In 1918 the paleontologists at the Smithsonian National Museum of Natural History were pretty busy. Rather than go out into the field to collect more specimens, they chipped away at specimens that had already been collected so that they could be put on display. When the famous fossil collector Charles H. Sternberg came calling with a few choice specimens, though, the Smithsonian staff couldn't help but acquire another spectacular skeleton for their display space.
Between about 100 and 65 million years ago, an inland sea cut what is now North America in half. Called the Western Interior Seaway, this body of water was home to some of the most impressive marine predators that ever lived: the mosasaurs. These creatures were more closely related to lizards than to dinosaurs, but they were powerful predators with lower jaws that could bow outwards to engulf particularly large prey. One of the largest genera was Tylosaurus, and in 1919 Sternberg just so happened to have a nearly complete specimen of this ancient marine predator for sale.
The Smithsonian did not have a complete mosasaur at the time and quickly bought the prize. As related by the paleontologist Charles Gilmore in a 1921 piece in Scientific American, the bones were removed from the chalk in which they were embedded, cleaned, and placed into a diving pose on a panel to go on display in the museum's paleontology halls. It can still be seen there today, but some of the associated material from the skeleton is not on display.
In 2004 paleontologist Mike Everhart reported that when Sternberg first found the Tylosaurus skeleton it had the partially-digested remains of a plesiosaur inside the body cavity. This mosasaur had died with bits from a plesiosaur meal still in its stomach! For an unknown reason, however, this was not commented on by Gilmore even though the plesiosaur parts were separated and placed in the Smithsonian collection. Perhaps the Smithsonian scientists did not know it at the time, but they had gotten two "sea monsters" for the price of one (even if there wasn't much left of the plesiosaur!).