A few posters got this one right. This is indeed the very tip of the tail of our iconic, resident plesiosaur, Styxosaurus snowii.
SDSM 451 was originally described by Welles and Bump (1949) as Alzadasaurus pembertoni. The specimen was discovered by Arthur Salzmann and excavated by J. Bump and H. Martin in October of 1945 near the town of Iona, SD. The specimen measures thirty-five feet three inches as mounted. One side was nearly complete including skull, vertebrae, girdle elements and flippers, and the opposing side included many complete elements as well. This specimen is the first thing most visitors notice when ascending the staircase to our displays.
Plesiosaurs like Styxosaurus lived during the Jurassic and Cretaceous periods of the Mesozoic Era, or the Age of Dinosaurs. Their remains can be found in marine deposits worldwide, including numerous specimens from Antarctica. They consist of a barrel-shaped body with four robust “flippers”, an elongate neck, skull with numerous long, thin, interlocking teeth, and a reduced tail. While their exact means of locomotion is unknown, it is presumed that they “flew” through the water by means of coordinated fore and aft flipper movements.
Some interesting aspects of the specimen include the presence of numerous gastroliths, or stomach stones, in place upon discovery. The exact function of stomach stones in plesiosaurs is still debated. They may have been used to facilitate physical breakdown of food, or could have helped the animals achieve neutral buoyancy at depth, similar to the function of a diver’s weight belt.
The very tip of the tail was not preserved with the specimen; only four caudal (tail) vertebrae were recovered near the base of the tail. These vertebrae at the very distal end of the caudal series are reproductions based on another specimen. Some additional portions of the skeleton have been reconstructed as well.
Welles, SP, and Bump, JD. 1949. Alzadasaurus pembertoni, A New Elasmosaur from the Upper Cretaceous of South Dakota. Journal of Paleontology. 23(5):521-535