Did Fungal Infections Help Mammals Survive the Dinosaur Extinction?

When the Chicxulub asteroid struck Mexico’s Yucatán Peninsula 66 million years ago, it triggered one of Earth’s most devastating mass extinctions and ended the reign of the nonavian dinosaurs. As ecosystems recovered, small mammals expanded into newly available habitats, evolving larger bodies, broader diets and more varied lifestyles.

However, new research suggests that Earth’s ecosystems may already have been under severe environmental stress before the asteroid impact. The evidence points to major fungal blooms both before and after the Cretaceous-Paleogene (K-Pg) mass extinction.

A study published May 12 in PNAS examined microscopic fossils preserved in North American rocks from around the time of the extinction. A research team led by Dr. Arturo Casadevall, chair of the Molecular Microbiology and Immunology Department at Johns Hopkins University, identified a fungal bloom approximately 30,000 to 100,000 years before the asteroid hit.

This earlier fungal increase coincided with a period of environmental disruption and cooling associated with intense volcanic activity in the Deccan Traps of what is now India. The researchers also found a second, much larger fungal spike immediately after the Chicxulub impact, when dead plants and animals would have covered devastated ecosystems.

The discovery supports earlier evidence from New Zealand suggesting that post-impact fungal growth was not limited to one region. Instead, fungal blooms may have occurred across much of the planet as dead organic matter accumulated and food webs collapsed.

What Is the Fungal Infection-Mammalian Selection Hypothesis?

The findings have renewed interest in a controversial idea known as the fungal infection-mammalian selection, or FIMS, hypothesis.

First proposed by Casadevall roughly two decades ago, the hypothesis does not claim that fungal infections directly wiped out the dinosaurs. Instead, it suggests that mammals may have gained an evolutionary advantage in the harsh conditions that followed the asteroid impact.

According to the theory, mammals’ live births, stable body temperatures and sophisticated immune systems may have helped them cope with an environment filled with decay, cold, starvation, polluted air and potentially harmful fungal spores. Egg-laying reptiles and many nonavian dinosaurs may have faced different vulnerabilities, particularly as their eggs and respiratory systems interacted with fungus-rich environments.

“The fungal infection mammalian selection hypothesis is focused on how the mammals became the dominant land animals,” Casadevall told Live Science via email, “not dinosaur extinction.”

Not all experts accept this explanation. Mary O’Connell, chair of the Zoology Department at the University of Manchester, told Live Science that the theory “emerges from a chain of plausibility arguments rather than direct evidence, and the links in the chain are weaker than the framing may suggest.”

Evidence of a Fungal World Before the Asteroid

To investigate ancient fungal activity, Casadevall and fungal researcher Rosanna Baker examined palynomorphs from the Denver Basin in Colorado.

Palynomorphs are microscopic fossilized organic remains, including fungal spores, fungal hyphae and plant pollen. These remains can survive in ancient sediment layers, providing scientists with clues about the organisms and environmental conditions present millions of years ago.

Baker analyzed between 100 and 500 microfossils from each sample. The sediment layers represented a period beginning approximately 60,000 years before the K-Pg extinction and extending to around 30,000 years afterward.

Most samples contained mainly plant material. Some layers, however, contained fungal spores representing at least half of the preserved microfossils. The researchers interpreted these unusually high proportions as signs of ecological disruption, when dead plants and animals provided abundant food for fungi.

The study identified one fungal bloom immediately after the asteroid impact and another roughly 10,000 to 30,000 years beforehand. The earlier bloom appeared to coincide with major Deccan volcanic activity, which likely contributed to climate cooling and ecosystem instability.

“What surprised us was the proliferation in association with Deccan volcanism,” Casadevall said. “This implies ecological disruption prior to the meteor impact.”

In other words, ecosystems may have already been weakened by climate change, volcanic pollution and food shortages before the asteroid delivered the final blow.

Why Might Mammals Have Had an Advantage?

When Casadevall first introduced the FIMS hypothesis, it was based on a basic biological observation: Mammals are generally more resistant to invasive fungal diseases than many other animal groups.

Fungi are important pathogens of plants, insects, amphibians and reptiles. In healthy mammals, potentially fatal systemic fungal infections are comparatively uncommon. Casadevall has argued that this resistance may be related to mammals’ complex immune systems and relatively high body temperatures.

Mammalian immune defenses include neutrophils, which can recognize fungal cell walls, engulf spores and attack invasive fungal filaments. The adaptive immune system can also produce targeted antibodies and retain immune memory, allowing the body to respond more effectively after a repeat exposure.

Most mammals maintain body temperatures between approximately 97 and 104 degrees Fahrenheit (36 and 40 degrees Celsius). Casadevall proposed that this consistently warm internal environment may prevent many environmental fungi from growing effectively inside mammals.

“The success of mammals made no sense given their high energy needs, and this idea provided a plausible mechanism for their success,” Casadevall said.

After the asteroid impact, forests burned, plants died and sunlight was blocked by dust and soot. Dead organic matter accumulated across the landscape, creating ideal conditions for fungi. Surviving animals may have inhaled large quantities of fungal spores while also coping with cold temperatures, hunger and toxic air.

The FIMS hypothesis suggests that mammals had several possible advantages during this crisis. Their stable body temperatures may have allowed them to remain active during the cold “impact winter.” Their immune systems may have helped them resist fungal infections, while their ability to shelter, burrow and eat a wide variety of foods could have improved their chances of survival.

Could Dinosaur Eggs Have Been Vulnerable to Fungi?

Reproduction may also have played a role. Many mammals protect developing embryos inside the mother’s body, where warmth and immune defenses provide a degree of protection from environmental hazards.

By contrast, all dinosaurs laid eggs. Dinosaur eggs developed in nests that were often exposed to soil, moisture and decaying plant material — precisely the conditions in which fungi can thrive.

Modern fungi can invade reptile eggs, and a 2008 study described fossilized fungal hyphae in dinosaur eggshells. These findings do not prove that fungus destroyed dinosaur nests. Fungal growth may have occurred after an egg died, rather than while it was developing.

Even so, the fossil evidence raises the possibility that fungal contamination affected some dinosaur eggs or hatchlings, particularly during a period when dead vegetation and unstable ecosystems allowed fungi to spread rapidly.

Evidence of a Sick Sauropod

Another possible clue comes from a dinosaur fossil showing signs of respiratory disease. In 2022, a team led by Cary Woodruff, curator of vertebrate paleontology at the Phillip and Patricia Frost Museum of Science in Miami, described unusual lesions in the neck vertebrae of a Late Jurassic sauropod from Montana.

The immature diplodocine, known as MOR 7029, lived approximately 150 million years ago — long before the Chicxulub impact.

Sauropods had long, air-filled neck bones connected to their respiratory systems. In healthy sauropod fossils, the areas where air-sac tissue met the bones are typically smooth. In MOR 7029, those areas were rough and irregular.

Woodruff compared the abnormal growths to “fossilized heads of broccoli florets.” Other anatomists and paleontologists suggested that the lesions resembled the effects of a respiratory infection.

By comparing the fossil damage with respiratory diseases in modern birds — which are living dinosaurs — the team tentatively identified the condition as airsacculitis with associated osteomyelitis. This involves inflammation of the air sacs that spreads to nearby bone.

However, the researchers could not determine whether the infection was fungal or bacterial. In modern birds, airsacculitis can be caused by fungal diseases such as aspergillosis, but bacterial infections can produce similar symptoms. Without preserved soft tissue or identifiable fungal structures, the diagnosis remains uncertain.

Woodruff believes the sauropod was likely sick enough for the infection to contribute to its death. The animal may have become too weak to eat, drink, keep up with its group or escape predators.

The Evidence Against the FIMS Hypothesis

Many paleontologists remain skeptical that fungal disease played a major role in the rise of mammals. The asteroid impact and its aftermath already provide powerful explanations for the extinction of large dinosaurs.

The Chicxulub impact generated tsunamis, earthquakes, wildfires and a global collapse in food webs. Large predators and herbivores at the top of those disrupted food chains would have been especially vulnerable to starvation.

Body temperature is another challenge for the theory. When the FIMS hypothesis was first proposed in 2005, dinosaurs were often described as “cold-blooded,” while mammals were considered “warm-blooded.” Subsequent research suggests that many nonavian dinosaurs, including Velociraptor, T. rex and Brachiosaurus, were likely capable of maintaining elevated body temperatures.

Birds, the only surviving dinosaurs, often have higher body temperatures than mammals. That raises a difficult question for FIMS: If dinosaur physiology made the group especially vulnerable to fungal infections, why did birds survive the mass extinction?

“If everything we know that suggests dinosaurs are more susceptible to fungal infections is based on living dinosaurs — birds — and they survived, doesn’t that rather undermine this hypothesis?” paleontologist Jingmai O’Connor of the Field Museum in Chicago told Live Science.

Andrew Flynn, an assistant professor of paleobotany at New Mexico State University, also noted that turtles and crocodilians survived the extinction event despite being ectothermic animals. This suggests that body temperature alone cannot explain why mammals later became dominant.

Could Dinosaur Respiratory Systems Explain Their Vulnerability?

Researchers supporting FIMS acknowledge that the survival of birds must be explained. In a newer version of the hypothesis, Casadevall and Johns Hopkins researcher Isabel Jimenez propose that dinosaur respiratory anatomy, rather than body temperature alone, may have increased susceptibility to fungal disease.

Modern birds — and likely many dinosaurs — have rigid lungs connected to a network of thin-walled air sacs. These air sacs act as bellows, creating a one-way flow of air through the lungs. Mammals, in contrast, breathe with flexible lungs filled with blood-rich alveoli.

Although the avian respiratory system is highly efficient, its air sacs have a relatively limited blood supply. Inhaled fungal spores could potentially travel deep into the respiratory tract and settle in air sacs, where circulating immune cells might have had more difficulty eliminating them.

Bird air sacs also extend throughout the body and, in some species, into the bones. This could allow a respiratory fungal infection to spread beyond the lungs and affect the skeleton.

Jimenez and Casadevall suggest that smaller, feathered and more ecologically flexible theropods may have been better equipped to survive. Their feathers could have helped insulate them, while generalist diets may have allowed them to use a wider range of food sources after the impact.

After the asteroid strike, soot and sulfur would have polluted the atmosphere, while darkness and cooling disrupted plant growth. These conditions could have weakened dinosaurs and made secondary fungal infections more likely.

“So secondary fungal infections are certainly possible,” Jimenez said.

More Evidence Is Needed

Although the revised FIMS hypothesis is more testable, critics say there is still insufficient evidence to show that fungal disease significantly increased dinosaur mortality after the asteroid impact.

To establish a stronger connection, researchers would need to identify clear signs of elevated fungal disease in dinosaur fossils from immediately after the Chicxulub impact. They would also need to distinguish disease-causing fungal species from the much larger number of harmless or decomposer fungi recorded in sediments.

“We would also need to be able to identify elevation of pathogenic fungal spores in the sedimentary record at that boundary, rather than just more spores,” O’Connell said.

At present, the evidence shows that fungal blooms occurred during periods of major ecological disruption. It does not yet demonstrate that fungal infections were a primary cause of dinosaur deaths or the main reason mammals became dominant.

The Chicxulub impact remains the central explanation for the K-Pg extinction, but fungal disease may eventually prove to have been one of several factors that shaped survival and recovery. Future fossil discoveries and improved analysis of ancient sediments could help determine whether fungi merely flourished after the disaster — or played a more active role in reshaping life on Earth.

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