Size and Weight:
During the latter part of the Mesozoic Era, plesiosaurs and pliosaurs (a populous family of marine reptiles) roamed the Sundance Sea, a shallow body of water that covered much of central and western North America. That explains the discovery of a huge, 35-foot-long Styxosaurus skeleton in South Dakota in 1945, which was given the name Alzadosaurus until it was realized to which genus it actually belonged.
Interestingly, this South Dakotan Styxosaurus specimen came complete with over 200 gastroliths--small stones this marine reptile deliberately swallowed. Why? The gastroliths of terrestrial, herbivorous dinosaurs aided in digestion (by helping to mash up tough vegetation in these creatures' stomachs), but Styxosaurus probably swallowed these stones as a means of ballast--that is, to enable it to float near the sea bottom, where the tastiest food was.