Researchers say habitat loss and tropical cooling were to blame for mass extinction

Apr 10, 2012 by Kimm Fesenmaier
Bedding surfaces covered in marine invertebrate fossils from the Late Ordovician. This photo shows part of the Ellis Bay Formation on Anticosti Island in Québec, Canada. Anticosti Island preserves one of the most fossiliferous and stratigraphically complete records through the Late Ordovician Mass Extinction in North America. Credit: Caltech

(Phys.org) -- The second-largest mass extinction in Earth's history coincided with a short but intense ice age during which enormous glaciers grew and sea levels dropped. Although it has long been agreed that the so-called Late Ordovician mass extinction—which occurred about 450 million years ago—was related to climate change, exactly how the climate change produced the extinction has not been known. Now, a team led by scientists at the California Institute of Technology (Caltech) has created a framework for weighing the factors that might have led to mass extinction and has used that framework to determine that the majority of extinctions were caused by habitat loss due to falling sea levels and cooling of the tropical oceans.

The work—performed by scientists at Caltech and the University of Wisconsin, Madison—is described in a paper currently online in the early edition of the Proceedings of the National Academy of Sciences.

The researchers combined information from two separate databases to overlay fossil occurrences on the sedimentary rock record of North America around the time of the extinction, an event that wiped out about 75 percent of marine species alive then. At that time, North America was an island continent geologists call Laurentia, located in the tropics.

Comparing the groups of species, or genera, that went extinct during the event with those that survived, the researchers were able to figure out the relative importance of several variables in dictating whether a genus went extinct during a 50-million-year interval around the .

"What we did was essentially the same thing you'd do if confronted with a disease epidemic," says Seth Finnegan, postdoctoral scholar at Caltech and lead author of the study. "You ask who is affected and who is unaffected, and that can tell you a lot about what's causing the epidemic." 

As it turns out, the strongest predictive factors of extinction on Laurentia were both the percentage of a genus's habitat that was lost when the dropped and a genus's ability to tolerate broader ranges of temperatures. Groups that lost large portions of their habitat as ice sheets grew and sea levels fell, and those that had always been confined to warm tropical waters, were most likely to go extinct as a result of the rapid climate change.

"This is the first really attractive demonstration of how you can use multivariate approaches to try to understand extinctions, which reflect amazingly complex suites of processes," says Woodward Fischer, an assistant professor of geobiology at Caltech and principal investigator on the study. "As scientists, we love to debate different environmental and ecological factors in extinctions, but the truth is that all of these factors interact with one another in complicated ways, and you need a way of teasing these interactions apart. I'm sure this framework will be profitably applied to extinction events in other geologic intervals."

The analysis enabled the researchers to largely rule out a hypothesis, known as the record-bias hypothesis, which says that the extinction might be explained by a significant gap in the fossil record, also related to glaciation. After all, if sea levels fell and continents were no longer flooded, sedimentary rocks with fossils would not accumulate. Therefore, the last record of any species that went extinct during the gap would show up immediately before the gap, creating the appearance of a mass extinction. 

Finnegan reasoned that this record-bias hypothesis would predict that the duration of a gap in the record should correlate with higher numbers of extinctions—if a gap persisted longer, more groups should have gone extinct during that time, so it should appear that more species went extinct all at once than for shorter gaps. But in the case of the Late Ordovician, the researchers found that the duration of the gap did not matter, indicating that a mass extinction very likely did occur. 

"We have found that the Late Ordovician mass extinction most likely represents a real pulse of extinction—that many living things genuinely went extinct then," says Finnegan. "It's not that the record went bad and we just don't recover them after that."

The team used data about North American fossils from the public Paleobiology Database as well as information about the sedimentary rock record from the Macrostrat Database developed by the University of Wisconsin, Madison. Along with Fischer and Finnegan, additional coauthors of the paper, " and the selective signature of the late Ordovician mass extinction" are Shanan Peters and Noel Heim of the University of Wisconsin, Madison. Finnegan will begin a new appointment at UC Berkeley in the fall. The work was supported by the Agouron Institute and the National Science Foundation.

Explore further: Synchronization of North Atlantic, North Pacific preceded abrupt warming, end of ice age

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User comments : 14

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NotParker
1.9 / 5 (9) Apr 10, 2012
"To the consternation of global warming proponents, the Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today-- 4400 ppm."

http://www.geocra...ate.html
slayerwulfe
3.4 / 5 (5) Apr 10, 2012
not ice age but glacial period, please.
Russkiycremepuff
1 / 5 (2) Apr 10, 2012
If the climatologists are right, then it would be best for the geologists, et al, to speed up their explorations to find the maximum amount of extinct species fossils before the flood gates open and all of that evidence is lost.
NotParker
2.5 / 5 (8) Apr 10, 2012
not ice age but glacial period, please.


Almost all references are to a "short ice age" of 500,000 years/
Feldagast
3 / 5 (4) Apr 11, 2012
Did they find any fossilized SUV's? Any Humans with coal in their pockets?
ubavontuba
2 / 5 (4) Apr 11, 2012
Researchers say habitat loss and tropical cooling were to blame for mass extinction
As I've been saying all along, a warm earth is a more livable planet. You can't farm icesheets.
Sanescience
3 / 5 (2) Apr 11, 2012
Change is bad. Warmer, colder, either way humans must assume bad things will happen with change. It's been pounded into our genome by eons of natural selection.

But why this issue gets so much more attention than the runaway train of geometric population growth barreling down at us is a very strange phenomenon.

Oh yea, because there is money to be made from climate change.
wwqq
1 / 5 (2) Apr 11, 2012
But why this issue gets so much more attention than the runaway train of geometric population growth barreling down at us is a very strange phenomenon.


Easy, there is no geometric population growth; it's barely linear(it has been ever since the 1960s!). As countries develop, death rates implode, lives get longer; after about a generation birth rates drop and they continue dropping until they eventually are below replacement levels.

Even draconian policies have had only limited effect on population growth and have bad side effects(China's one-child policy led to a large gender imbalance and a lot of children either murdered or in hiding).

The solution is business as usual and we're on track for a peak of about 9 billion mid century.
rubberman
not rated yet Apr 11, 2012
Well folks....clearly the data is incorrect or has been doctored as there couldn't possibly have been an iceage with CO2 ppm as high as NP suggests. Earth at that time was identical to the earth today other than the CO2 PPM as is clearly shown in the link below:

http//www.ignorelogic.com/denialistparadise/itsallok
rockwolf1000
5 / 5 (4) Apr 11, 2012
But why this issue gets so much more attention than the runaway train of geometric population growth barreling down at us is a very strange phenomenon.


Easy, there is no geometric population growth; it's barely linear(it has been ever since the 1960s!). As countries develop, death rates implode, lives get longer; after about a generation birth rates drop and they continue dropping until they eventually are below replacement levels.

The solution is business as usual and we're on track for a peak of about 9 billion mid century.


Are you serious? Population grows from 7 billion to 9 billion in less than 40 years and you think that's linear growth.
Give your head a shake.
rockwolf1000
5 / 5 (1) Apr 11, 2012

The solution is business as usual and we're on track for a peak of about 9 billion mid century.


Extrapolating backwards in a linear fashion as you suggest means the human population was zero a few centuries ago
NotParker
1.7 / 5 (6) Apr 11, 2012
Well folks....clearly the data is incorrect or has been doctored as there couldn't possibly have been an iceage with CO2 ppm as high as NP suggests. Earth at that time was identical to the earth today other than the CO2 PPM as is clearly shown in the link below:

http//www.ignorelogic.com/denialistparadise/itsallok


CO2 isn't the only GHG. Water vapor is #1. Methane is also a GHG.

Co2 is the wimpy, impotent GHG.
Sanescience
5 / 5 (1) Apr 12, 2012
Ok, something is an exponential growth/decline rates (geometric if measured in equal intervals)if it's change is a mathematical function proportional to it's current value. As in increasing or decreasing by an annual percentage.

Estimates do include the *possibility* that the growth rate of the world as a whole may plateau. However past estimates have under estimated population growth so far. The UN estimates from 2009 says 2050 will be between 8 billion and 10 billion.

Further, the real trouble comes in when the distribution of the population becomes heavily skewed by region and culture. Even if the world's annual average growth rate approaches a "plateau", that doesn't mean regions of high growth won't exist.
Sanescience
5 / 5 (1) Apr 12, 2012
Also, population growth rates seem to follow standards of living (or to be crass, women having control of their bodies). And assuming that standards of living will always be going up is also dangerous. If problems with the global economy causes a sustained contraction of standards of living, many regions of the world will probably swing back to more "barbaric" conditions where birthrates will trend higher again.