Why Did T Rex Get So

T. rex wasn't the only giant dinosaur of its day. The herding duckbills and horned dinosaurs were almost as hefty. So I'll rephrase the question: Why were the last dinosaurs so big? Well, maybe it's because the seaway in the middle of North America was drying out and there was more space for bigger animals to browse. And paleontologists have thought for a while that as the herbivores got bigger, the carnivores did, too.

Each group of animals seems to start out small and get bigger over time. Some species, however, remain the same. Those that do eat a lot of different things seem able to adapt to change more readily. Those that grow huge are more limited in the kind and amount of food and the extent of habitat they need, and so are more apt to go extinct than small animals. Throughout the geologic record we see trends toward bigger animals and then the extinction of those big animals. That doesn't mean big animals are more susceptible to extinction. Nearly everything that ever lived is dead. It might have taken a truly catastrophic event to wipe out the last dinosaurs, and the last dinosaurs, big and litde, seem to have disappeared at about the same time.

Mammals were no bigger than house cats all through dinosaur times. But when the dinosaurs vanished at the end of the Cretaceous period 65 million years ago, mammals started getting larger until there were giant mammals, bigger than elephants, in some groups in the Eocene (56 million to 34 million years ago). If dinosaurs had lasted longer, a killer even bigger than T. rex might have come along.

Canadian paleontologist Phil Currie has a different idea about why T. rex and its contemporaries, the last dinosaurs, were giants. The weather was becoming more and more harsh at the end of the Cretaceous. To Phil's thinking, big animals and primitive animals are the best adapted to surviving hard times, at least in the short term. Primitive animals aren't specialized in their behavior and can tolerate a wider range of environments.

T. rex was huge, bigger than almost every preceding carnivore, and far bigger than the meat-eating mammals that replaced it. Some scientists have suggested that tyrannosaurs could have become so big only by sea turtles are thought of as cold blooded, but their mass enables them to retain a consistently high body temperature. i suspect dinosaurs benefitted from the heat retained by their mass as well.

being slow-moving scavengers.

It isn't easy being huge. You've got to keep your population down so you don't use up the available food. But you've also got to keep enough of your kind around to prevent quick extinction. When you apply to dinosaurs estimates based on the number and size of modern mammal predators, as University of Indiana paleontologist Jim Farlow has done, the figures don't work. There wouldn't have been enough T. rexes to keep the species alive for the millions of years we know it endured. Something in the world of T. rex must have been quite different than it is, say, for lions on the Serengeti Plain today.

Paul Colinvaux, a well-known ecologist, asked and answered the same question in his 1978 book Why Big Fierce Animals Are Rare. Colinvaux drew on research that showed, in keeping with the second law of thermodynamics, that energy is lost with each step you take up the food chain. It's a much more efficient use of energy to eat plants than it is to eat the animals that eat plants. The giants of the earth today, blue whales, eat low on the food chain—plankton and shrimplike krill. We don't have many huge animals, Colinvaux figured, because "the energy supply will not stretch to the support of super-dragons."

But T. rex was a superdragon. Colinvaux figured it got that way by hoarding its energy:

The tyrannosaur was not a ferociously active predator... most of its days were spent lying on its belly, a prostration that conserved energy and from which it periodically roused itself Nothing like it has been seen since because the true active predators of the age of mammals were able to clean up the meat supplies before a sluggish beast such as a tyrannosaur could get to them. And active predators might even have eaten the tyran-nosaur itself.

Jim Farlow accepts Colinvaux's basic premise about energy loss up the food chain limiting the size animals can attain. But he finds the portrait of T. rex unflattering and unlikely, and so do I. There were smaller carnivores around in T. rex's day, and we have no evidence they ate T. rex. Certainly they didn't wipe it out. And just from appearances, T. rex and the other tyrannosaurs weren't built to be sluggish. Ostriches and emus, the large flighdess members of T. rex's closest living relatives, the birds, don't do much sitting around.

Think of what factors could have allowed T. rex to get huge, instead of what prevents animals from getting big. T. rex had some huge plant-eaters to dine on. Who knows what allowed them to get huge? Maybe there was more carbon dioxide in the atmosphere then, as several scientists have suggested. That would have allowed richer plant growth to sustain these gigantic browsers. Maybe duckbills and horned dinosaurs were better at getting energy from their diets than cows, deer, and other modern vegetarians are. A lot of maybes.

You also have to consider metabolism in any speculations of size, diet, and activity levels. If dinosaurian plant-eaters had a slower metabolism than modern mammals, they would have needed to eat less to survive, and so could have lived in greater populations without eating themselves out of existence. If dinosaurs grew slower than mammals do - based on bone studies it seems likely they did in adulthood - dinosaurs probably had a lower metabolism. So, Jim Farlow reckons, dinosaurs could have lived in denser populations than modern mammals can. If dinosaurs' metabolism slowed down when they grew up, there would be even more duckbills to the acre for T. rex to eat.

And maybe carnivorous dinosaurs ate less than large mammal predators do. Big animals can maintain a steady body temperature far more easily than little ones, simply by virtue of the size of their bodies. The bigger you get, the more your volume increases in proportion to the surface area of your body. The less surface area in comparison to volume, the less heat flow is pulled away from the body core and lost. So, small animals are more affected by the temperature around them than big animals are. Bigger creatures, even cold-blooded ones

dimetrodon. no animal is more often wrongly labeled a dinosaur. in truth, it is more nearly related to us, and tens of millions of years older than the oldest di nosaur.

dimetrodon. no animal is more often wrongly labeled a dinosaur. in truth, it is more nearly related to us, and tens of millions of years older than the oldest di nosaur.

like sea turtles, can survive prolonged periods of extreme weather because their size and ability to conserve heat, including the heat generated by the movements of their huge muscles, help to keep their temperature stable. This metabolic strategy is called gigantothermy. If T. rexwere a gigantotherm, it could have kept itself warm on the cold nights, or cool on the terribly hot days that might have killed smaller dinosaurs.

Jim Farlow thinks T. rex and other dinosaurs may have had a more variable metabolism. Perhaps they could shift their metabolic rates from low reptilian levels to higher bird and mammal levels according to their age, or the season (a sort of hibernation), or over even shorter time spans. The issue of dinosaur metabolism was a hot one in the 1970s, but was pretty much set aside by the 1980s, with a consensus scientists' answer of "maybe, for some dinosaurs" ("sometimes," Jim Farlow would add).

Now, though, we may be getting some hard answers about dinosaur metabolism. The most interesting new discovery about metabolism doesn't come from paleontologists, but from geochemists—Reese Barrick and Alfred Fischer at the University of Southern California and Bill Showers at North Carolina State. They developed a way of measuring temperature variations within an animal's body, whether it had died recendy or hundreds of millions of years ago. When they looked at T. rexribs, backbones, and lower and upper limb bones, the researchers found that little variation had occurred from bone to bone. The results, announced to North American paleontologists at our annual meeting in the fall of 1991, were intriguing to all of us. They suggested that T. rex had littie internal body temperature variation. Like modern hot-bloods, evidendy it had an efficient system for heat retention or maintenance. The same was true of other dinosaurs they sampled. It's not proof that dinosaurs were warm blooded like us. The difference seen in bone temperature might have more to do with how much blood gets out to the limbs than with differing metabolisms. Nor does it mean that T. rex was a fast runner, as the researchers have suggested. But it is some of the best hard evidence we have about dinosaur metabolism.

Maybe we can use these measuring techniques to test Jim Farlow's idea that dinosaurs might have had different metabolic strategies at different times in their lives. I've shown with Maiasaura and other plant-eating dinosaurs that young dinosaurs grew fast. And the evidence for shifting metabolic rates in dinosaurs is, so far, limited to T. rex and hadrosaurs. It does seem that big dinosaurs would have lived very inefficiendy if they were hot blooded. Why expend all that energy keeping warm when you can do so by the heat your own bulk produces? Dinosaurs had a lot smaller ratio of body surface area to volume than do smaller animals like in t. rex's time there were fish in the sea, mammals on land, and birds in the air, just as today. but the biggest animals in the sea were ichthyosaurs and plesiosaurs, pterosaurs in the air, and dinosaurs on land. this is ctenothrissa, a late cretaceous fish.

above: a cross-secti o n of primary bone from the leg of kathy wankel's t.

rex, magnified thirty times. the dark lines are the vascular openings for blood and nerves.

above: a cross-secti o n of primary bone from the leg of kathy wankel's t.

rex, magnified thirty times. the dark lines are the vascular openings for blood and nerves.

below: as the bone cells mature, the primary bone is eaten away and replaced with secondary bone with more dense and regularly shaped bone cells, also shown at thirty times magnification.

ourselves. That's not to say they couldn't have led high-energy lives as hot-bloods. It just doesn't seem like an efficient strategy for dinosaur life.

There are other prospects for one day figuring out T. rex's metabolism and how it grew so big. If you had a clear, consistent reading of the ratio between the population of predators and prey (or scavengers and potential carrion animals) in T. rex's day, you might be able to make an informed guess about T. rex's metabolism, population, and range. Bob Bakker finds the ratio of dinosaur predators to prey parallels that of modern warm-blooded predators to their prey: about one to three predators for every hundred prey animals. Coldblooded contemporary predators, like crocodiles, which require far less food, can live in densities ten times as high in relation to their prey. But I don't think Bob's estimates work for dinosaurs. We have too few fossils and too many complicating factors affecting which animals get preserved in different environments to make a reliable estimate of a predator/prey ratio for dinosaurs, or to draw any conclusions about dinosaur metabolism from those ratios.

We have found some evidence ourselves from Kathy's T. rex that suggests T. rexwas warm blooded, but not as we are. Through a generous gift from one of our donors, Anne Merck-Abeles, our museum has the only dinosaur histology lab in the country. That's a facility for studying dinosaur bone microscopically.

We've punched a few little holes in the T. rex, in places where it won't make much difference to anyone. From these cores we've cut razor-thin cross-sections. We stain these to show up well under the microscope and then put them in a special viewing microscope and camera at anywhere from 10 x to 100 x magnification. Then we can take pictures, in black-and-white or color, of what we see.

What we see in T. rex (and in many other dinosaurs) are two types of bone development. Both look a littie like rounded hollow pasta, especially after we optically color them red in our computer.

The rings, or holes, are sometimes round, but are more often elongated. These are cross-sections of vascular canals for blood vessels. The number of vascular canals suggests dinosaurs had an abundant blood supply, as warm-blooded animals today do. The hole, and the collagen bone tissue that develops around it, are called an osteon. When dinosaurs were young, they had what we call primary osteons. The fibers around the hole were circular and blended in with the surrounding bone. When dinosaurs matured, their bones featured secondary osteons instead. In these the hole expanded, and bone was laid down within the hole.

Dinosaur bone comes in two forms. When young, dinosaurs produced bone in a woven network, much like that of birds and mammals, a pattern we call fibrolamellar. When dinosaurs matured, their bone was laid down in layers, more like the bone of reptiles and amphibians. We call that pattern zonal lamellar. Between each layer of dinosaur bone is a line of arrested growth. Their first arrest line marks the time when I consider dinosaurs adults, though it's not the only possible definition of adulthood, and I don't know how or why that pause in growth happened. Dinosaurs didn't stop growing after that first pause, or any of the many subsequent pauses marked by other arrest lines— dinosaurs grew all their lives. But their rate of growth slowed down considerably after that first pause.

This sort of change happens in our bones about the time we reach forty, though we're long finished growing by then. In duckbills the change in bone structure probably happened when they were just four or four and a half years old. We don't know how old T. rexes were when the change in bone structure occurred. We do know that other dinosaurs' growth rates slowed substantially at this time and afterwards, enough for the difference to be recorded in their bones.

So does that mean dinosaurs were hot blooded like us and birds as youngsters, then cold blooded like reptiles as adults? Definitely not. For one, we've been talking about bone-growth patterns, not metabolism. A

dinosaur's metabolism didn't shut down when its bones stopped growing, or else it would have died right then. And the bone growth pattern of adult dinosaurs still looks different from that of reptiles. Dinosaurs' bones were fed by more blood vessels than reptiles' bones are.

What the bone studies do show about dinosaur metabolism is that dinosaurs were endothermic homeotherms like us or birds when they were young. After they reached a certain age, or more likely a certain mass, they switched strategies and became mass homeotherms, a bit like the gigantotherms I described earlier. That means dinosaurs' high volume and relatively low surface area, and their ability to utilize external heat and the heat generated by the movement of their muscles, enabled dinosaurs to stay active warm-bloods without burning so much fuel (comparatively) as we and birds must do to stay warm.

These bone studies are the best evidence I know of that dinosaurs had their own special metabolic strategy, unlike anything we know about before or since dinosaurs.

There are other things our bone studies can tell us the narrowness of T. rex's skull may have allowed its eyes to work stereoscopically, giving it depth perception.

Tyrannosaurus Rex Skull Muscle Pictures

the narrowness of T. rex's skull may have allowed its eyes to work stereoscopically, giving it depth perception.

about the life of Kathy's T. rex and other T. rexes. Sometimes dinosaur bones show changes, like arthritis, that point openly to a well-aged individual. We haven't found that on our T. rex. But when we looked at microscopic bone sections of its shin, we saw primary osteons. Why that area had new bone-growth cells we don't know. Maybe the dinosaur had injured its leg and had recovered shortly before it died. But there is no visible sign of an injury on the bone. As you might expect, well-rehealed bone shows up more in young dinosaurs. (If anyone figures that mystery out, it will be my graduate student Mary Schweitzer, who is working on T. rex bone histology.)

Perhaps there are other indications of T. rex's metabolism to be found in its skeleton. Bob Bakker, who sees dinosaurs as raging hot-bloods, thinks T. rex's head was ventilated for air conditioning. But a hot head could be a result of T. rex's huge mass, not a hot-blooded metabolism like ours. He also thinks that since some of these holes are located near the brain's blood supply, they might have worked as radiators to cool off T. rex's hot blood, a notion Phil Currie is dubious about. And the t. rex's skull design was well-ventilated. was this for air-conditioning or weight-saving?

Rex Skeleton

spiral bones discovered in the nose of T. rex's litde cousin and contemporary, Nanotyrannus, Bob thinks functioned to warm air for a hot-blood. Even if the holes and spirals in T. rex's head weren't radiators, many were conduits for carrying sophisticated sensory information.

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Responses

  • philip brooks
    Why were mammals bigger than now?
    8 years ago
  • paul
    Why are tranasaurus rex radio active?
    6 years ago
  • SEMRET
    Why did dinosaur grow so large?
    6 years ago
  • sophie
    Why is tyrannosaurus called t rex?
    6 years ago

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