From the Cretaceous sandstone of South Dakota a decade ago came a flabbergasting find: the fossilized heart of a sixty-six-million-year-old dinosaur. Not only is this just the second time fossilized internal tissues of a dinosaur had been discovered (a specimen unearthed earlier in Italy yielded fossilized intestines), but the Dakota fossil appears to confirm the controversial theory that the dinosaurs, though reptiles, were, in fact, warm-blooded.

This particular dinosaur was a thirteen-foot-long six hundred and sixty pound herbivore of the genus Thescelosaurus. Its discoverer, a fossil-hunter named Michael Hammer, noticing a large mineralized lump within the creature’s rib-cage, suspected that it might be one of the animal’s internal organs. He asked an Oregon physician, Dr. Andrew Kuzmitz, to run a tomographic scan on it, and that indicated the presence of a heart, neatly encased within a stone sarcophagus.

The North Carolina Museum of Natural Sciences in Raleigh acquired the fossil in 1996 and launched a more elaborate study. Computerized three-dimensional pictures made by imaging specialists from the North Carolina State College of Veterinary Medicine led to the conclusion that the stone lump did indeed contain a heart, a four-chambered one, with an aorta-like struc-ture emerging from the left ventricle–thus providing the best evidence thus far for the warm-blooded-dinosaur theory.

Warm-bloodedness has been considered a biological advantage since Aristotle’s time. He wrote, "The thicker and warmer the blood is, the more it makes for strength." Since cold-blooded creatures have no internal temperature-regulating devices, they become torpid as the air temperature around them drops, and below a certain critical level enter a state of dormancy. When warmth returns they become active again, but if the day grows too hot they are in danger of overheating, and must find cool hiding places. All of this limits their ability to gather food, their responsiveness to unexpected challenges, and their ability to adapt to environmental change.

The high-powered metabolism and greater adaptability of warm-blooded creatures permits far more strenuous functioning of heart, lungs, and muscle tissue. They are more versatile than cold-blooded ones in every way, which is why the forests, meadows, and deserts of the world swarm with mammals and birds, whereas amphibians and reptiles skulk in odd corners and fish are confined to the sea.

That the circulatory systems of dinosaurs might differ from those of modern reptiles is an old idea. The pioneering British paleontologist Richard Owen had by 1842 recognized that the pelvic structure of the giant beasts set them apart from living reptiles. Owen had no doubt that they were cold-blooded, but he did not see them just as oversized lizards or crocodiles with unusual pelvises. Their hearts, he argued, must have been of different design also.

Modern reptiles have three-chambered hearts: two atria, one for oxygenated blood coming from the lungs and the other to receive deoxygenated blood from the rest of the body, and a ventricle in which both streams of blood are mixed to be pumped forth again. This mixing reduces the efficiency of the animal’s metabolism. Dinosaurs, Owen said, must have had a more "highly organized center of circulation" to operate their vast bodies, requiring hearts with two atria and two ventricles to avoid the mixing of stale blood with fresh.

Later scientists also found dinosaur metabolism puzzling. Frederic A. Lucas, in 1929, raised the issue of how dinosaurs, if they had been as sluggish as modern reptiles, could ever have managed to gather enough food to keep their enormous bodies functioning. Ultimately he fell back on the point that reptiles, precisely because they are sluggish cold-blooded creatures, need nowhere near the quantity of fuel required by mammals of the same mass. "Still," he added, "it is dangerous to lay down any hard and fast laws concerning animals . . . and in the present instance there is some reason, based on the arrangement of vertebrae and ribs, to suppose that the lungs of dinosaurs were somewhat like that of birds, and that, as a corollary, their blood may have been better aerated and warmer than that of living reptiles."

But no serious challenge to the classic view of dinosaurs as cold-blooded developed until 1964, when the Yale paleontologist John Ostrom discovered a mini-dinosaur he called Deinonychus, "terrible claw" –a fast-moving carnivore that weighed about 150 pounds and chased its prey on powerful hind limbs. Running in an upright position, Ostrom pointed out, requires a tremendous output of energy, incompatible with a cold-blooded metabolism. In today’s world, the only animals that walk upright are warm-blooded ones, mammals and birds; reptiles and amphibians are waddlers and sprawlers. Yet here was the upright Deinonychus: how, Ostrom wondered, did it manage to keep itself on the move if it had had a reptilian metabolism? For that matter, where did the other, larger dinosaurs find the energy to pump blood from their hearts to brains that might be located twenty or thirty feet away?

Then the French anatomist Armand de Ricqles, examining thin sections of dinosaur bone, discovered them to be rich with canals that facilitate the passage of calcium from the blood to the skeleton. Contemporary reptile bone has no such canals, and Ricqles concluded that this "indicates rate of bone/ body-fluid exchange at least close to those of large, living mammals."

Next Robert T. Bakker, a former pupil of Ostrom’s, launched an all-out offensive in favor of saurian warm-bloodedness. Bakker studied the numerical predator/prey ratios in fossil beds where dinosaurs were found. Warm-blooded carnivores like lions and cheetahs must live among vast surrounding populations of herbivores to survive; otherwise their own voraciousness would cause them to consume their whole food supply. But large predatory reptiles like the Komodo dragon, being cold-blooded and thus of sluggish metabolism, need no more than three times their body weight in food a year. Bakker showed that the predator-prey relationship for dinosaurs was two or three carnivores per hundred herbivores, about the same as it is for modern mammals. But for non-dinosaurian reptiles and amphibians the predator/ prey ratio is closer to 40 per hundred.

For Bakker this proved that the dinosaurs had been alert, fast-moving warm-blooded creatures with the high energy needs that a physiology with internal-temperature-regulating capacity demands. His ideas have gained a wide following, despite the opposition of more conservative paleontologists such as Nicholas Hotton, who concedes that dinosaur physiology probably differed from that of modern reptiles, but will not go as far as accepting warm-bloodedness, declaring, "Alternative thermal strategies and life-styles available to dinosaurs may well have been as exotic as their body form, the like of which no man has ever seen."

And now comes the discovery of a dinosaur fossil containing a four-chambered heart. That clinches the issue–or does it?

Well, not exactly. Some scientists, like Dr. Paul C. Sereno of the University of Chicago, wonder whether the stone lump within Thescelosaurus really is its heart, or just a coincidentally heart-shaped mass. He has questioned the whole concept of preservation of internal organs in fossils.

Others accept the idea that the fossil does contain a heart, but are skeptical about its four-chamberedness. Here the evidence is unclear. The Carolina scientists admit that only the ventricles and aorta are distinguishable with the imaging methods that have been used so far. But they insist that the two upper chambers–the atria–must also have been present, since all reptilian hearts have two atria.

The development of more sophisticated scanning techniques will surely settle some of these questions. I suspect that we’ll find that Thescelosaurus’ heart did have four chambers, which will support the idea that dinosaurs had some sort of system for internal metabolic regulation. The dinosaurs, after all, dominated the world for a hundred million years, until–so it is widely thought–they were wiped out sixty-five million years ago by the apocalyptic climatic changes resulting from the collision of an asteroid or comet with the earth. For one group of animals to have maintained supremacy for such a length of time argues for their great biological adaptability.

This is not to say that the dinosaurs necessarily were warm-blooded animals the way lions and squirrels and sparrows and human beings are. As Richard Owen pointed out long ago, and such scientists as Nicholas Hotton and Armand de Ricqles have reiterated in our own time, dinosaurs may very well have had unique metabolic systems that at present we can’t understand, because the fossil evidence is inadequate.

If so, it widens the range of probabilities for life in the universe in general. Up to now we’ve tended to assume that the system of categories with which we define terrestrial vertebrates–amphibians, reptiles, birds, mammals–would probably hold true on all planets that have environmental conditions analogous to ours. Such thinking pretty well forecloses the possibility of non-mammalian intelligent life, which would be precluded by the metabolic limitations of reptiles and amphibians.

But we know that our own planet once harbored a race of reptilian beings that managed to maintain their position at the summit of creation for an immense period of time, until a cosmic catastrophe destroyed them, and it is beginning to appear that their metabolic systems were significantly different from those of the reptiles of today. We already know that it is a grave error to lump the dinosaurs together with such sleepy creatures as alligators and tortoises simply because similarities of their skeletal structures lead us to class them all as reptiles. There are reptiles and reptiles, evidently, and some were quicker and smarter than others.

Intelligent reptiles have long been a staple of science fiction, going back as far as E.E. Smith’s Lensman novels of sixty years ago, one of whose heroes is the fearless Worsel of Velantia, "a nightmare’s horror of hideously reptilian head, of leathern wings, of viciously fanged jaws, of frightfully taloned feet." Smith ignores the question of Worsel’s metabolism, but surely a four-chambered heart must have beaten in that saurian bosom. The "wise and noble reptiles who had mastered superluminal physics" that James Patrick Kelly gave us a few years ago in his award-winning story "Think Like a Dinosaur" had to have been warm-blooded as well. And the fossil heart from South Dakota–if indeed that is what it is–provides us with the first substantive evidence that Earth’s own dinosaurs may have been dynamic and intelligent animals. When and if the aliens come from space to visit us, it may very well turn out that they have the beady eyes and scaly skins of the reptilian critters SF long has loved to conjure up.