Mass Extinctions Face Downsizing, Extinction
Richard A. Kerr

Paleontologists are wondering whether mass extinctions were really all that
massive --or, in the case of a 94-million-year-old example, whether they existed
at all A bunch of sea urchins turned up in the Cretaceous like a bad penny,
millions of years after they were believed to have gone extinct. Their
reappearance casts doubt on the existence of one long-presumed mass extinction
and by implication that of several others. "This is going to shake up the paleo
world for a while," says paleontologist Lisa Park of the University of Akron in
Ohio.

Scattered among the five major crises in the history of life--such as the
Cretaceous-Tertiary (K-T) extinction 65 million years ago that marked the end of
the dinosaurs--are a half-dozen lesser extinction events, including the
Cenomanian-Turonian (C-T) mass extinction 94 million years ago. These smaller
events have long been prominent mileposts in the geologic record and examples of
how life on Earth suffers under stress. But a new study threatens the very
existence of the C-T. And because the other second-tier mass extinctions are
defined by similar evidence, they may be suspect, too. Even the scale, although
not the existence, of the Big Five mass extinctions is coming under scrutiny.
All this because a group of British paleontologists, in a paper in the latest
issue of Paleobiology, has removed European sea urchins from the list of C-T
victims.

Paleontologists have estimated that 26% of marine genera known in the Cenomanian
stage had disappeared by 94 million years ago when the Turonian stage began.
That's hardly in the same class as the Big Five's 47% to 84% losses at the genus
level, but it's still a pretty big deal.

But paleontologists Andrew Smith and Neale Monks of The Natural History Museum
in London and geologist Andrew Gale of the University of Greenwich at Chatham
Maritime wondered just how reliable the fossil record of the C-T is and thus how
real the C-T extinctions were. They saw two potential problems. The known C-T
record depends heavily on fossils recovered from rock outcrops in just two
areas, Western Europe and the western interior of North America. These areas
loom large in all of paleontology not because they harbor ideal fossil records
but because they happen to be near the world's major institutions of
paleontology of the past 2 centuries. And these records may be suspect because
they come from sediments laid down when sea level was high, so high that Europe
turned into an archipelago. Shallow seas on the continent would accumulate much
less sediment than the continental shelf, and the fall in sea level since the
C-T has given erosion 90 million years to remove the C-T fossil record deposited
on the continents.

To see just how well the fossil record fared, Smith and his colleagues made a
detailed study of sediments and echinoderm fossils--primarily sea
urchins--already collected from southeastern England by Smith, Gale, and other
colleagues. They also worked on sites in France and Germany. Twelve genera found
in the 6-million-year-long Cenomanian made it through to the Turonian, but they
found 29 Cenomanian genera that had disappeared from the rock record by the
start of the Turonian. Apparently, the C-T extinction event claimed 71% of
existing echinoderm genera.

But appearances at the C-T are deceptive, Smith and his colleagues say. Fifteen
of the 29 apparently extinct genera reappeared in the record as much as 20
million years after the Turonian, they found. These "Lazarus taxa" obviously had
not disappeared from the world, only from the rock preserved in Western Europe
around the C-T boundary. Another seven Cenomanian genera disappeared and never
reappeared, but new genera so much like them later appeared that the new genera
must have evolved from the Cenomanian genera, which therefore must not really
have gone extinct, Smith concludes. By Smith's count, then, the C-T extinction
of echinoderms shrinks from a catastrophic 71% to 17%, a loss that might simply
reflect the background extinction that inevitably occurs at any time.
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In the meantime, even the big ones are coming under closer scrutiny, because,
like the C-T, most major extinction events come at times of rising sea level and
are documented largely in Western Europe and North America. Even the biggest
mass extinction, the Permian-Triassic of 250 million years ago, may not be as
big as the 96% species extinction claimed for it, Park says.

In fact, last month paleontologist Johnny A. Waters of the State University of
West Georgia in Carrollton and colleagues reported at the Earth System Processes
meeting in Edinburgh that another of the Big Five--at the Frasnian-Famennian
boundary 364 million years ago--isn't as big as it once  seemed. Based on their
recent finds in remote northwestern China and finds elsewhere, the number of
echinoderm taxa following the boundary in the Famennian is five times greater
than previously thought, they said, greatly diminishing the extinction event.
"I'm not convinced the Frasnian- Famennian is a big deal," says Christopher
Maples of Indiana University, Bloomington, a co-author of the work. Getting the
scale and even the existence of mass extinctions right will require more
explorations beyond merry old England.

POWRÓT