Of the five mass extinctions in the Earth’s past, one stands above the rest in magnitude: the Permian-Trassic extinction, known as the Great Dying. It saw the disappearance of almost 60 percent of all families, and over 80 percent of all genera — in the ocean, that added up to about 96 percent of all species. The cause of this event, 250 million years in the past, is still a matter of debate.
The most likely culprit is the prolific volcanism of the Siberian Traps — the erupted basalt still covers about 2 million square kilometers — but other events may have also played a role. Evidence for a massive destabilization of methane hydrates on the seafloor (a phenomenon described as “The Big Burp”), ocean anoxia and even contemporary asteroid impacts have all been found.
A couple of recent papers in the journal Geology have brought some new information to the discussion, and may help make the picture just a little bit clearer.
One source of significant mystery has been the nature of the organic microfossils that are common in rocks dated to the time of the extinction worldwide. The tiny fossils resemble filamentous colonies of cells, but have evaded positive identification.
Some researchers think they are the remains of fungi, while others argue that they are algae instead. There’s evidence on both sides, but the two scenarios represent very different conditions. The fungus indicates a widespread dying of woody vegetation, while algae suggest extensive swamps forming along river systems.
A paper published this month shows that the microfossils are almost identical morphologically to a group of pathogenic soil fungi that can infect trees. If its authors have identified these correctly, it fits in well with an overall picture showing loss of forests and topsoil. The demise of tree species is clear in pollen studies, and there is a lot of evidence for greatly accelerated soil erosion, including increased sediment deposition in deltas with lots of soil-derived organic debris.
Modern studies show that drought stress and UV damage, both of which could be caused by the massive releases of volcanic gases from the Siberian Traps, can make trees susceptible to fungal infection.
Connecting a fungus to a global mass extinction may seem tenuous, but the authors point out that processes down in the world of the very small are often overlooked in any extinction discussions. They summarize by saying, “There may have been a variety of other globally operating environmental stress factors, but whatever sequence of events triggered ecosystem destabilization on land, the aggressiveness of soil-borne pathogenic fungi must have been an integral factor involved in Late Permian forest decline worldwide.”