LIFE & EVOLUTION
Story of dinos’ die-off not a simple tale
Dinosaurs’ numbers were already dwindling before asteroid hit
B Y MEGHAN ROSEN
Neither a giant asteroid nor a gradual
die-out can take full blame for dinosaurs’
demise. Rather, the culprit may be both,
two new studies suggest.
Tens of millions of years before the
asteroid delivered its killer blow some
66 million years ago, the number of dinosaur species had already begun to drop,
researchers report online April 18 in the
Proceedings of the National Academy of
Sciences. But not all dino groups were in
decline, other researchers suggest online
April 21 in Current Biology.
At first glance, the two studies seem
to conflict, but “they can coexist,” says
paleontologist Michael Benton, who
coauthored the PNAS paper. Both add to
what has become an increasingly intricate picture of dinosaurs’ final days.
In the 1960s and ’70s, scientists generally believed that dinosaurs petered out
after a long, gradual decline. That view
took a U-turn in 1980, when researchers proposed that, instead, an asteroid
impact suddenly triggered the extinction.
What actually happened is probably more nuanced, says Benton, of the
University of Bristol in England. He
and colleagues analyzed the number of
Dinosaurs called toothed maniraptorans (one
in flight) thrived until their sudden extinction
66 million years ago. Modern birds’ ancestors
(one in log) may have survived by eating seeds.
dinosaur species emerging and going
extinct in a roughly 175-million-year fossil record. Up to 50 million years before
the mass extinction, dinosaurs started
losing species faster than they gained
new ones. This loss in diversity may have
made it harder to bounce back from the
“This doesn’t in any way attack the
importance of the impact,” Benton says.
But in most dinosaur groups, species
numbers were already dwindling.
According to the analysis in Current
Biology, toothed maniraptorans (small
birdlike relatives of velociraptors) were
an exception. An analysis of over 3,000 of
these dinosaurs’ fossilized teeth suggests
that these dinos’ ecosystem was stable up
until the extinction, says study coauthor
Derek Larson of the Philip J. Currie Dinosaur Museum in Wembley, Canada.
Larson and colleagues looked for varia-
tions in the teeth’s dimensions and in ser-
ration size. Big changes in variation over
time could be a hint that the dinos were
on the decline, Larson says. But “things
basically stayed the same through the last
18 million years of the Cretaceous.”
Toothed maniraptorans “seemed to be
doing just fine right up until the extinc-
tion,” says University of Oxford paleo-
biologist Roger Benson.
Larson’s team suspects that diet may
explain why toothed, meat-eating maniraptorans went extinct after the impact
while some of their relatives — the beaked
ancestors of birds — didn’t. In analyzing
modern birds’ diets, the team determined
that ancestral birds probably ate seeds,
Larson says, a hardy food source that
could have lasted for decades or longer.
Seeds might have sustained ancestral
birds through the nuclear winter that
could have followed the asteroid impact.
When hoards of plants and animals died
out, and dinos ran out of food, Larson
says, “the only resource that would have
been reliable and available would have
been seeds.” s
to the liver where it can be made into bile
and escorted out of the body in feces.
Hempel e-mailed Latz and suggested
that cyclodextrin might melt the cho-
lesterol crystals in arteries. Latz and
colleagues tested the idea by feeding
mice genetically prone to atherosclero-
sis a high-fat diet and giving them regu-
lar injections of cyclodextrin under the
skin. The sugar kept cholesterol plaques
from building up in the mice’s arteries.
The scientists also found that cyclodex-
trin reduced already established plaques
by about 45 percent, even though mice
were still eating a high-fat diet.
Cyclodextrin could be used in combination with other drugs, such as statins,
says Eran Elinav, an immunologist at
the Weizmann Institute of Science in
Rehovot, Israel. Statins inhibit cholesterol production. “Potentially, combining cholesterol lowering with dissolution
of preformed cholesterol in plaques
could be additive,” he says.
Although cyclodextrin is already
approved by the FDA for use in people, it
may be years before it’s known whether
injecting the sugar will soften people’s
hardened arteries. No pharmaceutical
companies have come forward to sponsor expensive clinical trials needed to get
approval for this specific use, Latz says. s
will increase overall. And the fraction of
such events that could condition corals to withstand bleaching will fall from
75 to 22 percent, the team reports. The
team predicts most reefs that have experienced preconditioning in the past will
lose the ability to prepare when average water temperatures increase by
0.5 degrees. Warming trends suggest that the added half degree should
appear within 40 years. “If that protective mechanism does get lost going into
the future, then what we’ve seen so far as
being bad impacts could become worse,”
For now, preparation may help
some corals survive in warming seas,
but reduced carbon emissions will be
required to sustain coral cover throughout the century, the team’s data suggest. s