A Johns Hopkins graduate student may have figured out
why rates of extinction were so low for many of the major
groups of marine life during one of the greatest ice ages
of them all, which occurred from about 330 million to 290
million years ago, late in the Paleozoic Era.
The likely answer: because those aquatic life forms
that did survive during this era were singularly equipped
to endure severe fluctuations in temperature and sea
levels. Those that were not died in a mass extinction that
heralded the ice age's onset.
"These results not only clue us in to what happened
many millions of years ago, but they also have implications
for understanding the modern marine ecosystem," said
Matthew Powell, a doctoral candidate in the Morton K. Blaustein Department of Earth and
Planetary Sciences in the Krieger School of Arts and
Sciences. His paper on the topic appears in the May issue
of Geology, published by the Geological Society of
"If the patterns I detected also are true for the
modern ice age — and other researchers' results
suggest that they may be — then modern marine life
ought to be relatively resistant to extinction," he said.
"Yet species are dying off at an alarming rate. It may be
that humans have altered the environment so much that we
are now causing the extinction of species that should be
relatively immune. Though it's difficult to say exactly
what the implications are for the world we live in, what I
can say is that it is worrisome."
Powell looked at extinctions during an age when
glaciers reached to within 35 degrees of the equator,
roughly as far south as a line between present-day Raleigh,
Memphis and Albuquerque or nearly as far north as Buenos
Aires. Powell tackled the question of why extinction rates
were so low during that great ice age by closely examining
geographic patterns of evolution and extinction in
brachiopods, simple shelled sea creatures that were
abundant and well fossilized during the Paleozoic. He
constructed a database that charted latitudinal patterns of
evolution and extinction through the late Paleozoic.
"This database is the first ever; no other database of
this kind exists for any interval of geologic time from
which to study geographic patterns of macroevolution,"
According to Powell's analysis, brachiopods that lived
primarily near the equator suffered the highest extinction
rates and did not reappear in great numbers until the ice
"The absence of these particular brachiopods during
the ice age left the oceans populated almost entirely with
those who lived over a wider geographic area," Powell said.
"What I found is that the uniquely low global rates of
evolution and extinction for brachiopods during the late
Paleozoic ice age were caused by the loss and lack of
recovery of those that had existed in narrow latitudinal
Powell believes that those brachiopods that existed
within narrow latitudinal ranges became victims of the
extremes in the annual minimum and maximum temperatures
that were typical of the late Paleozoic. During that era,
"seasonality" — the difference between annual
temperatures' highs and lows — was amplified by the
presence of glaciers.
"I've suggested that those brachiopods which
eventually became extinct had adapted only to small
temperature changes, and thus did not survive," he said.
"The other competing hypothesis is that large fluctuations
of sea level, driven by the melting and reforming of
glaciers, disrupted marine communities, and the ones which
survived were those able to adjust."
Powell's research was funded by the Department of
Earth and Planetary Sciences.