Mushrooms in Science

Fungi Revived the Earth After Impact
March 08, 2004

The asteroid impact theory for mass extinctions supposes that the present Gulf of Mexico began as a 'wet' crater, eventually changing the climate enough to end the dinosaur's reign 65 million years ago.

Astrobiology Magazine -- The catastrophe that extinguished the dinosaurs and other animal species, 65 million years ago also brought dramatic changes to the vegetation.

Artist's depiction of the Chicxulub asteroid impact event. Image credit: NASA.

In a study presented in latest issue of the journal Science, the paleontologists Vivi Vajda from the University of Lund, Sweden and Stephen McLoughlin from the Queensland University of Technology, Australia have described what happened to the vegetation month by month.

They depict a world in darkness where the fungi had taken over.

It´s known that an asteroid hit the Yucatan peninsula in Mexico at the end of the Cretaceous Period. It left a 180 km wide crater and from the impact site tsunamis developed and the Caribbean region was buried in ash and other debris. The consequences of the asteroid impact were global.

The outlines of the structure, called the Chicxulub crater (named for a local village), are visible in the above representation of gravity and magnetic field data from the region. Credit: NASA.

Vajda and her colleagues have previously studied the broad-scale changes in the New Zealand vegetation following the impact, but now they have dramatically improved our view of the timing of events.

At the end of the Cretaceous the vegetation on New Zealand was dominated by conifers and flowering plants.

Many of these species disappeared suddenly at the end of the Period and were instead replaced by fungal spores and fungal threads preserved in a four millimeter thick layer of coal. The layer coincides with fallout of iridium, an element rare in Earth's crust but which abounds in asteroids.

"We have managed to reconstruct the event month by month, with a very high time resolution," says Vivi Vajda. "During a very short period - from between a few months to a couple of years - the fungi and other saprophytes which live on dead organisms must have been the dominating life form on Earth. Atmospheric dust blocked the sunlight and led to the death of plants that are dependent on photosynthesis."

The layer of fossil fungi is followed by a 60 cm thick interval containing traces of the recovery flora, which re-established relatively quickly, ground ferns at first, followed after decades to hundreds of years by more diverse, woody vegetation.

A similar layer of fungi and algae is known from a previous catastrophe which happened 251 million years ago at the Permian-Triassic boundary. This was an even greater mass extinction: about 90% of the existing species disappeared.

Research will now focus on whether the similar biological signatures at these mass extinctions reflect similar causal mechanisms.

As Clark Chapman of the Southwest Research Institute in Boulder introduced in Astrobiology Magazine's "Great Impact Debates": "Even the catastrophic influence of asteroids has been mainly beneficial to mankind. We mammals have definitely benefited from the evolutionary competition unleashed 65 million years ago when the Chicxulub impact caused the Cretaceous-Tertiary (K/T) mass extinction of dinosaurs and other dominant species. My understanding of the post-Cambrian (last ~600 million years) evolution of life on this planet is that evolution has been profoundly influenced by major epochs of sudden upheaval due to mass extinctions."

Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994). Credit: NASA .

Peter Ward, a paleontologist at the University of Washington, put an evolutionary perspective on asteroid risks, "The 1980 discovery that a mass extinction had been caused by an asteroid impact was revolutionary. Questions then arose regarding the frequency of asteroid and comet impacts on Earth. By examining the size and frequency of meteor impact craters, Gene Shoemaker and others calculated that we might expect a K/T-sized impact every 100 million years. This frequency roughly fits the facts on Earth: there have been five major extinctions in the past 500 million years. But the K/T extinction is the only one undoubtedly caused by an impact. And a salient fact remains - the K/T asteroid came nowhere near wiping out all animals and plant species. We took this hit, reeled a bit, and got back to business relatively quickly."

"While such events have wiped out many species of life," said Chapman, "they have provided the environmental niches for evolutionary change. As the late Stephen Jay Gould argued, evolution favors the more randomly selected species that are able to adapt to unexpected sudden changes rather than those that slowly evolve in competition with their competitors in a nearly constant world."

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