When you’re a paleontologist seeking to solve an ancient mystery–in this case, the demise of Ice Age ‘megafauna’ like Woolly Mammoths and Giant Sloths–you sometimes have to get down and dirty. Specifically, you have to collect and analyze fossilized poop.
A scientific team from the University of Wisconsin at Madison, led by Jacquelin L. Gill, chose to analyze fossilized dung samples (known as coprolites) to detect the presence of spores from a dung fungus called Sporormiella. The fungus requires ingestion by animals (herbivores and some birds) as part of its life cycle. The resulting digestion leaves fungal spores in the animals’ dung. The spores are also found in lake sediments where, it has been determined, they end up via “dung loading” (the washing of dung into the water after defecation). The team found that the decline of fungal spores closely tracked the decline of megafuana populations. The spores are found in abundance in late-glacial sediments–when megafauna were also abundant–but then decline rapidly as the Holocene (our current geological period) proceeded. The spores rebound again after the introduction of domestic grazers by settlers.
In North America, the end of the last Ice Age–known as the Pleistocene-Holocene deglaciation–lasted from 18 ka (years ago) to about 6 ka. This warming period was accompanied by “massive biotic upheaval”, and included the extinction of 34 megafauna genera, as well as mass migrations, the rise and decline of novel plant communities (so-called “no-analog” communities), and increased biomass burning (from natural fires).
Previously, “rapid extinction” theories were put forth to explain the demise of these large mammals. These theories included an extraterrestrial impact scenario (somewhere around 12.9 ka) and a “blitzkrieg” of over-hunting by Paleo-Indians (such as the Clovis People).
But, according to these proxy record analyses, the decline of megafauna populations was gradual–taking several thousand of years before the giant mammals became “functionally extinct” (i.e., their off-spring numbers being insufficient to maintain their long-term survival), and then to reach “final extinction”. According to the paper, this decline began around 14 ka–before the Clovis people were widespread on this continent. Anthropologists believe that the Clovis People–known by their distinct, notched, arrowheads–to be the first established “Paleo-Indian” culture in North America.
It has also been theorized that climate forcing (from increased insolation, or earth-reaching solar radiation) caused a change in vegetation, and consequently, in “fire regimes”, which then negatively impacted the megafauna, forcing them into extinction. While there is some evidence of climate change impact, this new evidence suggest that it was more likely the opposite: that the decline of these largest land mammals preceded the vegetation shift (or in fact, their decline caused the rise of new plant and tree communities), and the subsequent change in wild fire regimes (seasonal/spatial frequencies).
The samples were collected from Appleman Lake in Indiana, with additional samples taken from similar glacial ponds and lakes in upstate New York. Also collected were samples of wood, pollen and charcoal (the result of biomass burning).
The paper (Gill et al, ‘Pleistocene Magafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America’, Science, 20 November, 2009) is careful to point out that the evidence does not completely rule out the human factor in megafaunal collapse, only that it excludes “rapid extinction” hypotheses–whether human or extraterrestrial. The authors assert that their work shows “close connection among the late-glacial histories of fire, vegetation and mammalian herbivores” and that the loss of these herbivorous consumers “contributed to substantial restructuring of plant communities and fire regimes.”
By illuminating the causes and consequences of Pleistocene, megafaunal collapse, such research can be used to address widespread declines amongst contemporary megafauna (such as the African elephant and rhinoceros).
image: Mauricio Anton
About Michael Ricciardi
