The Pierre Shale is an upper Cretaceous marine formation, which is well known for its ammonite fossils. Most of the fossils below were found in Boulder and Pueblo Counties. A very complete and up-to-date reference for the ammonites and nautiloids of the Pierre Shale is Ammonites and the Other Cephalopods of the Pierre Seaway, by Neal L. Larson, Steven D. Jorgenson, Robert A. Farrar, and Peter L. Larson (Geoscience Press, Tucson, AZ, 1997). Another good reference, which also includes marine invertebrates other than cephalopods, is E. G. Kauffman, 1977, "Illustrated Guide to Biostratigraphically Important Cretaceous Macrofossils, Western Interior Basin, U.S.A.," The Mountain Geologist v. 14, p. 225-274.
Some photographs of ammonites, including many from the Pierre Shale, can be seen at the website of the "Worldwide Museum of Natural History". Look there to see better specimens than mine.
Click on the links below to view the specimens.
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