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. Image: Caltech |
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 online in 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% 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 mass extinction.
“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 sea level 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 earth 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.”
