Scientists claim they have identified an ancient fungus that flourished about 250 million years ago, feeding on dead trees as it spread across the planet. Those remains could provide a crucial clue to the identity of what killed off much of Earth's plant and animal life at the time, although some researchers remain skeptical.
Earth's history is marked by several mass extinctions. Probably the best-known of these is at the so-called Cretaceous-Tertiary boundary, about 65 million years ago, the catastrophe that wiped out the dinosaurs and many other terrestrial and marine species. All over the world, samples of the sediments that were deposited then show traces of iridium, an element that is rare on Earth but common in asteroids, pointing to a massive impact.
A more mysterious mass extinction happened about 250 million years ago, marking the end of the Permian period and the beginning of the Triassic. Almost all marine life vanished, as did nearly three-quarters of land animals--almost all of which resided on a single, giant continent known as Pangaea. But the cause of the extinctions has remained elusive. There's no evidence of an impact, only scattered signs of lava flows and hints of possible sea-level rise or changes in ocean circulation.
In 1996, researchers tested the chemical remains of a genus of micro-organisms called Reduviasporonites, which are common in the Permian-Triassic (P-Tr) boundary layer. They concluded that the organisms were fungi, but later analyses suggested that the organisms were a form of algae. Now, members of an international team say they have confirmed not only that the ubiquitous Reduviasporonites were fungi but also that their primary diet was dead trees--something that might provide a decisive clue about the type of catastrophe that ended the Permian period.
The team analyzed carbon and nitrogen isotopes from samples of Reduviasporonites. As the researchers report today in Geology, those analyses identified chemicals unique to Reduviasporonites, whose reign spanned the Permian and Triassic periods, and other compounds associated with dead tree matter, within the same microfossils. Astrobiologist and lead author Mark Sephton of Imperial College London says the analyses show that the organism fed on dead wood. In addition, he notes, the Reduviasporonites microfossils have been found in sediments deposited at the P-Tr boundary all across what was then Pangaea. "This suggests that we are looking at something truly global in effect."
Sephton explains that for Reduviasporonites to be so common at the P-Tr boundary, they must have thrived on a disaster that brought about "a dramatic change in the environment." The most likely cause, he says, is a massive release of sulfur dioxide and other noxious gases from volcanic eruptions. Those gases would have caused highly acidified rain, enough to poison most of the planet, killing trees and creating a global feast for Reduviasporonites. "When things turned really bad," Sephton says, "they were most at home."
It's a good story if it turns out to be true, says paleontologist C. Kevin Boyce of the University of Chicago in Illinois. The analyses of the carbon isotopes provide the "strongest evidence" of a fungal lifestyle for Reduviasporonites, he says. They do "a much better job than previous work" in identifying components of Reduviasporonites versus bits of organic matter from fossilized dead trees. So was Reduviasporonites a fungus? "Maybe," Boyce says. "What [the team's] work does is at least reopen that door."