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Were Dinosaurs Warm-Blooded?
Discover, Dec, 2000 by Karen Wright
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.
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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.
Scientists have found the soft-tissue remains of just one other dinosaur, Scipionyx samniticus. The arrangement of its liver and intestines hints that it may have relied on a diaphragm to inhale--like warm-blooded animals--instead of expanding its rib cage to take in air, as most reptiles do.
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