By studying fungal microfossils in 66-million-year-old rock samples from the Denver Basin in Colorado, Johns Hopkins University microbiologists have confirmed that the dinosaur-killing asteroid impact triggered a worldwide fungal takeover, and uncovered a second, previously unknown ecological crisis just before it.
The end-Cretaceous mass extinction was marked by both the Chicxulub asteroid impact and the ongoing eruptions of the Deccan Traps volcanoes.
“Fungal proliferation in geologic samples can signify major ecosystem disruptions,” said Johns Hopkins University researchers Rosanna Baker and Arturo Casadevall.
“Such spikes are documented globally for the Permian-Triassic extinction but for the end-Cretaceous extinction have been reported previously only in New Zealand.”
“In this study, we revisited the question of whether fungi proliferated on a global scale following the end-Cretaceous mass extinction event using samples from North America.”
In the study, the researchers focused on rock samples from several paleontological sites in the Denver Basin, Colorado, and the Williston Basin, North Dakota.
They processed and analyzed the samples to quantify fungal microfossils and distinguish them from pollen and other plant-derived microfossils.
The analysis of the Colorado samples showed a clear spike in the presence of fungal microfossils, relative to plant-derived microfossils, in the strata known to correspond to the asteroid impact.
This represents the first direct confirmation of the New Zealand findings, bolstering the idea that the Cretaceous period ended not just with a bang but also with a global overgrowth of fungi.
In an unexpected finding, the samples bore evidence of an additional, more extended period of fungal prominence approximately 30,000 to 10,000 years before the asteroid impact.
The scientists showed that this corresponded to a known interval of relatively low temperatures at the site — which followed a period of intense volcanism in what is now western India.
“There is other evidence from the fossil record that some species were dying off already at this time,” Baker said.
“It’s plausible that this volcanism in Asia was stressing ecosystems worldwide, essentially setting it up for the final blow when the asteroid struck.”
The analysis revealed another, roughly 2,000-year period of fungal overgrowth of unknown cause in the Early Paleocene epoch, about 10,000 years after the asteroid event.
“Fungi are life forms that often thrive on environmental calamities,” Professor Casadevall said.
The North Dakota samples did not show a fungal overgrowth at the time of the asteroid strike, but the researchers suggest that this may have been due to differences in the type of rock compared to the Colorado samples.
Their analysis of the North Dakota samples corroborated the Late Cretaceous period tens of thousands of years before the asteroid event and Early Paleocene findings about 10,000 years after the asteroid event.
“The results are consistent with the hypothesis, that a proliferation of fungi after the Cretaceous period gave mammals — with their warmer, more fungi-resistant body temperatures — a critical advantage over reptiles, allowing them ultimately to dominate the planet,” the scientists concluded.
The findings appear in the Proceedings of the National Academy of Sciences.
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Rosanna P. Baker & Arturo Casadevall. 2026. Fungal proliferation before and after the Cretaceous-Paleogene mass extinction event in North America. PNAS 123 (20): e2536899123; doi: 10.1073/pnas.2536899123
