Size and Weight:
To show how different the late Cretaceous period was, geologically speaking, from modern times, the first fossil of the plesiosaur Elasmosaurus was discovered in 1868 in landlocked Kansas--not the first place you'd think to dig up a marine reptile. The fact is that, 70 million years ago, much of North America was submerged beneath the relatively shallow Western Interior Sea, which has yielded countless remains of icthyosaurs, plesiosaurs and mosasaurs.
Elasmosaurus was distinguished by its enormously long neck: this plesiosaur had 71 vertebrae, compared to as little as 28 for earlier members of the breed. This unusual feature has occasioned some disagreement about how Elasmosaurus hunted for fish: some paleontologists think it bent its head sideways around its body, while others believe this reptile swam on the surface, holding its head high above the water to scope out prey.
By the way, Elasmosaurus occasioned one of the pettier disputes in 19th-century paleontology, the Bone Wars, which started with a Cretaceous bang when the famous fossil-hunter Edward Drinker Cope mistook this plesiosaur's long neck for its tail--and placed the head on the wrong end. Cope's rival Othniel C. Marsh none-too-graciously pointed out the error, and the two paleontologists spent the rest of the 19th century in a bitter feud.