In life, 70 million years ago, the five-foot-long animal would have resembled Nessie, the long-necked creature reported to inhabit Scotland's Loch Ness.
The new fossil skeleton is one of the most complete of its type ever found, researchers said. It will go on display Wednesday at the South Dakota School of Mines and Technology's Museum of Geology.
Plesiosaurs lived for millions of years in the then-warm southern ocean surrounding Antarctica, with adults growing as large as 32 feet long. With diamond-shaped fins they could "fly" through the water much as penguins do now.
The National Science Foundation said researchers battled freezing conditions and 70 mile-per-hour winds in recovering the fossil, which was too heavy to be carried out and had to be moved by helicopter.
Leaders of the 2005 expedition that recovered the plesiosaur were James E. Martin, curator of vertebrate paleontology at the museum, Judd Case of Eastern Washington University and Marcelo Reguero of the Museo de La Plata, Argentina.
The researchers said the animal's stomach area was well-preserved, including forked ribs, sometimes into three prongs, and numerous small, rounded stomach stones probably used to help maintain buoyancy or to aid digestion.
The skeleton was found in an area covered with volcanic ash, leading them to speculate that the plesiosaur was killed in an eruption, either by the blast or by ash dumped in the ocean.