|Left: Traces of two ventricles hint that a lump of rock found with fossilized bones contains a dinosaur heart. |
Image by Paul Fisher, NCSU College of Veterinary medicine biomedical imaging resource facility
Dead men may tell no tales, but dead dinosaurs are even less likely to spill their guts. That's because dinosaur guts and hearts and lungs and skin and the rest of their soft tissue usually decomposed before they could fossilize, leaving paleontologists in the dark about certain critical aspects of the bygone beasts' appearance and behavior. Now the surprising discovery of a dinosaur's stony heart has revived one of the perennial questions of dinosaur lore: Were 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.
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 that 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 of 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 of birds, which are warm-blooded.
|Researchers who studied the bones and stony heart of Willo believe the dinosaur was a highly mobile herbivore.|
Ed Heck/AMNH; NC ,Museum of Natural Sciences
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.
Ruben contends that the only anatomic features directly related to metabolic rate are bony or soft cartilaginous scrolls, called respiratory turbinates, that curtail the loss of heat and moisture in the nasal passages of almost all warm-blooded animals. Because fast metabolisms demand more oxygen than slow ones, warm-blooded creatures must take in more air than cold-blooded animals and, without the turbinate scrolls that trap exhaled water, they could easily become parched. Trouble is, turbinates are soft tissue, so no one expects to find them in dinosaur fossils. But Ruben and his colleagues say they've identified a bony ridge where the turbinates attached in 70-million-year-old bird skulls. So far, not one of the dinosaur skulls that Ruben has examined shows any signs of a turbinate ridge. "Very clearly, they just aren't there in dinosaurs at least not in theropod dinosaurs," he says.
Theropods are the big, active predators whose highly cinematic style of aggression has captured the imagination of paleontologists and the public. Some dinosaurs in the lower-profile order to which Willo belongs may have been warm-blooded, Ruben concedes but a stone softball in a fossil chest isn't enough to convince him. "Even if you assume it's a heart, you cannot tell from what's preserved there whether it's more like a reptile's or more like a mammal's or a bird's," he says.
The four-chambered hearts of mammals and birds have two large cavities called ventricles that keep the oxygen-poor blood returning from the body separate from the oxygen-rich blood supplied by the lungs. The typical reptile heart, in contrast, has one ventricle in which oxygen-rich and oxygen-poor blood commingle, so oxygen delivery is far less efficient. But crocodiles and alligators are exceptions: They have two aortas and two ventricles that can switch between segregation and mixing modes. Critics like Ruben maintain that the North Carolina team's CT scans aren't sufficient evidence to rule out the possibility of a second aorta in Willo's heart and thus the possibility that the organ resembles a crocodile's.
"Is this lack of information disturbing? Yes," Russell concedes. "Is it compelling? No. Because this is a decayed organ it's not complete." Russell and his colleagues are conducting a fresh round of high-resolution scans to see if they can find more telling details of the heart's structure. Meanwhile, there may still be room for a middle ground in paleontologists' metabolic musings. At least one expert has speculated that dinosaurs were an intermediate between cold- and warm-blooded animals, and Ruben has proposed that they mixed attributes of both metabolic styles in a unique physiology that ultimately went nowhere.
"They're not like anything alive today," he says. They were the peerless icons of a vanished past, when reptiles were huge, and mammals were scared.
For more, see www.dinoheart.org