Geologic Era:  Eocene
Location Found:  Green River Formation (Utah)
Size:  Undetermined, possibly up to 1.7 meters in length

This is a fragment from a freshwater turtle shell.  On close examination, one can see the wave-like pattern that tends to be seen with water dwelling turtles as opposed to tortoises.  My best guess is that this piece would have come from somewhere on the carapace, or upper shell of the turtle.  I am not able to assign a genus or species to this fragment, though several species of turtles have been documented in the Green River Formation. 

I feel that our specimen is more similar to the first turtle shown.  The wave like pattern on the shell looks similar, and the curvature of our piece indicates that it came from a relatively large animal.  The particular turtle pictured was 1.7 meters in length, which would dwarf any modern freshwater turtle.

Image Credits:

Green River Turtles:  http://geology.com/articles/green-river-fossils/animal-fossils.shtml  (photos listed as originally being from the National Park Service)