This is a section of a Cretaceous Trionyx Turtle Shell. These fragments were found while surface collecting in Cretaceous deposits of the Hell Creek Formation.
The Cretaceous Trionyx Turtle shell was found while prospecting sites within the Badlands of Montana. Because the badlands have little vegetation, when it rains, the rains wash away sediment. So when prospecting bone fragments as well as turtle shell, fish scales and other fossils are exposed. These fragments are from a Trionyx turtle, a soft shelled turtle of the Cretaceous Period. There are also several other kinds of turtles found there so this points to a warm and wet environment. It is possible that the area was a shallow pond or a river. Because gar fish scales are also found nearby, the evidence points to a river system.
Because of the intense sunlight any fossil exposed on the surface will begin to bleach white. Such is the case with this piece of Cretaceous Trionyx Turtle Shell. The normal color of Hell Creek fossil material is a dark brown color. This deposit is from the time of T-rexes and Triceratops dinosaurs. Many fossils have been found in the Hell Creek Formation and most are in exceptional condition. But any fossil on the surface that is exposed to harsh conditions will rapidly begin to decay.
This deposit has been known for many years and fossils such as this turtle shell are continuously being eroded to the surface. Because these shells were found during a surface collecting mission, they are well bleached.