CENSUS STUDY OF FOSSIL PLANTS, INSECTS AND INVERTEBRATES IN THE LOWER SHALE UNIT OF THE FLORISSANT FORMATION OF COLORADO

A section of the commercial Florissant Fossil Quarry was donated to enable Florissant Fossil Beds National Monument, the Denver Museum of Nature & Science, and a local school district to conduct a detailed excavation that provides a taxonomic census of the lower shale unit of the Florissant Formation. The upper and middle shale units of the Florissant Formation are now part of the Monument. Because the lower shale unit is not exposed within the Monument, this study is providing new information on plant, insect, and fresh water gastropod and mollusk diversity. A 1.5 m thick stratigraphic section was collected using 10 cm thick aggregate sampling intervals over a surface area of approximately 1 m². The 10 cm sections were chosen to record high-resolution changes in diversity, abundance, and composition of fossil material between sampling intervals. This effort has been compared with floral census data published in MacGinitie’s 1953 Florissant monograph. Our taxonomic abundance data differ from that of MacGinitie’s four census quarries, indicating a much lower population of Fagopsis and a much higher population of Chamaecyparis. Fluctuations in composition of plants and insects as well as deeper water invertebrates have been observed in the 10 cm sections, possibly signifying variation in the position of the lake margin and water depth. This change in composition is easily observed above and below a thick tuffaceous mudstone layer that lacks fossils. Below this marker, plant fossils, including the relatively abundant "Typha," are concentrated in distinct layers, possibly indicating a near-shore depositional environment. Above this marker layer, we noted that fresh water gastropods and mollusks dominate, which is similar to MacGinitie’s locality 4. We also noted that the floral diversity and sedimentary facies below this mudstone marker layer are similar to MacGinitie’s localities 1-3.