AMBER OF THE HELL CREEK FORMATION: A RECORD OF PRE-K-PG ECOSYSTEMS IN SOUTHEASTERN MONTANA

Well-studied amber deposits throughout the Cretaceous have yielded important data about Mesozoic insects. However, approaching the Cretaceous-Paleogene Boundary there is a considerable gap in sampling for insect body fossils, largely due to the absence in deposits conducive to their preservation. Therefore, any research towards late Maastrichtian insect fossils is crucial in understanding extinction dynamics in insect communities of the past and future. One such sampling gap is in the amber deposits of the Hell Creek Formation (HCF), largely overlooked because of the amber’s notorious fragility and small specimen sizes. This study aims to explore amber deposits of HCF, develop an amber conservation methodology, and to help fill in this crucial lapse in the history of one of our planet’s most ecologically important groups. Multiple HCF amber deposits in Southeastern Montana were sampled and specimens were conserved using epoxy embedding. HCF amber preserves in multiple lithologies and taphonomic modes including lower energy mudstones, lignites, and higher energy siltstones possibly preserving transported specimens. Carbonaceous mudstone deposits preserving original exudation resin morphologies may also indicate autochthonous deposits, a rarity in global amber study. Sampling from these sites also revealed greater diversity in amber specimen sizes, with sizes ranging from a few millimeters to 30 centimeters in length. It was found that epoxy embedding was a necessary step in conserving samples for study, prior to inclusion surveying. Optical microscopy and micro-computed tomography (micro CT) were used to study inclusions. Often arthropod inclusions have translucent limbs or are completely translucent, impeding surveying and definitive identification of taxa. Micro CT scans revealed preservational diversity in arthropod inclusions: some inclusions are preserved as endocasts with partial cuticular preservation, while others appeared to be infilled with resin showing almost no sign of cuticular preservation. This amber captures multiple orders of insects, as well as feathers, plant matter, and possibly fungal material. This diverse assemblage of deposits may have implications in understanding amber taphonomy, Late Cretaceous biodiversity, and extinction dynamics.