It was a beautiful Sunday morning, the air full of Spring, a perfect time for my wife and I to go to the Zoo and check out the new dinosaur exhibit. Set up in an enclosed area within The River’s Edge, the temporary exhibit presents a dozen dynamic dinosaur models, scaled down to child size, that bend their necks and make interesting noises. Each model comes with a brief description of just what critter is being seen, and how big it really was when it was alive and lumbering about. Even at 9:30AM the exhibit was alive with children, and it was a joy to watch them relate to these mini-dinos with such immediacy and glee.
This exhibit does what it sets out to do very well: excite the interest of childern. With luck, the children — and the parents joining in the fun — will be lead by this experience to learn more about dinosaurs. The families that embark on this journey are in for a world of fun, for the story of dinosaurs is exciting, interesting, and full of unexpected twists and turns.
For years I taught a course on dinosaurs to Washington University undergrads, and a lot of them weren’t so different from the kids I saw last Sunday, just as full of curiosity and wonder, and eager to learn more. College students and parents are lucky, for dinosaurs can offer us older kids so much more: We can get our intellectual feet dirty with the arguments among scientists about what dinosaurs were really like. Scientists are a scrappy lot, by and large, and thoroughly enjoy the give-and-take of contesting ideas. Few things in science are more fun than a good argument about dinosaurs.
One of the fiercer debates among dino-palentologists (scientists who study dinosaur fossils) has focused on wether or not dinosaurs were “warm blooded.” Humans like you and I are said to be warm blooded because we maintain a relatively constant body temperature higher than the surrounding air. A lizard, by contrast, adopts the temperature of its surroundings and is said to be cold blooded.
There are other more formal terms that professional biologists use to talk about body temperature, like homeotherm, endotherm, and pokliotherm, but the basic sense of the issue is clear: warm blooded animals like mammals keep their body temperature constant, while cold blooded ones like reptiles don’t.
Because dinosaurs are classified as reptiles, they have been traditionally viewed as cold blooded. This view began to be seriously challenged four decades ago by John Ostrom, an influential professor of paleontololgy at Yale University, and his vocal student Robert Baaker.
Ostrom and Baaker marshalled a battery of indirect but attractive arguments to support the contention that dinosaurs were in fact warm blooded. Their argument about the growth rate of dinosaurs will provide some of the flavor of the case they made, and can be illustrated with the models at the Zoo’s dino exhibit.
Warm blooded and cold blooded animals grow at different rates, Ostrom and Baaker point out. Warm blooded animals grow quickly, while cold blooded animals do not. A lion grows to 200 pounds in two years, while it takes an alligator 20 years to grow to 200 pounds.
Did dinosaurs grow slowly like other reptiles, or fast like warm blooded animals? It seems they grew very fast indeed. Studies of tiny dinosaur baby bones in hadrosaur nests by Jack Horner indicate that some of the young reached considerable size while still in the nest, direct evidence that baby dinosaurs grew quite fast. The Parasaurolophus model dinosaur, second in line at the Zoo exhibit, illustrates this quite well. A hadrosaur (“duck billed” dinosaur) like those studied by Horner, this Cretaceous dinosaur is shown with a nest full of eggs, some of them hatching. Alongside the nest is a baby, already quite large. To have grown this large this quickly, it would have to have grown at a prodigeous rate.
Additional evidence for rapid dinosaur growth can be found in the microscopic structure of the bones. Dinosaur bones are rich in the passage ways called Halversian canals that transport nutrient-laden blood to the cells that lay down new bone. The fast-growing bones of warm blooded mammals have this same high density of Halversian canals, while the slow-growing bones of cold blooded reptiles have far fewer.
For many years, indirect arguments like these were batted back and forth, sustaining an intense and scientifically rich dispute. Of course, the only way to truly settle the matter would be to somehow take the temperature of a dinosaur, but as they are extinct, this didn’t seem an option.
Two scientists from North Carolina State, Reese Barrick and Bill Showers, however, found a way.
They started with the observation that the extremities of a cold blooded animal like an alligator or other reptile are colder than its torso — the tip of the tail is colder than the interior of the chest cavity because the tip looses heat more readily to the surrounding air. By contrast, a warm blooded mammal like a cow or deer constantly circulates warming blood to its extremities, keeping its tail at the same temperature as the body core.
So Barrick and Showers set out to determine if the extremities of a dinosaur were maintained at the same temperature as its body core (warm blooded), or not (cold blooded).
How could one possibly make such a determination? The researchers were very clever. Bone is composed of the mineral calcium phosphate (CaPO4). The oxygen atoms in calcium phosphate exist in nature in the form of two isotopes, 16O and 18O. Because the bonds formed by 16O atoms break a little easier than ones formed by 18O atoms (physicists call this the “isotope effect”), the oxygen atoms taken up into newly-formed bone tend to be preferentially more 16O at higher temperatures.
The ratio of 16O to 18O can be accurately measured. However it may vary from one animal to another, it should be the same at the extremities and the core of one animal, if that animal is warm blooded. That is just what Barrick and Showers found: In both deer and cow, the 16O/18O ratios are similar in tail and rib bones. In cold blooded animals, by contrast, the 16O/18O ratio was much more different, comparing tail to rib.
So what about dinosaurs? In six different Cretaceous dinosaurs, there was little or no difference between tail and rib in 16O/18O ratios. Comapring the unmineralized interior of T rex bones to rule out possible artifacts, the same result was obtained. Fossil reptiles found in the same deposits with T rex, by contrast, exhibited the difference characteristic of cold blooded animals.
Could this result be just a matter of large body size retaining heat? No. Small dog-sized dinosaurs show the same constancy of 16O/18O ratio. It looks like Cretaceous dinosaurs were indeed warm blooded.
You see the point. Dinosaurs and those who study them have interesting stories to tell us adult kids, once we get old enough to get our mental fingers around the arguments. In the meantime, the Zoo is to be congratulated on doing such a good job igniting the fires of interest in our next generation of dinosaur lovers.
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