaurus called, Thalassomedon, from a slightly earlier part of the Late Cretaceous (the two cousins probably met)
An Elasmosaurus appears in the main Walking With Dinosaurs series.
Elasmosaurus was one of the hazards in Hell's Aquarium (Nigel's nickname for the Most Dangerous Sea of all time), in Sea Monsters: A Walking with Dinosaurs Trilogy Even so, they seem to be benign and passive creatures, gently migrating through the sea; however, this could be due to their task of migrating and nothing else. Still, as their primary prey is small fish, there is no apparent reason for them to attack the humans (though one is seen to investigate the ROV Camera with its jaws open)