The strange contradiction of asteroids going extinct, like the infamous Chicxulub impact that ended the dinosaurs, is that life simply goes on. For example, the terrifying Chicxulub space rock that crashed into the Yucatán with a staggering force of 100 million megatons also simultaneously created a rich marine ecosystem built from life-giving hydrothermal vents.
And now, two researchers who study fungal pathogens have discovered geological evidence of what they call a “global fungal bloom,” which literally spread across the dusty, acrid expanses following the Chicxulub attack. The debris kicked up by this impact, along with other sun-blocking soot eruptions from the volcanoes that formed the Deccan Traps in western India, eventually transformed Earth’s climate into something not unlike a humid basement. Although this environment was terribly inhospitable to most megafauna, it created almost perfect conditions for a global network of fungi to feast on the remains of these dying species.
“Fungi are great degraders of organic matter,” as microbiologists and immunologists Rosanna Baker and Arturo Casadevall write in the preprint of their new study, published Tuesday in the Proceedings of the National Academy of Sciences.
The duo – who have focused in the past on fungal pathogens that afflict people with autoimmune diseases and weakened immune systems – also noted that this sprawling fungal network could have also feasted on the living.
“Mass mortality may not be necessary for fungal proliferation since ecological upheavals may also weaken the resistance of existing species to fungal diseases,” they noted.
The kingdom of mushrooms
Baker and Casadevall turned to lithostratigraphy, the geological study of rock layers stacked over millennia, to prove this theorized global fungal invasion during the so-called Cretaceous-Tertiary (K/Pg) extinction event, about 66 million years ago. Before their research, only one study of ancient spore fossils, excavated from the Moody Creek Mine in New Zealand, had documented strong evidence for this increase in fungi.
The researchers focused on two well-preserved geologic sites in the Denver Basin, Colorado, and the Williston Basin, North Dakota, sampling material related to the Cretaceous, as well as some K/Pg-era “boundary clay” and material deposited in the Paleocene.
They found that most of the layers at Bowring Pit in the Denver Basin were rich in tiny fossil remains of plants and animals, called palynomorphs, except for the K/Pg boundary clay and another surprising Late Cretaceous layer, “where fungal forms made up 50 percent or more of the total assemblage,” the scientists wrote in their study.
According to Baker and Casadevall, this surprise was “correlated with a period of climatic cooling” and “curiously coincided with the Poladpur phase of the Deccan Traps,” a similarly apocalyptic catastrophe that helped accelerate the demise of the dinosaurs.
The pair’s samples collected in the Williston Basin of North Dakota revealed two corroborating fungal spikes before and after the K/Pg boundary layers, samples rich in fungal roots or hyphae, and various fossilized spores.
The lost diapers
Baker and Casadevall attributed their success to a gentler-than-usual approach during their analysis of geologic sediments. Standard sieving for palynomorphs, they write, typically involves the removal of unrelated minerals “with hydrochloric and hydrofluoric acids, oxidation, alkaline treatment, and acetolysis,” all of which can dissolve and remove telltale old fungal traces from field samples.
Instead, the researchers worked with a non-acidic alternative, sodium hexametaphosphate, a compound that reacts in a targeted manner and breaks up clay particles without much damage to the fungal microfossils embedded within them.
“We also omitted the common 10 µm (micrometer) sifting step, which removes amorphous organic matter but also risks losing smaller fungal spores,” they added (a simple but powerful filtering change that anyone who grinds their own coffee can imagine).
Baker and Casadevall hope these updated methods could also help uncover additional episodes of fungal dominance in the fossil record or “additional periods of ecological stress throughout geologic history, including regional rather than global events.”