The Younger Dryas period was an era of extinctions and ecosystem change that occurred just prior to the end of the last ice age. It's also a hot area of research right now, with some researchers suggesting that a comet or meteor struck the earth over North America, killing off megafauna like mammoths and mastodons. That prompted a response that suggested the evidence for an impact might just be a product of bad lab techniques. Now, a new study is out that tracks the decline of these giant herbivores using what the researchers term a "dung fungus." It turns out they may have been dying off well in advance of the Younger Dryas.

The problem with figuring out cause and effect when it comes to the events of the Younger Dryas is that so many things happened in a geological blink of an eye. The planet warmed rapidly about 15,000 years ago, bringing on the catchily named B�lling-Aller�d warm period. But, instead of bringing the ice age to a close, the Younger Dryas arrived, returning glacial conditions to North America for over 1,000 years. The ice age didn't truly end until about 12,000 years ago. Somewhere during this climactic rollercoaster, humans arrived in North America in significant numbers, entire ecosystems that look nothing like the modern one came and vanished again, and every mammal that weighed more than 1,000kg went extinct—fully half of the mammals over 35kg died off, in fact.

There also may have been that meteor, although that's looking less likely.

As the authors of the latest paper point out, the events of this time period might be a bit easier to sort out if we actually had a better sense of the timing of events. Fossils of the megafauna are fairly abundant, but don't necessarily provide a clear picture of their population levels, or indicate precisely when the last ones died out. It's also challenging to link fossils to a general picture of the prevailing ecosystem.

So, the authors don't bother with mammoth and mastodon fossils at all, instead focusing on their poop. Or, more specifically, the fungus Sporormiella. This fungus creates spores that don't mature unless they've been through the digestive track of a herbivore, and they've already been found in the remains of mammoths. Although there were clearly other herbivores in North America, there have been past studies that indicate the general levels of the spores track the decline of the megafauna that used to dominate the ecology.

Conveniently, these spores are very tough, and they tend to get washed into the local bodies of water, where they get preserved along with the pollen and other debris that can be used to track the progression of different ecosystems. Add some carbon dating to get temporal information, and it's possible to track the changes in herbivores and the ecosystems they once lived in. The authors also include an analysis of carbon deposits associated with fires, which became more common as the Younger Dryas drew to a close.

At an Indiana site near Lake Michigan, the dung fungus started its decline over 14,000 years ago. That predates the Younger Dryas by a significant margin, and actually seems to correspond with the start of the B�lling-Aller�d warm period. The decline seems to correspond with a rise in the pollen from ash and ironwood in the same samples, and the start of a gradual decline in spruce. Spruce doesn't seem to move off the scene entirely until the end of the Younger Dryas, though; by that time, it appears that mammoths and mastodons had already been gone from the ecosystem for over 1,000 years.

After about 700 years of pine dominance, something approaching the modern ecosystem began to appear. It was only at this point that evidence of regular fires becomes common in these samples.

A single site is obviously not going to satisfy anyone, so the authors perform a similar analysis at three lakes in New York. Two of them show a similar pattern of declining fungal spores in advance of significant ecosystem changes, and one of those shows a clear association between fires and the end of the Younger Dryas; the third seems to have a very sparse record of the time span under study.

The authors say that a broader sampling of sites will be necessary to get a better sense of how well this pattern holds up. Still, they say that their data can rule out at least two ideas that have been circulating about the Younger Dryas changes. For one, ecosystem changes didn't drive the megafauna decline, since that decline started before changes in the foliage took place. It also was nearly complete before the unique ecosystems of the Younger Dryas appeared, indicating that the megaherbivores couldn't have contributed to the distinct characters of this plant life.

The data is consistent, however, with the onset of fire arising because of the different plant coverage that appeared after the herbivores vanished. It's also possible that humans enhanced this process.

The slow decline of megafauna also seems to rule out two of the more dramatic scenarios proposed about the Younger Dryas: the extraterrestrial impact, and humans hunting the animals to extinction. Hunting undoubtedly wouldn't have helped a population in the midst of slow decline, but it doesn't appear to have set off the decline.

Science, 2009. DOI: 10.1126/science.1179504