PLANT COMMUNITY CHANGE ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN NORTHEASTERN MONTANA

The Cretaceous-Paleogene (K/Pg) boundary is marked by a major mass extinction resulting in global biotic turnover, notably the extinction of non-avian dinosaurs. The Hell Creek Area in northeastern Montana contains some of the most well-studied vertebrate localities recording this mass extinction; however, very little is known about floral changes in this area. As part of an effort to reconstruct floral changes across the K/Pg in northeastern Montana, this study presents data from a sequence of macrofloral assemblages across the K/Pg boundary. Four latest Cretaceous and three earliest Paleogene plant macrofossil localities from Garfield and McCone Counties, Montana, are described and placed in a high-resolution chronostratigraphic framework spanning approximately 2 m.y. around the K/Pg boundary. Each site preserves leaves, reproductive structures, as well as stems and other vegetative structures. These fossils are preserved in siltstone and sandstone lithologies, interpreted as riparian depositional environments. Our results indicate that many of these assemblages have taxa in common (e.g., Metasequoia occidentalis, Glyptostrobus europaeus, and “Dryophyllum” subfalcatum), but overall preserve distinct plant communities through time. In particular, there is a large turnover in plant taxa between the latest Cretaceous and earliest Paleogene communities, supporting the interpretation that environmental changes at the K/Pg boundary also led to major regional extinction among plants. In comparing this series of assemblages with the well-studied Williston Basin macrofloral sequence in North Dakota, we find that many common taxa (e.g., Metasequoia occidentalis, “Dryophyllum” subfalcatum, Leeierceia preartocarpoides, “Populus” nebrascensis) are shared across these study regions, but also note differences in floral composition. In particular, many Cretaceous angiosperm taxa recovered from our Montana sites appear to be unique, pointing to geographic heterogeneity in forest make-up. This record of plant communities leading up to the K/Pg boundary lends insight into the dynamics of Late Cretaceous ecosystems, plant community composition change across the K/Pg boundary, and variation between local and regional floras of the northern Great Plains region.