MIDDLE CAMBRIAN FOSSILS OF SOUTHWESTERN MONTANA IN A STRATIGRAPHIC CONTEXT

The middle Cambrian Flathead Sandstone, Wolsey Shale, and Meagher Limestone are exposed throughout southwestern and central Montana. This succession is Wuliuan (509–504.5 Ma) in age, and spans the Albertella, Glossopleura, and Ehmaniella trilobite biozones. Despite being coeval and proximate to the Burgess Shale, relatively few paleontological studies have been conducted on these units. Each formation contains variable depositional settings spanning both carbonate and siliciclastic environments (e.g., tidal flat, shoreface, shelf, deep-subtidal). It has been recognized that fossil preservation in these units is highly variable and the controlling processes are not well understood. Some areas with soft-bodied preservation would be classified as a Tier 3 Burgess Shale Type deposit, while others preserve no fossil material. This variation has been previously explained as a biologic signal with organisms only living in their preferred environments, or as a taphonomic process where organisms are preserved only where environmental conditions allow. New stratigraphic sections and fossil assemblages from the Big Belt Mountains and Bridger Range in southwestern Montana can shed light on how preservation changes with geographic location and depositional settings. There are multiple observed taphonomic modes in these units, with the most abundant material coming from near the Wolsey Shale-Meagher Limestone contact. With this new stratigraphic framework, we can better understand the distribution of fossil materials in the formations, and whether variable preservation represents taphonomic processes, ecophysiological constraints, or other processes.