On the Rocks Field Trip: The Kremmling Giant Ammonite Locality

Title: On the Rocks Field Trip: The Kremmling Giant Ammonite Locality

Trip Leader: Emmett Eavanoff, Researcher, University of Colorado Museum

Date: Saturday, June 24, 2000

Publication: The Outcrop, May 2000, p. 11

The Kremmling giant ammonite locality is remarkable for the abundance of huge shells of the ammonite Placenticeras costatum. Most ammonite shells are less than a foot in diameter, but the Kremmling site has over one hundred shells up to 3 ft. in diameter. Most of the shells have either been eroded or collected from the site, but large concretions containing molds of the shells are scattered all over the site.

The ammonites are found with an exceptionally diverse normal-marine biota of over 60 species of rare tropical and subtropical forms, including snails, clams, nautiloids, and other ammonoids. The site also contains the bones of marine fish and reptiles and shipworm-bored wood. The site occurs in the boundary between the Baculites compressus and Baculites cuneatus zones of the Pierre Shale. It is in a sand sheet that was offshore to a major delta situated to the northwest of the site.

Placenticeras shells include abundant, large, smooth-shelled forms and less abundant, small, ribbed forms, representing female and male shells. The large (female) shells are typically complete with intact living chambers and complete early whorls. The concentration of female shells may indicate that the site was a brooding area where females came to lay eggs; modern female squid and octopi typically mate, lay their eggs in a specific area, then die there. Larger shells typically have broken margins or large holes in the living chambers that are interpreted as shell breaks from crabs or lobsters that scavenged the ammonite body. A variety of organisms lived on or within the empty shells, including limpets, oysters, polychaete worms and a variety of burrowing animals.

The field trip will meet at 8:30 a.m. in the 6th and Simms Park-n-Ride, located on the south side of the West 6th Ave. and Simms exit. Participants are responsible for their own food and drink, as well as dressing appropriately for weather and field conditions. There is a quarter mile hike up a steep hill to the site! Plan to return to the Park-n-Ride by approximately 5:00 p.m. For reservations call Jeanette Dubois. Trip Limit: 20 participants.

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