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Were Dinosaurs Warm-Blooded?

Discover, Dec, 2000 by Karen Wright

The tricky art and science of interpreting CT scans of fossils

DEAD MEN MAY TELL NO TALES, BUT DEAD DINOSAURS ARE EVEN LESS to spill their guts. That's because dinosaur guts--and hearts and lungs and and the rest of their soft tissue--usually decomposed before they could fossilize leaving paleontologists in the dark about certain critical aspects of the beasts' appearance and behavior. Now the surprising discovery of a stony heart has revived one of the perennial questions of dinosaur lore: the giant reptiles cold-blooded, like snakes and crocodiles, or warm-blooded, like kangaroos and kittens--and us? There's more at stake than a mere factoid of physiology, for the metabolic status of dinosaurs holds clues to what they ate, how they looked, and even why they became extinct.

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The newfound heart belongs to "Willo," a 66-million-year-old herbivorous Thescelosaurus unearthed in South Dakota by a private collector in 1993. Last April, researchers announced that CT scans of a grapefruit-sized rock in the dinosaur's chest cavity revealed the outlines of two ventricles and a single aorta hallmarks of the four-chambered heart that today is found only in warm-blooded animals. "None of us ever thought it could be a heart, because a heart's made of muscle, and it should have decayed" says Dale Russell, a paleontologist at North Carolina State University and curator at the North Carolina Museum of Natural Sciences in Raleigh, where the remains of Willo are on display. Russell and his colleagues think mineral traces of the heart may have survived because Willo perished on a submerged sandbar, and a waterlogged carcass buried in sediment would resist decomposition.

Skeptics are questioning both the CT evidence and the Raleigh team's interpretation. Their objections typify a debate that has been raging for decades. Paleontologists once assumed dinosaurs, like all present-day reptiles, had slow metabolisms that didn't maintain a constant body temperature. Then in the 1970s, self-styled dinosaur heretic Robert Bakker pointed out that a cold-blooded metabolism probably couldn't have supported the high-energy lifestyle implied by the erect posture of predatory dinosaurs such as the fearsome Tyrannosaurus rex. Bakker also noted that there wasn't a shred of proof for the cold-blood prejudice---only the circular argument that if dinosaurs were reptiles, then they must be cold-blooded, since all reptiles are cold-blooded.

"It matters because we're trying to figure out what these animals were like as living creatures," says Jack Horner, a paleontologist at the Museum of the Rockies in Bozeman, Montana. Because cold-blooded creatures are warmed by their environment, they need less energy and oxygen than warm-blooded ones, and they tend to be less active. A different metabolism conjures a different lifestyle: Warm-blooded animals eat more, breathe faster, and laze less; they're more likely to be insulated with feathers, hair, and fat; they can travel long distances; and they need to brood their eggs. Above all, the evolution of warm-bloodedness gave animals an expanded repertoire of behaviors that help them withstand minor fluctuations in food and climate. If dinosaurs were cold-blooded, their extermination may simply have been the gradual result of competition with the upstart warm-bloods; but if they were warm-blooded, says Russell, nothing short of a catastrophic asteroid impact could have put an end to their 150-million-year reign. It's no coincidence, he says, that the emergence of an impact theory for dinosaur extinction has coincided with growing support for the idea of warm-blooded dinosaurs.

And until the putative discovery Willo's heart, fossilized bone served as the only physical evidence paleontologists could use to plumb the mystery of dinosaur metabolism. Horner, for example, has studied microscopic features of bone tissue that can reflect the differences in growth rates between warm- and cold-blooded animals. Fast growth generally requires a speedy, warm-blooded metabolism. And blood vessels create many more spongelike spaces in fast-growing bones than they do in slow-growing bones. Horner's studies of fossil bones suggest that even the most sluggish species of dinosaur had growth rates similar to those birds, which are warm-blooded.

Analyses of oxygen isotopes in dinosaur bones have also been used as! evidence of a warm-blooded metabolism. Working with tyrannosaur bones in the early 1990s, Russell's colleagues at North Carolina State looked for changes in isotope ratios that occur when bone forms at varying body temperatures; they found none.

But a stable body temperature isn't necessarily proof of warm-bloodedness, says John Ruben of Oregon State University in Corvallis. Big, cold-blooded animals like sea turtles and Komodo dragons have stable body temperatures because the ratio of their surface area to volume is relatively low, minimizing heat exchange with the environment. And reptiles are masters at basking and burrowing to compensate for changes in air temperature.

Were Dinosaurs Warm-Blooded?

Discover, Dec, 2000 by Karen Wright

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