FOSSIL FISH SCALES FROM THE CENOMANIAN (UPPER CRETACEOUS) COLERAINE FORMATION OF NORTHERN MINNESOTA: INSIGHTS FROM INDUSTRIAL MICROSCOPY

The Cenomanian (Upper Cretaceous) Coleraine Formation exposed on the Mesabi Iron Range in northern Minnesota contains sediments and fossils from the Western Interior Seaway and its shores. It was known to contain fossil fish material since shortly after the first mining operations cut through it in the early 1900s. Historically the focus of research was on the diverse invertebrate material with other fossil types being noted but not heavily studied. The Hill Annex Paleontology Project (HAPP) has collected fossil fish scales of teleost fish from iron mining spoil piles of the Coleraine Formation at Hill Annex Mine State Park (Calumet, Minnesota). Microscopes used in Minnesota’s medical device industry have capabilities that exceed what is typically available for paleontological research. These microscopes are being utilized through citizen science to study all fossil types collected by HAPP. Applying industrial microscopy to Coleraine Formation material presents an opportunity to discover microfossils from multiple taxonomic groups and study the features of macrofossils. Original fossil fish scale material and scale impressions in iron-rich sandstone were imaged using high magnification optical and laser confocal microscopes. Scale structures such as growth rings and surface ornamentations on the order of 10 micrometers were measured and 3D scanned using the surface metrology features of these microscopes. Original scale material was also tested for elemental composition using SEM-EDS. Elemental analysis is consistent with the expected elemental composition of fish scale hydroxyapatite. Hydroxyapatite could eventually be tested for oxygen isotope composition, which is correlated to habitat and climate. The fish scales of the Coleraine Formation may provide an additional avenue for learning about the extinct taxa and paleoenvironment of this part of the Western Interior Seaway.