T REX How Fast How Heavy

You can't answer one of these questions without addressing the other. And to make an informed guess

Rex Skeleton Drawing

about either, you need to take a close look at T. rex's complete skeleton, something only lately possible.

When you put T. rex's bones back together, there aren't a lot of ways to get them to fit. You can make a skeleton, and better yet a cast, go wherever you want it to by forcing and bending. A good example is the dramatic new Barosaurus mount at the American Museum of Natural History. Defending its young from a hungry Allosaurus, that huge plant-eater is rearing up on its hind legs with its neck held high, like Flicka on the old horse movies, only a lot more impressive. It's the tallest dinosaur mount ever. It's also an unlikely pose for a huge dinosaur in life, as the scientists at the American Museum would tell you. And Peter May, who built the exhibit, had to do a fair bit of bending and pulling to get the casts of the bones of Barosaurus to fit the desired pose.

As for our T. rex, the bones show an S-curve in the neck. Some curve in the neck is a feature common to many dinosaurs—one of the things that sets them apart from other animals. The exaggerated "S" in T. rex's neck drew its huge head farther back toward the hips, a way of pulling the center of gravity back, so the animal wouldn't fall forward.

With the body horizontal, the pelvis had to be aligned parallel to the ground. And the tail, which emerged from the pelvis, would have started out from the body well off the ground. But how did it end up? At our museum, we have only the proximal half of the tailbones from a T. rex, the part closest to the body. But we have a clear answer anyway. There were huge transverse processes, grooves and buttresses on the individual tailbones that would have supported large muscles. Those muscles would have been sufficient to keep the tail off the ground. And we see prints of a dragging tail on some of only the earliest of the many dinosaur footprint sites. That's negative evidence, the worst kind, but it still suggests that most dinosaurs kept their tails up.

With its thick tail muscles, T. rex would have been able not only to keep its tail well off the ground, but to

Tiger And Tyrannosaurus Rex

t. rex weighed little more than a bull elephant, but it was considerably longer. next to t. rex, the largest of the cats, the tiger, looks like a tabby.

swish it around for balance (even use it as a weapon) when it ran. It isn't hard for me to imagine a T. rex striding along with its tail held out behind it. You have only to look at crocodiles in the swamp to picture the tail of T. rex. Crocs don't drag their tails when they run—they keep them raised. And crocs can go at a pretty good clip, even gallop with a bent-leg waddle. With its long and powerful legs and its balanced two-legged stance, T. rex could probably run a lot faster than a crocodile.

With a close look at T. rex's bones and some pretty sophisticated thinking, we can make some reasonable guesses about how fast this dinosaur moved. Estimating how fast T. rex might have run is something we'll be able to do better after fully measuring Kathy's T. rex. Then we'll be smack in the middle of a good, long-standing argument about dinosaur speed.

When we made T. rex the fiercest of a stupid bunch of sluggards, it didn't need to be very fast to survive. Now that so many people think of dinosaurs as hot-blooded speedsters, T. rex is the swiftest of the swift— Carl Lewis in lizard skin.

The new image of the hot-running dinosaur was popularized by Bob Bakker. He says that the T. rex has

Bob Bakker Tyrannosaurus

a large bone surface below its knee, the attachment point for a huge mass of muscle that would have propelled it quickly as a runner. Its thigh bone was indeed massive, seventeen inches around.

Bob then contrasts that thigh dimension with his estimate for the weight of T. rex—four tons. That's more bone strength per pound, Bob argues, than a white rhino. Bob says he's driven a jeep in Africa at twenty-eight miles an hour and not put any distance betweeen himself and a white rhino. So T. rex, according to Bob, could run forty to forty-two miles per hour. Period.

I think Bob is great for paleontology. He makes a lot of intriguing statements that get other scientists riled up, and sets them to work disproving him. And like much of what Bob has to say, I think his estimate of T. rex's speed is extreme.

Just because something sounds logical doesn't mean it is true. Otherwise, as Ralph Molnar says, we could all be philosophers and not have to grub around in the dirt doing messy research. Scientists look for ways to test a hypothesis like the idea that T. rex moved at near highway speeds. Urifortunately, we don't know enough from looking just at bones to be able to tell how fast T. rex or any animal moved. It's not good enough to say that because a bone is big, with plenty of room for big muscles, that those muscles and bones were built for speed and not for support or other functions. Nor can you compare a two-legged dinosaur to a four-legged mammal of the same mass and suppose they must have moved at similar speeds. What's more, you can't be sure T. rex weighed four tons and not twice as much or half as much. But I'll get to that in a minute.

We can estimate an animal's speed fairly well when we have some other fossil evidence, such as a good set of tracks. If we compare the length of the stride to the size of the animal's leg according to a formula developed by a British zoologist, R. McNeill Alexander, we get a pretty good estimate. Apply his formula to some clear and well-spaced dinosaur tracks in Texas and you

bob bakker considers himself a maverick among paleontologists, and looks the part.

jim farlow studies dinosaur footprints, such as this carnivorous dinosaur trackway he subjected to computer analysis.

find that the fastest dinosaur we have footprints for, a medium-sized predator much smaller than T. rex, ran twenty-five miles an hour. But we can't say how fast bigger dinosaurs like T. rex moved because we don't have any footprints from them.

Speed aside, I've never been a great believer in dinosaur footprints as indicators of dinosaur behavior. But many of my colleagues are. They use footprints to draw conclusions about dinosaur herding and hunting as well as speed.

I think we don't know enough about what animal made those footprints, and under what circumstances. Maybe it was one animal using the same path over and over. Who knows if the animal was running at top speed? A T. rex probably couldn't sprint over the sort of soft sand or gooey mud that fossil footprints are made from. It may well be that none of the thousands of dinosaur trackways we have were made by a sprinting dinosaur. We don't have any footprints we know for sure came from T. rex. So anyone's estimate of how fast T. rex moved is just a guess.

Still, trackways do show us things about how dinosaurs moved that skeletons can't. From tracks, we know bipedal dinosaurs walked in an erect fashion, putting

Rex Footprint

their feet down almost direcdy one in front of the other.

Jim Farlow, who measured the Texas speedster dinosaurs (probably ornithomimids), does some very careful laboratory studies of dinosaur footprints. He compares them with tracks made by big flighdess birds of today, which are very similar to those made by bipedal dinosaurs (of which T. rex was one of the biggest). Jim "persuades" emus from the local zoo to run across prepared ground. Then he makes all kinds of measurements of the footprints and converts them to computer graphics. He hopes to use that information one day to tell him how dinosaurs might have distributed their weight while walking and running. And he hopes by studying patterns made by different birds he'll be better able to distinguish among dinosaurs by their footprints. Jim has also tested R. McNeill Alexander's formula for measuring dinosaur speed with his emus. Though Jim's results aren't final yet, videotapes of the emus suggest that they did in fact move at the same speed Jim estimated.

Among living animals the fastest runners are animals in the mid-sized range. Jim's footprint analysis shows the same for dinosaurs, or at least those we have tracks for.

left to right: dinosaur hindfeet and possible footprint shapes: t. rex, iguanodon, apatosaurus

Running Dinosaur TracksRex Dinosaurs Eating

emus bear many resemblances to meat-eating dinosaurs. but could t. rex have sat this way? i doubt it. i'm surprised the emu could.

emus bear many resemblances to meat-eating dinosaurs. but could t. rex have sat this way? i doubt it. i'm surprised the emu could.

Mid-sized dinosaurs were the fastest.

But as Jim points out, the fossil footprint record is biased. Many footprint sites are on small patches of land eroded by modern-day rivers. If T. rex were moving at twenty-five miles per hour or more, as fast as the fastest mid-sized dinosaurs, its stride would have stretched ten yards or more. There aren't many trackway sites large enough to preserve two such prints. And the squishy mud that takes good footprint impressions isn't a good surface for running fast.

However quickly T. rex might have run, it is likely it didn't travel at top speed often. Most animals walk relatively slowly most of the time they are moving, and most dinosaur footprints come from animals just ambling along, not racing.

Without footprints, how can you gauge the speed of T. rex?You can get some information from the bones. And if you go just by bones, it is by no means clear that T. rexwas a fast runner.

Fast-running animals have relatively long legs. They run on their toes, which are few in number and symmetrically arranged (either four or three toes). They have tightly linked foot bones, and their feet and lower legs are long compared with their upper legs. All meat-eating dinosaurs have these features, though T. rex doesn't show them as strongly as smaller bipedal dinosaurs do.

But bigger animals may have developed different shapes to support the greater weights of their bodies. If an animal is twice as long as another of the same shape, it will weigh eight times, not twice as much as its smaller version. (Weight is proportional to the cube of the length.) If a T. rex needed to be as fast as its mid-sized predator cousin, it might accomplish that, Jim writes, "by making its skeleton stouter, with thicker, more massive leg bones."

T. rex's bones were far stouter than those of the midsized predators that were thought to have been the fastest-known dinosaurs. Maybe these heavier limbs are designed to compensate for the stresses associated with fast ninning. Jim finds that in general, big animals tend to stand, walk, and run in a more stiff-legged, erect way than smaller animals. Jim's compared the angle formed by a leg from where it is when it's farthest back in the stride to where it is when it is swung forward. That angle is far smaller in large animals than it is in small animals. The greater range of motion of the small animals means that they can extend their stride relatively farther when running than a large animal can. To Jim that means T. rex probably couldn't extend its stride much. So it probably ran in a relatively stiff-legged fashion.

On R. McNeill Alexander's strength index, T. rex ranked along with the elephant. Alexander doesn't think T. rex was a fast runner. Not all scientists accept Alexander's formula as reliable. But even those who do subscribe to it have some questions about Alexander's conclusions for dinosaur speeds. Jim Farlow thinks Alexander, as a conservative scientist, may have underestimated the running abilities of dinosaurs. And the shape of T. rex's body is so different from that of any of the modern animals with which Alexander compared it that using the index to estimate T. rex's speed is a pretty risky business—something Alexander admitted.

The diciest part of estimating T. rex's speed by using Alexander's "athletic index" is getting a good estimate of the mass of the animal. That's easy enough when you have a living creature you can stick on a scale. But how do you weigh a T. rex?

You can estimate a dinosaur's mass from the limb proportions themselves. You measure the bone circumference and diameter and multiply that by the kind of bulk modern animals add to their skeletons.

Recendy, while a graduate student at Yale University, paleontologist Tom Holtz found that the mass of T. rex and other meat-eating dinosaurs can be closely estimated from the application of a mathematical formula to the length of a single bone, the femur. Holtz has found that the tyrannosaurs and the ostrich-mimic dinosaurs (and a few others) are distinct from other meat-eating dinosaurs in the proportions of their legs. Their limbs

Rex Dinosaur Eating Other Dinosaurs

knowing the leg length of a dinosaur is essential to calculating its speed.

knowing the leg length of a dinosaur is essential to calculating its speed.

the best estimates of dinosaur mass come from seemingly crude experiments such as measuring the water displaced by a toy dinosaur model. but if the model is good, the measurement can be sound.

are longer and slimmer near the feet than the legs of other predatory dinosaurs. Because scientists didn't make the scaling adjustment for the slim builds of the tyrannosaurs and ostrich-rnimics, Holtz thinks T. rex's mass has been overestimated at seven to eight tons when it should be about four and a half tons. But even Holtz's measurements require an independent estimate of mass. Usually that comes from one source, a toy.

By "toy" I mean an accurate model of T. rex. You make a scale model of the bones, then look at the muscle attachments and try and figure out what the musculature of the animal would have been like in life. Maybe you look at large ground-running birds for comparison. Then you put some plastic skin on the model.

When all these features are put together, you can get an estimate of the volume of the model. You do that by measuring the amount of water the model displaces. That's the way Archimedes discovered the concept of measuring volume when he sat in the bathtub, saw the water rise, and yelled "Eureka!"

Since crocodiles, which are fairly closely related to dinosaurs, are about the density of water, Alexander thought it reasonable to assume dinosaurs weighed about what their volume of water weighs. For a model dinosaur, the bath is an immersion in a calibrated container of water. Record the volume of water displaced, convert it to mass, and scale that back up to come up with a measurement of a full-sized T. rex's mass.

The problem with this method is that no one had made an accurate T. rex model, in part because before Kathy Wankel found her T. rex, no one had anything like a complete skeleton to build from. Now Matt Smith, who sculpted many of the display models in the Museum of the Rockies, has made a model that conforms to the new skeleton (see cover photograph). Jim Farlow, who is now doing the displacement experiment on Mart's model, gets a weight of about six and a half tons from his first of several measurements. I was surprised. This T. rex looks so much slimmer than previous ones that I imagined it was going to come out less than four tons. But I accept Jim's conclusions, and his methods, since that's the best we can do for now in estimating dinosaur weight. Since some elephants weigh four tons, and T. rex is a far longer animal, Jim's estimate makes sense. And it is still a lot less than the eight tons I've seen listed previously as T. rex's weight.

Our estimates of T. rex's mass are never better than the models we measure from. There's so much uncertainty in this good science that those who study dinosaur speed most closely, like Farlow, say we can't be sure from any anatomical evidence or 'mathematical projection we have just how fast T. rex could run. But if you ask Jim Farlow for his opinion, he'll tell you that he thinks T. rex could have run as quickly as twenty-five miles an hour, maybe even thirty-five miles an hour, though Jim doesn't think that likely. Anything faster is highly unlikely. I agree.

For me, it's simply a matter of common sense. To survive, predators need to be only a bit faster or more maneuverable than the fastest animal they hunt. Even if T. rex were the most active of hunters, its chief prey would likely have been the most common plant-eaters of its day, TriceratopsandEdmontosaurus. Looking at those bulky animals (in most reconstructions), you can't imagine their moving very quickly. T. rex didn't need to be fast to catch a Triceratops, so there's no evolutionary reason for an anatomy designed for speed.

Animals try to conserve energy. The bigger an animal is, the more it costs in energy to move its mass. The faster it moves, the more it costs. Furthermore, an animal as big as T. rex wouldn't be very maneuverable at high speeds. If it fell, it was a long way down for a head that stood twelve or more feet off the ground. Drop a skull from that height and there's a chance of causing a nasty injury, even a fatal one. And once a T. rex fell over, I think it would have had a hard time getting up.

Even at twenty-five miles an hour, T. rex is still frightening in my imagination, because at that speed a T. rex could still outsprint any of us.

We'll never have that race, but with all we know, and can reasonably guess from our T. rex about how that dinosaur sensed, and moved, we can begin to put together a new picture of T. rex as a living animal.

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  • Primula Tunnelly
    How fast was the t rex in its day?
    8 years ago
  • Thorsten
    What are the measurements of a apatosaurus footprint?
    8 years ago
  • nicholas
    How do tyrannosaurus move ?
    8 years ago
  • Arron Henderson
    What do t rex look like?
    8 years ago
  • jonathan
    How fast was the trex?
    7 years ago
  • mark
    How heavy was Tyrannosaurus rex?
    7 years ago
    How fast do tyrannosaurus rex go?
    7 years ago
  • Cora Underhill
    How to estimate trex speed?
    5 years ago

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