A study published in May 2026 in the Proceedings of the National Academy of Sciences reports a finding that complicates the standard one-line version of the dinosaur extinction. Two researchers at the Johns Hopkins Bloomberg School of Public Health, microbiologists Rosanna Baker and Arturo Casadevall, examined sediment samples from three well-documented sites that span the boundary between the Cretaceous and Paleogene periods, the moment around 66 million years ago when the Chicxulub asteroid struck Mexico’s Yucatán Peninsula and triggered the global extinction event that ended the age of non-avian dinosaurs. They found three distinct spikes in fungal abundance in the rock. Two were after the impact, as expected. One was before. The pre-impact spike, dated to roughly 30,000 to 10,000 years before the asteroid struck, suggests that the dinosaurs’ world was already under significant ecological stress when the rock arrived.
What fungi can tell you about a dying ecosystem
The reason the researchers were looking at fungi at all is that fungi tend to flourish when ecosystems collapse. Fungi are decomposers. When plants and animals die in large numbers, fungal populations expand to feed on what is left behind. Major fungal blooms have been documented in the rock record after the Permian-Triassic mass extinction 252 million years ago, the largest known extinction in Earth’s history. That post-Permian bloom was global and is one of the standard signatures palaeontologists use to identify ecological collapse. Casadevall has called this field “disaster microbiology.”
Until recently, evidence of a similar post-impact fungal bloom following the Chicxulub asteroid had been limited to a single site in New Zealand. The Baker-Casadevall study set out to test whether the pattern appeared elsewhere. They analysed rock from three sites in the western interior of North America: the Bowring Pit in the West Bijou study area in the Denver Basin, Colorado, roughly 2,500 kilometres from the Chicxulub crater, and the Mud Buttes and John’s Nose sections in the Williston Basin in North Dakota, roughly 5,500 kilometres from the crater. The samples were provided by Tyler Lyson, a palaeontologist at the Denver Museum of Nature and Science.
According to the Johns Hopkins news release, the Colorado section showed a clear spike in fungal microfossils in the strata corresponding to the asteroid impact itself, in the boundary clay layer. The North Dakota sections showed spikes before and after the impact but not a distinct spike in the boundary clay, attributed to rock-type differences between the two regions. A second, sustained fungal spike appeared in the strata laid down between roughly 2,000 and 10,000 years after the boundary across both regions. Together, the results corroborate the New Zealand finding and support the interpretation that the post-impact fungal bloom was global.
The earlier spike, and what it might mean
The unexpected result was the third spike. In the Late Cretaceous sediments laid down before the boundary, the researchers found a prolonged interval of elevated fungal abundance, dated to roughly 30,000 to 10,000 years before the impact. The PNAS paper describes this earlier spike as “temporally correlated to a period of climatic cooling at the site and intriguingly coincident with the Poladpur phase of the Deccan Traps.”
The Deccan Traps are one of the largest volcanic provinces on Earth, formed over roughly a million years through repeated flood basalt eruptions in what is now western India. The eruptions began roughly 400,000 years before the Chicxulub impact and continued for several hundred thousand years afterwards, releasing enormous quantities of carbon dioxide, sulfur dioxide, and other climate-altering gases into the atmosphere. How much they contributed to the end-Cretaceous extinction, alongside the asteroid, has been a long-running debate in palaeontology.
The authors are careful about the chain of inference. The fungal spike is a signature of ecological disturbance, not a direct measurement of the cause. The temporal coincidence with the Poladpur eruption phase is suggestive of a volcanic driver but does not establish causation. As Baker and Casadevall write in the paper, the pre-impact fungal episode “suggests ecological upheaval occurring tens of thousands of years before the bolide impact, which may have contributed to the Cretaceous-Paleogene extinction event.”
The Chicxulub impact remains the proximate cause of the mass extinction event. What the new study supplies is independent biological evidence, in the form of fungal microfossils preserved in sedimentary rock, that the biosphere the asteroid hit was not in robust health at the moment of impact. Plant communities were stressed, decomposition was elevated, and the climate had cooled in the run-up to the asteroid’s arrival.
