The asteroid impact thought to have wiped out the dinosaurs 66 million years ago was followed by surges of fungi in North America, according to a new paper by Johns Hopkins Bloomberg School of Public Health researchers.
The findings , based on analyses of microfossils in rock samples from the Denver Basin in Colorado, are the first to place fungal proliferation in North America following the Cretaceous Period that ended with the asteroid impact. These are also the first findings to provide direct evidence that this post-asteroid fungal bloom may have been a global event. A 2004 study by another research team reported a spike in fungal microfossil prevalence in rocks from New Zealand from the same era.
In the new study, the researchers also found signs in the same set of samples of an extended fungal bloom tens of thousands of years before the asteroid impact. This coincides with intense volcanic activity in what is now India and supports the idea that volcanism was a factor in the mass extinctions in that period.
Fungal surges are presumably due to the availability of dead plants and animals as food sources following disasters and disruptions.
The findings overall reinforce the idea that fungal microfossils can be useful markers for ancient ecosystem upheavals.
The study was published online May 12 in the Proceedings of the National Academy of Sciences .
“If you ask most people what killed the dinosaurs, they’ll say it was that asteroid, but our fungal microfossil-based results suggest that the world already had been undergoing a cataclysm when the asteroid struck,” says study senior author Arturo Casadevall , MD, the Alfred and Jill Sommer Professor and Chair of the Department of Molecular Microbiology and Immunology at the Bloomberg School.
The study’s first author was Rosanna Baker , MSc, an assistant scientist in the Casadevall Laboratory.
Prior research in this field—which Casadevall calls “disaster microbiology”—found that a global fungal bloom accompanied the greatest known mass extinction in Earth’s history, terminating the Permian Period 252 million years ago. There has been little direct evidence of a similar global fungal proliferation nearly 200 million years later before these new findings.
For the study, Casadevall and Baker worked with rock samples from paleontological sites, including a site near Denver, that were provided by paleontologist Tyler Lyson, PhD, of the Denver Museum of Nature and Science. Baker 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 from a different site, 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 researchers 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 says. “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,” Casadevall says.
The researchers also analyzed samples from two paleontological sites in North Dakota. 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 study results are consistent with the hypothesis, first proposed by Casadevall in 2005 and subsequently refined in later publications, 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.
“ Fungal Proliferation Before and After the Cretaceous–Paleogene Mass Extinction Event in North America ” was co-authored by Rosanna Baker and Arturo Casadevall.
Casadevall was supported in part by the National Institutes of Health (R01-HL059842, R01-AI152078, and R01-AI052733).
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Proceedings of the National Academy of Sciences