These fossils come from an Inland Sea in the Late Cretaceous that covered a large portion of North America around 90 million years ago. The inland sea deposited a formation found in the western USA known as the Mancos, or Tropic Shale. Numerous types of fossils are found in this formation including, Baculites, Ammonites, Pliosaurs, Turtles, numerous species of Sharks, and a wide variety of crustaceans just to name a few. Below are some of the fossils I've come across in the Tropic.
External Paleo Links:
The Plesiosaur Directroy, Plesiousauria.com
A great resource for information on Plesiosaurs and Pliosaurs.
Oceans of Kansas website
Another great resource for information on Plesiosaurs and Pliosaurs, and a lot more information on the Tropic, Mancos shale.