TREX predator or scavenger

AAA AS T.REX A VICIOUS killer? Ask mostanyone, including most paleontologists, and they'll say yes. Ask me, I'd say no. You may have noticed that I haven't referred to it as a predator once in this book, only as a carnivore. We're all guessing, but I think those who cast T. rex as a predator are letting some common prejudices cloud their thinking.

For sure, T. rex ate a lot. It takes a lot of hamburger to feed a six-ton, or bigger, animal. But it shouldn't have mattered to T. rex whether lunch was meat caught on the hoof or ripped off a carcass. Either way, T. rex had to go get its food. If it were a scavenger, it would have had to roam around looking for carrion. If it were a hunter, it would have had to track down its prey. The chase probably wasn't easy. The big hunters of today, from lions to wolves, miss their quarry nine times out of ten.

What did T. rex eat? Pete Larson of the Black Hills Institute thinks he might have the answer in purported T. rex fossil scat found at the site of his crew's T. rex discovery in South Dakota. Coprolite researcher Karen Chin is going to analyze that material to see what it contains, but she's dubious of its identity. We might be able to determine the diet of the animal that produced it, but there's no way to say for sure that T. rex made those coprolites.

A more certain way to determine a dinosaur's diet is to study the isotope concentrations inside the bones themselves. Fossils still have original organic matter in thpm Tf vr\n p-vtmrt thr»«e r»rr»tf»in« vr»n ran rnmrvire one dinosaur with another. The relative abundance of amino acids in fossils indicates what the animals ate, as geochemist Peggy Ostrom at Michigan State University found, first with sea creatures, and lately with dinosaurs.

Peggy took samples from a lot of dinosaur teeth from the Judith River Formation of Alberta, laid down several million years before T. rex's time. The higher the animal's place on the food chain, the higher the nitrogen content in its bones, or the bones of the animal that ate it. For instance, duckbilled dinosaur bones are not as nitrogen-rich as horned dinosaur bones are. Maybe that's because duckbills ate different plants than the horned dinosaurs did. Peggy isn't sure. She did find that the tyrannosaurids of that time (such asAlbertosaurus) seemed to prefer to eat duckbills. The carnivores' bones had a level of nitrogen that corresponded to a diet of duckbills, not horned dinosaurs. Smaller dinosaurs, such as the ostrich-rnimic ornithoniimids, show evidence of a more varied diet.

That doesn't mean T. rex liked eating duckbills better. From the Hell Creek Formation it looks like there were a lot more Triceratops around to eat than duckbills, and I don't expect that T. rex was very choosy about what this is the likely result of dinosaur's digestion, a carnivore's fossilized waste we call a coprolite.

it ate, especially if what it ate was already dead.

How much did T. rex eat? Some of the questions of metabolism that enter into

. , , .. j. scavenger begins here that calculation were discussed in the last chapter. Artist Greg Paul has done a lot of speculating about T. rex. He figures it would have eaten about four hundred horned dinosaurs if it lived sixty years, or one Triceratops every six weeks. Since a Triceratops might have weighed twelve thousand pounds, that's more than three hundred pounds of meat a day, maybe twice that. All that T. rex feasting, Paul suggests, would have held down the numbers of plant eaters and preserved the vegetation from overgrazing. Paul figures there may have been a quarter of a million T. rexes at any one time scrounging around the western floodplain. He estimates that two to five of every hundred dinosaurs that lived then in the West were T. rexes.

With an appetite like Paul estimated for T. rex, that many T. rexes might have eaten one out of every five big plant-eaters every year.

I don't know about that estimate. It assumes T. rex was feasting on live young dinosaurs, not on old, sick, and dead ones. And you can't tell populations from percentages of fossils. There are all sorts of built-in biases in the fossil record. Depending on where you look, big dinosaurs may be easier to find than small ones. Certain habitats favor some kinds of dinosaur fossils over others. We get fossils from lowland stream channels, but many kinds of dinosaurs didn't live or die near there.

Accurate diet figures would be based on estimating T. rex's and Triceratops'weight, something we can't do very reliably yet, and T. rex's metabolism. We don't know how much running around T. rex did or how much energy it burned keeping its temperature steady.

T. rex could have gone most any place in search of a meal. It might well have been a good swimmer. Lots of land animals today are good swimmers, from dogs to horses to ostriches. Greg Paul thinks T. rex's long toes, strong limbs and maneuverable tail would have helped it wade and swim faster than the more hulking prey it might have chased. Canadian paleontologist Phil Currie has found footprints in the Peace River of Alberta that seem to show three mid-sized carnivores chasing some iguanodonts into the water from land. Then again, T. rex could have gone into the water just to take a bite of a carcass.

Vhether a predator or a scavenger, T. rex would have made good use of keen senses to get a meal. Forward-facing eyes, and perhaps stereo vision as a result, would certainly have helped it fix distances when it was tracking down an animal. That visual acuity would help it make precise bites when it was trying to kill, especially in twilight or night, when T. rex's eyesight advantage would be most useful. But acute eyesight would also have helped in spotting distant carcasses. As paleontologist Jim Farlow points out, T. rex was capable of standing tall enough to have a big advantage over other dinosaurs in surveying the countryside. A keen sense of smell would help find rotting meat as well as sniff out nearby live prey.

We could just leave it at that—T. rex was a hunter or a scavenger, and we'll never know for sure. But it seems to matter a lot to us which T. rex was.

Just looking at T. rex, most of us would assume it was a hunter, and a darned good one. There are plenty of anatomical indications to suggest T. rex was indeed big and powerful: massive legs to chase down prey, teeth half a foot long in huge jaws that could rip off and swallow five hundred pounds of meat in a single bite and gulp. But scavengers can be big and rx>werful, too.

Paleontologist Don Baird thinks T. rex's size argues for its being a predator, or at least an opportunist that killed when it needed to. Says Don, "There's no evolutionary reason to grow that big if you're a scavenger. Common sense says it evolved that size to be top predator." Top predators today are not the biggest

t. rex as scavenger and predator.

animals. But some of the biggest animals are scavengers, certainly among the birds, the dinosaurs' closest living relatives.

Historically, T. rex was first thought of as the top predator, the king, like the lion today. One of the first news stories ever written about T. rex, in the New York Times in 1906, refers to a "swift, two-footed 'tyrant,'" so ravenous that its appetite might have explained its very extinction "simply as a result of an insufficiency in the food supply of those days to appease the cravings of his enormous hunger."

Of course we know now that animals don't eat themselves out of business in their own environment. But that journalist's view of T. rex was borrowed from the scientist who named it, Henry Fairfield Osborn. "Evidently this was an active, aggressive hunter that relied upon its strong jaws and teeth and upon the heavy claws of its hind legs for bringing down its prey." The man who found T. rex, Barnum Brown, said it was "the most formidable fighting machine ever devised by nature."

To folks in the early 1900s, T. rex had to be fierce, not just because it looked mean, but because it was old, and therefore primitive and savage. Osborn wrote about "the tendency for the older forms to be the more quarrelsome and wage their combats with greater persistence."

Prejudices about T. rex have also run the other way, to thinking of it as a scavenger. In 1917, Canadian paleontologist Lawrence Lambe suggested that T. rex and other big dinosaur meat-eaters could only have chewed soft, rotten meat. He based his conclusion on his observation that the crowns of the dinosaurs' big teeth didn't show any wear. We now know that T. rex's teeth were unusually sturdy, and that worn and broken teeth were replaced regularly.

Primitive and savage is how most of us would like to see T rex. That could mean a scavenger as well as a predator. But a predator is more the kind of movie monster that satisfies our need to be scared out of our minds. I think that is why T. rex is usually depicted as hunting, running full speed, chasing down an animal, flashing huge teeth, and making bloody kills.

But when you try to figure out how a hunting T. rex actually worked, it's still hard to imagine how it caught its dinner. If you look at lions and dgers, ariimals that catch their prey, you'll see they use their front legs to hold down the victim and their back legs to stabilize it. Then these predators reach over with their mouths and bite and kill their meal. They also don't eat anything nearly as big as they are, like a full-grown Triceratops for a T. rex.

What T. rex did with its arms, nobody knows. In the past, those littie arms were commonly assumed to be vesdges. But that doesn't mean they were useless. We have a gready reduced littie toe, but it still is important to us for balance.

Then again, these arms weren't long enough to reach T. rex's mouth, so it can't have used them to put food in its face. Nor could it have used its arms to help it rise from a reclining position. Nearly a century ago, Osborn suggested T. rex used its arms to hold onto its date during mating. Jim Farlow thinks T. rex's claws could have assisted its jaws in positioning a mate for sex.

To me, T. rex's using its arms for foreplay beats the heck out of preparator Ken Carpenter's idea. Ken thinks that once T. rex caught an animal and brought the victim close to its chest with its jaws, it could use its arms to secure the prey. Ken also examined the hands of Kathy Wankel's T. rex, the first ever found intact. He saw that those fingers were positioned very diflferentiy from how scientists and artists had guessed in reconstructing T. rex. The T. rex we see in museums and in books has two hooks set side by side, that would have moved in parallel, like two of our fingers (but not our opposed thumb).

But when Ken and preparator Matt Smith fitted the actual bones together and manipulated them, they found that the claws (if they had been found with the

t. rex's arm motions were so limited and its arms so short that despite their power it is difficult to imagine them being of much use.

t. rex's arm motions were so limited and its arms so short that despite their power it is difficult to imagine them being of much use.

hand) would have spread apart from the hand. Ken, like Henry Fairfield Osborn, compared the claws to meat hooks with which T rex would have impaled prey and held it fast while lashing out with its arms.

With its big arms, AHosaurus could have held onto prey while feeding. Littie carnivorous dinosaurs had long and highly mobile arms. But T. rex didn't have long arms or a long neck, so it couldn't have held its prey, even a corpse, at a distance with arms outstretched. It needed to get its body up close to its meal for its jaws to do their work. It could have stood on one foot and grabbed its meal with the other foot while tearing off meat with its mouth. I suspect it could have eaten just as well even if its arms were lopped off.

To hold onto its food with its littie arms, T. rex would have had to have brought its big head way back to take a bite. I don't think that would have been possible, so I don't think Ken's idea is worth a hill of beans.

I certainly don't think those little arms could have been used to catch a living animal, such as a Tricer-atops—by far the most abundant large animal in T. rex country. If those hands were useful at all for feeding, and I don't think they were, they'd have been just as good for grabbing onto carcassas the way an eagle uses its legs and claws.

Use your imagination and try to conjure up what it would be like if a Triceratops were running down the street and a Tyrannosaurus rex tried to catch it. Of course, the Triceratops wouldn't want to be caught. If T. rex couldn't use its littie front legs to catch the Triceratops, what could it use?

Well, T. rex did have big hind legs. Some have suggested T rex might have run ahead of its prey. Then, with its long tail, T. rex could have knocked the animal over. That seems absurd to me. A Triceratops would just run off in another direction. Or, perhaps T. rex used one leg to jump on its prey. That would mean T. rex would have been hopping on the other leg, an action that just doesn't make sense for an animal that large.

Nowadays, you can reconstruct an animal almost any way you want, just as when you go to put plastic flesh on it you can make it fat or skinny. But from what we know about the bone structure of the pelvis of T. rex, even if it weighed four tons (at the lower end of scientific estimates), it is very (lifficult to visualize how it could jump up and down.

We don't have any land animals as big as most dinosaurs today, except for elephants. You can see elephants doing all sorts of weird things with their bodies, including standing on one foot in the circus. But they never jump off their platforms. If an elephant jumped, it would break or dislocate its legs. And until we know more about what happens to dinosaur bone under really stressful conditions, we won't be able to tell whether T. rex ever jumped up in the air to get on top of its prey. As for kicking and slashing, the toe claws of tyrannosaurids (as artist Greg Paul has pointed out) were proportionately smaller and duller than those of other meat-eating dinosaurs.

What T. rex did have for a weapon was a huge mouth full of enormous teeth. No meat-eating dinosaur had a more powerfully made head than T. rex. Those who've studied the skull of T. rex see a lot of killing potential to it, but they can't agree on how T. rex killed. Greg Paul thinks that T. rex's bite was like a giant "cookie-cutter." Its uniquely thick teeth, arranged in a U-pattern within a powerful jaw, allowed it to snap lumps of flesh a yard wide, down to and sometimes into the bones of its prey. Whereas other tyrannosaurs were slashers, T. rex was a chomper, according to Paul. Others who've studied T. rex's jaw come to other conclusions. Paleontologist Bob Bakker says, "The head of T. rex was not the head of Godzilla. It was not designed for biting huge chunks of the Chrysler Building." Skull expert Ralph Molnar thinks T. rex's jaws were designed for both slashing and chomping, and opened wide to take especially big bites. To Jim Farlow, a T. rex tooth is a fair compromise between greg erickson is the first to study t. rex toothmarks on bone, here the sacrum of a triceratops. the T. rex left raking marks throughout the bone.

a cutting blade, a puncturing tool, and a sturdy support.

It's not easy to find hard evidence for how T. rex used its teeth. Broken teeth with smoothed edges have been found in the jaws of several meat-eating dinosaurs. That means the tips of the teeth probably broke off in the process of feeding, and that the teeth were strong enough at the base to stay rooted in the jaw and to get ground down by subsequent chewing.

Paleontologist Tony Fiorillo at the University of California, Berkeley, notes that you don't find tooth marks from any dinosaurs in nearly the same percentage of fossils as you do mammal tooth marks in mammal bone, though tooth-marked dinosaur bone is common. To him, that suggests dinosaurs didn't bite into bone routinely on purpose as hyenas do today.

My former student Greg Erickson has come up with the best evidence yet of T. rex teeth marks. He's studied a huge Triceratops sacrum found by paleontologist Ken Olson in the Hell Creek Formation in 1990, the first evidence of a Tyrannosaurus munching on a Tricer-

atops. Greg also looked at a leg bone of a duckbill and a toe bone of another, both bitten by T. rex. How does he know it was T. rex?We don't know any other animal from that time and place with teeth big enough to leave such grooves. And the marks are shaped, in relief, just like a T. rex's tooth.

To Greg, the gnaw marks suggest T. rex was eating the flesh off bones from meaty places like the butt and calves. Of fifty-eight bites on the sacrum, only two show serrations. To Greg, that indicates T. rex's bite had a forty- to sixty-degree angle of teeth meeting flesh. T. rex wasn't sawing the bone, it was puncturing and pulling back.

To me, the bite marks on the sacrum show that this T. rex was scavenging an animal it may or may not have killed. It couldn't have gotten its teeth all around a bone four feet long like that sacrum unless the animal was already dead. And if it was trying to kill a Triceratops, it would have gone for a more lethal area, like the neck, the muzzle, or the rib cage.

Maybe some other T. rex could have reached over elephants can do lots of tricks, but they can jump only once in their lives. after that they'd be crippled. I suspect t. rex wasn't built to jump either.

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    What are some of trex predators?
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