The Western Interior Seaway (also known as the Pierre Seaway) covered much of the western interior from approximately 69 to 80 million years ago, during the Cretaceous (late Campanian to early Maastrichtian (Scott, G.R. & W.A. Cobban, 1986 referenced in 1). The Pierre Shale represents deposition from this marine environment. Well-preserved fossils are somewhat rare in the Pierre Shale of Colorado, but a location known as Baculite Mesa near Pueblo is one of the exceptions. The Pierre Shale at Baculite Mesa ranges from fine shades of gray to light beige/tan. The area is loaded with invertebrates such as ammonites, baculites, inoceramus clams, scaphites, pelecypods, gastropods, and nautiloids.
Specific locality information will not be provided here at the request of the private land owners. There is generally no public access to this land though the land owners have been willing to allow some access by paleontological groups or other groups of a scientific/academic nature. One group, the Western Interior Paleontological Society (WIPS), has an annual field trip to the site.
The purpose of these pages is to provide specimen images and identification which may be of use to people studying the Pierre Shale and to future WIPS field trips to the area. Special thanks to Malcolm Bedell Jr. for organizing and leading the field trip and for providing the field trip guide containing great information and references. This guide is the source of much of the information found here.Note: I'm new to identifying invertebrates of the Pierre Shale. Some specimens may have errors in identification. Responsibility for these errors rests entirely on myself, Steve Wagner. I would greatly appreciate any help in accurately identifying these specimens (see, The Fossils section).
For more images from the trip, see The Fossils and Geology & People menu options (below, left).
Baculite Mesa, near Pueblo, Colorado
The preservation of some of the ammonites is quite good. The small specimen below, Placenticeras intercalare, shows great detail in its sutures.
The fossils can be found either in very hard concretions (spherical or oval, example image below), as shown below, or floating freely in the Pierre Shale (example image above). Considerable preparation work is often required to expose fossils found in concretions. Safety glasses and a large hammer are a must when cracking these open! Usually, the fossil will also crack or even shatter into many pieces when the concretion is split. The Denver Museum of Nature & Science (DMNS) has a fantastic Didymoceras specimen on display that came from one of these concretions. When it was split open, the fossil shattered into hundreds of tiny pieces, requiring many hours of careful repair (and lots of glue!).
Unfortunately, many freely floating fossils found are only portions of the original creature, such as the beautiful baculite below. However, with some patience and good eyes, you can sometimes find other portions of the fossil in the surrounding shale or by looking directly above the find or downstream in the creekbed. In the Fossils images, you can see two sections of a nice baculite. I found the first piece at the bottom of a tiny 5" wide ravine in one of the side canyons. I climbed upward searching the tiny ravine and found the second section approximately 25 feet above the first. It's like detective work - you find one piece and must deduce why erosional forces caused it to come to rest there. Then, you try to determine where it must have come from and, with luck, you may find the rest of it. The various baculite species are distinguished by their suture patterns, cross-sectional shapes, degree of taper, size, and surface ornament (Itano, 2001) Also, see ref. 5 for a key to the baculite species of the Pierre Shale.
The huge baculite below is an example of how large these sea-dwelling creatures grew during the Cretaceous. The sections recovered are approximately 26 inches long, yet they are only a portion of the original creature. It's possible that it was once 4-6 feet in length. This specimen was excavated from the wall of a side canyon. Initially, only the oval shape of the large end (left in image below) was exposed. However, it was easy to see the structure present in the visible portion. After about 15 minutes of digging, the rest of the pieces towards the smaller end were recovered. Finally, deterioration had consumed the smallest sections.
The sand dollar, below, was recovered on a previous trip to Baculite Mesa by WIPS member Roger Palmer. Sand dollars are very rare in the Pierre Shale of Colorado.
The image below was taken in one of the many side canyons to the east of Baculite Mesa. A lot of hiking and watchful eyes are required to be successful here. Also, the Pierre Shale erodes to a very loose material. As you can see in the images in Geology & People, it's not uncommon for sections of the wall weighing thousands of pounds to collapse. Great care must be exercised while navigating these canyons. In other places, there are huge holes and overhanging ledges waiting to bury the unsuspecting hiker. It's critical to search these areas with a buddy!
1. Bedell Jr., Malcolm W., 2003, "Baculite Mesa Field Trip Guide", Western Interior Paleontological Society (WIPS).
2. Scott, G.R. and W.A. Cobban, 1986, "Geologic and stratigraphic map of the Pierre Shale in the Colorado Springs-Pueblo area, Colorado". USGS Miscellaneous Investigations Series, Map 1-1627.
3. Tweto, Ogden, 1979, "Geologic map of Colorado: U.S. Geological Survey Special Map."
4. Larson, P.L., N.L. Larson, and R.A. Farrar, 1991, "The Pierre Shale and its macrofauna." Black Hills Institute Geol. Res., Inc., 1-18.
5. Larson, Neal L., Steven D. Jorgenson, Robert A. Farrar, and Peter L. Larson, "Ammonites and the Other Cephalopods
of the Pierre Seaway", Geoscience Press, Tucson, AZ, 1997
6. Kauffman, E.G., "Illustrated Guide to Biostratigraphically Important Cretaceous Macrofossils, Western Interior Basin, U.S.A.," 1977, The Mountain Geologist v. 14, p. 225-274.
7. Itano, Wayne M., "Colorado Pierre Shale Fossils", fossils from Boulder and Pueblo Counties, 2001.
8. Moore, Lalicker, and Fischer, 1953 - "Invertebrate Fossils", Macgraw Hill
9. Clarkson, E.N.K., "Invertebrate Palaeontology and Evolution", 3rd Ed., 1993, Chapman & Hall.
10. Worldwide Museum of Natural History (WMNH), Pan Terra Inc., PO Box 556, Hill City, SD, USA 57745.