The First Plants to Recolonize Western North America Following the Cretaceous-Paleogene Mass Extinction Event

Premise of research. A two-phase fern spike occurred immediately after the Cretaceous-Paleogene (K/Pg) mass extinction event. Solely on the basis of palynological evidence, researchers have traditionally attributed the first phase of this spike to the proliferation of a single species of a Cyathea-like fern in the earliest Danian. This traditional perspective is challenged by recent investigations linking Anemia-like fern foliage with Cyathidites spores at K/Pg boundary localities in the Raton Basin, where the fern spore spike was first discovered by R. H. Tschudy. Although evidence emerging from neighboring basins appears to corroborate this new perspective, it remains to be seen whether this generalization applies to the northern Great Plains.Methodology. A comprehensive list of the most common fern megafossils collected from earliest Danian plant localities across western North America was compiled to determine the number and potential identity of Cyathidites-producing ferns.Pivotal results. Three K/Pg survivors commonly collected from basal Paleocene strata in western North America conceivably produced psilate, trilete (Cyathidites) fern spores essentially identical to those observed at the K/Pg boundary fern spore spike: Anemia elongata (Newberry) Knowlton, Dennastra sorimarginata McIver et Basinger, and the Coniopteris-like fern “Dennstaedtia” americana Knowlton.Conclusions. Arborescent ferns were not among the first plants in western North America to thrive in the immediate wake of ecological collapse at the K/Pg boundary. Basal polypod ferns can be linked to Cyathidites spores in the northern Great Plains, perhaps because of a latitudinal climatic gradient. These results could explain taxonomic inversion in the dual-phase fern spore spike between western North America and New Zealand.