While visiting the Capitol Reef National Park I stopped off at a site outside of the park to collect some prolific oyster fossils. They are Pycnodonte newberryi and come from the Mancos shale. the shells are generally small but when found they cover large areas. They are considered rudists by some and this is a good example of how they could form reef like structures. Here is a typical specimen with both valves.
Left valve top
Left valve anterior
Left valve left side
Left valve posterior
Left valve right side
Left valve interior
Right valve profile (life position?)
The Mancos shale is Cretaceous in age and is dated to the late Cenomarian stage. The site these were found at is outside of the park boundaries on federal BLM land.