HELL CREEK AMBER: A NEW FOSSIL ARCHIVE FOR LATEST CRETACEOUS TERRESTRIAL ECOSYSTEMS PRIOR TO THE K-PG BOUNDARY

The uppermost Maastrichtian Hell Creek Formation (HCF) is the most heavily studied terrestrial section yielding paleobiological and paleoecological information leading up to the Cretaceous-Paleogene extinction. While the HCF preserves a diverse terrestrial biota, this record of a fluvial system also suffers from a taphonomic size bias in the preservation of complete body fossils. Noted, but largely overlooked in the last century of study, are the fossil resins that can be found throughout the HCF, particularly in the lignite and carbonaceous shales. The lignites are the remains of swampy peat mires that have historically yielded few fossils. Furthermore, inclusions in amber represent organisms that were sampled in-situ, in their palaeohabitat. This is in contrast to sedimentary fossil deposits of the HCF, in which the fossils can be allochthonous. This study reveals that the HCF amber is inclusion rich and can provide a record of well-preserved fossils prior to the K-Pg boundary. Amber occurs throughout the HCF in a variety of taphonomic modes. Of interest to this study are the in-situ blebs and runnels of amber that occur in the lignite beds in the Upper HCF. Although small (less than 3 cm in diameter) these resins are inclusion rich, yielding an average of 10 organismal inclusions per 250 grams. This study reveals remarkable soft-tissue preservation and new records of insects from the Coleoptera, Diptera, and Hymenoptera.

The recognition and collection of exceptionally preserved soft-tissue fossils from the HCF will provide insights into the paleohabitat of subtropical to warm-temperate swamps prior to the K-Pg extinction. The lignites of the conformably overlying Paleogene Fort Union Formation also bear amber, providing potential for preservation of a record of recovery within marshland ecosystems across the K-Pg mass extinction. Results from this ongoing research will be greatest for the record of insects, which occur with the highest frequency in the Hell Creek ambers but have an extremely poor body fossil record in other sediments at the K-Pg boundary.