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Caltech geochemists Rob Eagle (left) and John Eiler display a dinosaur tooth, which was used to determine the body temperature of the extinct creature. Credit: Caltech/Lance Hayashida
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Were
dinosaurs slow and lumbering, or quick and agile? It depends largely on
whether they were cold or warm blooded. When dinosaurs were first
discovered in the mid-19th century, paleontologists thought they were
plodding beasts that had to rely on their environments to keep warm,
like modern-day reptiles. But research during the last few decades
suggests that they were faster creatures, nimble like the velociraptors
or T. rex depicted in the movie Jurassic Park, requiring warmer, regulated body temperatures like in mammals.
Now,
a team of researchers led by the California Institute of Technology
(Caltech) has developed a new approach to take body temperatures of
dinosaurs for the first time, providing new insights into whether
dinosaurs were cold or warm blooded. By analyzing isotopic
concentrations in teeth of sauropods, the long-tailed, long-necked
dinosaurs that were the biggest land animals to have ever lived—think Apatosaurus (also known as Brontosaurus)—the team found that the dinosaurs were about as warm as most modern mammals.
"This
is like being able to stick a thermometer in an animal that has been
extinct for 150 million years," says Robert Eagle, a postdoctoral
scholar at Caltech and lead author on the paper to be published online
in Science Express. (Click here for video and additional images.)
"The
consensus was that no one would ever measure dinosaur body
temperatures, that it's impossible to do," says John Eiler, a coauthor
and the Robert P. Sharp Professor of Geology and professor of
geochemistry. And yet, using a technique pioneered in Eiler's lab, the
team did just that.
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A Jurassic sauropod. Credit: Illustrated by Russell Hawley, Tate Geological Museum
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The researchers analyzed 11 teeth, dug up in Tanzania, Wyoming, and Oklahoma, that belonged to Brachiosaurus brancai and Camarasaurus. They found that the Brachiosaurus had a temperature of about 38.2 C (100.8 F) and the Camarasaurus
had one of about 35.7 C (96.3 F), warmer
than modern and extinct crocodiles and alligators but cooler than
birds. The measurements are accurate to within one or two degrees,
Celsius.
"Nobody
has used this approach to look at dinosaur body temperatures before, so
our study provides a completely different angle on the longstanding
debate about dinosaur physiology," Eagle says.
The
fact that the temperatures were similar to those of most modern mammals
might seem to imply that dinosaurs had a warm-blooded metabolism. But,
the researchers say, the issue is more complex. Because large sauropod
dinosaurs were so huge, they could retain their body heat much more
efficiently than smaller mammals like humans.
"If
you're an animal that you can approximate as a sphere of meat the size
of a room, you can't be cold unless you’re dead," Eiler explains. So
even if dinosaurs were "cold blooded" in the sense that they depended on
their environments for heat, they would still have warm body
temperatures.
"The
body temperatures we've estimated now provide a key piece of data that
any model of dinosaur physiology has to be able to explain," says
Aradhna Tripati, a coauthor who's an assistant professor at UCLA and
visiting researcher in geochemistry at Caltech. "As a result, the data
can help scientists test physiological models to explain how these
organisms lived."
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Camarasaurus tooth from the Jurassic Morrison Formation of North America that was analyzed in the study by Eagle et al. Credit: Thomas Tutken (Bonn University)
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The
measured temperatures are lower than what's predicted by some models of
body temperatures, suggesting there is something missing in scientists'
understanding of dinosaur physiology. These models imply dinosaurs were
so-called gigantotherms, that they maintained warm temperatures by
their sheer size. To explain the lower temperatures, the researchers
suggest that the dinosaurs could have had some physiological or
behavioral adaptations that allowed them to avoid getting too hot. The
dinosaurs could have had lower metabolic rates to reduce the amount of
internal heat, particularly as large adults. They could also have had
something like an air-sac system to dissipate heat. Alternatively, they
could have dispelled heat through their long necks and tails.
Previously,
researchers have only been able to use indirect ways to gauge dinosaur
metabolism or body temperatures. For example, they infer dinosaur
behavior and physiology by figuring out how fast they ran based on the
spacing of dinosaur tracks, studying the ratio of predators to prey in
the fossil record, or measuring the growth rates of bone. But these
various lines of evidence were often in conflict.
"For
any position you take, you can easily find counterexamples," Eiler
says. “How an organism budgets the energy supply that it gets from food
and creates and stores the energy in its muscles—there are no fossil
remains for that," he says. "So you just sort of have to make your best
guess based on indirect arguments.”
But
Eagle, Eiler, and their colleagues have developed a so-called
clumped-isotope technique that shows that it is possible to take body
temperatures of dinosaurs—and there's no guessing involved.
“We’re
getting at body temperature through a line of reasoning that I think is
relatively bullet proof, provided you can find well-preserved samples,"
Eiler says.
In
this method, the researchers measure the concentrations of the rare
isotopes carbon-13 and oxygen-18 in bioapatite, a mineral found in teeth
and bone. How often these isotopes bond with each other—or
"clump"—depends on temperature. The lower the temperature, the more
carbon-13 and oxygen-18 tend to bond in bioapatite. So measuring the
clumping of these isotopes is a direct way to determine the temperature
of the environment in which the mineral formed—in this case, inside the
dinosaur.
"What
we’re doing is special in that it’s thermodynamically based," Eiler
explains. "Thermodynamics, like the laws of gravity, is independent of
setting, time, and context."
Because
thermodynamics worked the same way 150 million years ago as it does
today, measuring isotope clumping is a robust technique.
Identifying
the most well-preserved samples of dinosaur teeth was one of the major
challenges of the analysis, the researchers say, and they used several
ways to find the best samples. For example, they compared the isotopic
compositions of resistant parts of teeth—the enamel—with easily altered
materials—dentin and fossil bones of related animals. Well-preserved
enamel would preserve both physiologically possible temperatures and be
isotopically distinct from dentin and bone.
The
next step is to take temperatures of more dinosaur samples and extend
the study to other species of extinct vertebrates, the researchers say.
In particular, taking the temperature of unusually small and young
dinosaurs would help test whether dinosaurs were indeed gigantotherms.
Knowing the body temperatures of more dinosaurs and other extinct
animals would also allow scientists to learn more about how the
physiology of modern mammals and birds evolved.
Study abstract
SOURCE
