Application of a novel schema for describing fossil fern foliage and investigating taxonomic and morphological change across the Cretaceous–Paleogene boundary in western North America

PREMISE: The Chicxulub asteroid impact at the Cretaceous-Paleogene (K-Pg) boundary triggered a mass extinction 66 million years ago. Ferns thrived in the aftermath of the extinction, evidenced by the well-documented "fern spore spike." However, fern macrofossil records across the K-Pg boundary are less understood. METHODS: We analyzed 698 fern macrofossils from 19 Cretaceous and 83 Paleogene localities in the Raton (Colorado and New Mexico), Denver (Colorado), and Williston basins (North Dakota and Montana), grouping specimens into morphotypes using a newly developed descriptive schema. We examined familial affiliations and documented taxonomic and morphological turnover across the boundary. Additionally, herbivory and pathogen damage were categorized on each specimen. RESULTS: We identified 33 morphotypes: eight were found only in the Cretaceous, 12 in both the Cretaceous and Paleogene, and 13 only in the Paleogene. Two fern families were in the Cretaceous only, nine in both the Cretaceous and Paleogene, and five in the Paleogene only. Taxa that occur in both the Cretaceous and Paleogene occupied less morphospace than taxa that occur only in the Cretaceous or the Paleogene. Sixty-three specimens had herbivory or pathogen damage; one damage type was found in the Cretaceous only, nine in both the Cretaceous and Paleogene, and seven in the Paleogene only. CONCLUSIONS: Our data indicate possible changes in morphotype composition, family representations, morphological disparity, and insect herbivory across the K-Pg boundary. This research is important for understanding K-Pg recovery dynamics, fern adaptation, and their relation to deep time and modern catastrophic events.