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Middle Cambrian Trilobites of the Horseshoe Hills, Montana: Taxonomy and Taphonomy

Hunter C. Olson, and Dr. David Varricchio, Earth Science Department, Montana State University, 226 Traphagen Hall, Bozeman, MT 59717-3480

Although Cambrian outcrops are extensively exposed in western Montana, relatively few studies have been conducted on the trilobites from these strata. The last thorough description of the trilobites of Montana was published by Charles F. Deiss in 1939 and focused on specimens from the northwestern part of Montana. Since that publication, many taxa have been synonymized or reclassified. Here, I present new information on the trilobite fauna known from the Middle Cambrian rocks near the town of Manhattan in southwestern Montana. The composition of 174 samples in the paleontology collection at the Museum of the Rockies, Montana State University includes three orders (Corynexochida, Agnostida, Ptychopariida) and four families of trilobites (Dolichometopidae, Dorypygidae, Alokistocaridae, Ptychopariidae). These trilobites, though typically found as shed elements, are generally well preserved, facilitating identification. The quality of preservation and coloration of these trilobite shells exhibits some variation due to mineral replacement. In addition to trilobites, brachiopods, a scenellid mollusk and one sponge are also represented. These trilobites and other organisms were deposited on a shallow marine shelf, on the western margin of the North American craton. The dataset includes species indicative of the Bathyuriscus-Elrathina biozone and are comparable to those of the Wheeler Shale in Utah, the Spence Shale in Utah and Idaho, and the Burgess Shale in British Columbia. As Montana is situated between the well-explored Middle Cambrian strata of northwestern Canada and the southwestern United States, these specimens are crucial to understanding faunal and biogeographical transitions between these regions.




Additional Abstract Information

Presenter: Hunter Olson

Institution: Montana State University Bozeman

Type: Poster

Subject: Geography/Geology

Status: Approved


Time and Location

Session: Poster 7
Date/Time: Tue 3:30pm-4:30pm
Session Number: 5068