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 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.”

Separately, another recent paper has pinned down the timing of the extinction. It’s not considered to have been as sudden as the End Cretaceous extinction that killed off the dinosaurs, but the precise timeline has been tough to get a handle on, and estimates have varied.

The research group looked at some marine Permian-Triassic rocks in China that recorded cyclical global climate patterns. Climate controlled the amount of terrestrial sediment that was deposited in this area, which shows up as changes in grain size through the rock layers. Using a device that measures magnetic susceptibility, they were able to precisely quantify changes in grain size across the rock layers. Together with some uranium-lead isotopic ages, they were able to pick out the orbital cycles that control climate, including the prominent 400,000 year eccentricity cycle, and use them to precisely date the extinction interval.

A couple of interesting things show up in the data. For one, minima in several of the orbital cycles coincide shortly before the start of the extinction period. (Think of three sine waves with different wavelengths—at certain points in time, all three troughs will line up by chance.) That could have made for some unusual climatic conditions. Additionally, the effect of the 100,000 year orbital cycle on climate seems diminished for as long as 2 million years afterward.

It’s dangerous to extrapolate to the big picture from records like this, but there’s enough there to warrant further investigation of the orbital forcings.

In the end, they found that the extinction took 600,000-700,000 years to play out. This is consistent with the idea that several events acted in concert to destabilize ecosystems and cause the loss of so many species, meaning a significant length of time would be needed. It was simply a nasty time to be a living thing on planet Earth. Some advice for any time travelers out there—steer well clear of the Great Dying.

Geology, 2011. DOI: 10.1130/G32126.1 and Geology, 2011. DOI: 10.1130/G32178.1 (About DOIs).