Freitag, 20. November 2009

Extinctions & Excrements

"dal letame nascono i fior
dai diamanti non nasce niente"
From dung flowers are born
From diamonds nothing comes
"Via del Campo", Fabrizio de André (Italian poet-musician)

Until 20.000 years ago North America showed a biodiversity of large mammals c
omparable with modern Africa, if not greater. 10.000 years later 34 genera with animal-species weighing more than a ton were extinct.

The extinction of the Pleistocene Megafauna is still an unsolved mystery. The proposed hypothesis range from overkill by human hunters to a meteor impact and climate change at the end of the last glacial maximum. Geologically speaking it happened suddenly, but a new study now maybe can date more precisely the extinction pattern and duration, using an unusual data source - fossil excrements and the inhabitants of this "biotope".

In 2005 and 2006 sediment cores with a complessive length of 11,7m were taken from Appleman Lake and compared with other cores of lakes in the U.S. State of Indiana.
Thirteen wood, pollen and charcoal samples, recovered from the lacustrine sediments, were dated by radiocarbon method on ages between 7.000 and 14.000 yr BP and used interpolate an age-depth model of the core.

The fungus-genus Sporomiella lives on animal dung and the spores have to pass the digestive tract of large herbivores to germinate. The spores can also accumulate in sediments along with other micro- and macrofossils like pollen and charcoal, so the presence of the fossil spores in sediments correlates with the amount of excrements - "Lots of dung means lots of spores" (JOHNSON 2009), and the amount of dung can give a hind to extrapolate the size of the population of herbivorous animals like mastodon or mammoth.

The timing of the Sporomiella decline and the first major charcoal peak are well constrained by two dates between 14.6 and 14.7ka. The wood pollen (Quercus and Pinus) increases between 10.7 and 12.2 ka.

Figure from GILL et al. 2009: Appleman Lake time series for (A to F) percent pollen abundances of selected taxa (NAP, nonarboreal pollen), (G) Sporormiella and (I) charcoal counts.

Applying this method, Gill et al. found that the amount of spores first decreases slowly, and only in 14.800 years old sediments the number of spores decreases significantly. To old for the proposed impact, and also to old for a climatic or environmental change - vegetation change, interpolated from the pollen assemblage, namely happens only after the faunal demise, and is more probable caused by the extinction of large herbivore, then the cause of extinction.

The greatest impact of humans - in form of the Clovis Culture - on the Pleistocene la
ndscape in North American was supposed in a time interval between 13.330 and 12.900 years ago. The new data predates the Clovis, nevertheless archaeological findings support a lesser tool specialised pre-Clovis culture in the time interval of the Megafauna collapse, so human influence could not be ruled completely out.

Figure from JOHNSON 2009.

The changing environment after the Megafauna collapse, from an open savanna with scattered trees to a spruce-broadleaf woodland, was the result of ceased pasture of shrubs and trees by Mammoth and Co. The expansion of woodlands is also supported by a larger amount of charcoal in the sediments, from time to time the woodlands caught fire, and the ash was eroded, transported and finally deposited in the examined lakes.

Even if the new method con not give us the definitive answer, at least it's provide some new data to better understand the temporal progress and the environmental change of the late Pleistocene extinction event.


GILL et al. (2009): Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America. Science 326: 1100-1103

GILL et al. (2009): Supporting Online Material for Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America. Science 326.

JOHNSON (2009): Megafaunal Decline and Fall. Science 326: 1072 - 1073.

Interview to Dr. Jacquelyn Gill by the Canadian Broadcast: mp3 (4MB)

--------- Thanks to Ole Nielson for linking to the post----------

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