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Deducing the Feeding Habits of Tyrannosaurus rex

Keywords: bite force, coprolite, diet, Dinosaur, trace fossil, Triceratops, Tyrannosaurus rex.

STEVE BRUSATTE Note 1

Ottawa, IL, USA, brusatte@theramp.net

Published April 1, 1999

Ever since the first dinosaur fossil was unearthed by Robert Plot in 1676, dinosaur fever has infected almost everyone. Dinosaurs captivate us not only because of the enormous size that some of them attained, but because of their extraordinary diversity. Encompassing such astonishing creatures as the fearsome Tyrannosaurus rex, the majestic Triceratops and the cunning raptors, dinosaurs are truly fascinating.

From such fascination many questions arise. How were the dinosaurs able to survive for such a long period of time? Why did they become so large? Just what was their everyday existence like?

These questions have puzzled scientists for years. But by looking at fossils, paleontologists have been able to learn much about the dinosaurs. They can infer the appearance of these creatures in life-like detail as they existed millions of years ago. They can investigate their diet. And they can find out about the way dinosaurs interacted both among themselves and with their environment.

Despite the frequent media attention given to discoveries of dinosaur bones, especially skulls, the truth is that such fossils are rare, much rarer than fossils of most plants and invertebrate animals. Thus, although bones are of great interest, if we had no other evidence, we would have a much less accurate picture of the dinosaurs than we in fact possess.

How, then, has it been possible to learn so much about these terrible lizards when we have so few bones to study? The answer lies in trace fossils. As the name implies, a trace fossil is a "trace" of an ancient organism such as a footprints, a tooth or bite mark, or a coprolite.

Footprints and tooth marks, yes, but coprolites? As any paleontologist will tell you, a coprolite is a piece of fossilized dung. Why would anyone study fossilized dung? Well, if you really want to know, you would do well to ask Karen Chin, visiting scientist with the U.S. Geological Survey, Menlo Park, CA, who is the world's foremost expert on dinosaur dung.

Chin is considered a rising star of paleontology. At Montana State, she studied under the prominent and controversial John Horner. Now she is hitting the lecture circuit, explaining to enthused audiences around the nation the importance of dino dung.

From the University of California, Santa Barbara, Ph.D. in hand, Chin took up the seemingly unusual notion of studying the fecal remains of Triceratops and other herbivorous dinosaurs. Soon, however, she was diverted by a project beyond her chosen area of study.

In 1995, Wendy Sloboda and Tim Tokaryk were hunting fossils near the town of Eastend, Saskatchewan on behalf of the Royal Saskatchewan Museum. There they discovered a mass 42 cm long, 12 cm high, and 15 cm wide close to the Tyrannosaurus skeleton now known as Scotty. At the time, Sloboda and Tokaryk did not consider the mass to be of any particular importance, but thinking that it might be of some interest, sent it to Chin for examination.

In her lab, Chin gingerly observed the lump for weeks. She broke the specimen into pieces and studied thin cross sections under the microscope. The lab work was challenging, she admitted, although as she put it, "everything came out alright in the end."

Indeed, everything did come out alright. Chin identified the mass as a coprolite, a very large one indeed. But, that was the easy part. Now she had to identify the creature responsible: the "species feces" question, as she put it.

To answer this question, Chin did a great deal of hard paleontological labor. More sections were examined, and other experts were called in, among them former classmate from Montana State, Gregory Erickson, currently a post-doc at Standford University.

Erickson is yet another rising paleontological star. In one article, he described the horrifying bite force of the Tyrannosaurus rex. In the research for this paper, he constructed a mechanical representation of the jaws of a rex and applied it to the pelvic bone of a cow. By noting the force applied by the robotic jaw and comparing the damage caused with that tooth marks on fossilized bones of Triceratops, he was able to estimate the bite force of the living beast.

But why would Chin ask for assistance from Erickson, a specialist in fossilized bones? Because, in her study of the coprolite, Chin located many minute masses that she thought might be bones. With Ericksonís help, she was able to confirm this and identify some of them as the bones of a juvenile Triceratops and a juvenile Edmontosaurus.

Bones mean carnivore, so Chin narrowed the list of candidates species to the five carnivores previously discovered in the rock formation from which the coprolite was unearthed. Just who were the suspects of this foul deed? The cunning and birdlike Troodon and Dromaeosaurids were two possibilities, ad then there were the crocodile, Leidyosuchus, Tyrannosaurus rex and the Elimisaurids.

The coprolite was very large: the largest from a carnivore ever excavated, according to Chin. But of the candidate species, only Tyrannosaurus rex weighed more than 100 kg. Thus it was no contest. Only Tyrannosaurus was available to take the unwanted crown.

Actually, Chin cannot be absolutely positive about the identification, but as Erickson put it, "[it was] either a TyrannosaurusÖ or a large, unknown animal."

The story garnered much attention from the media. But, notwithstanding the scatological humor it prompted, it represents a major paleontological development. "We've always guessed that Tyrannosaurus rex and their cohorts must have been able to crush the bones of the animals they fed on, but now we have the first hard evidence that they actually did," Chin told reporters following publication of her study in the journal Nature (1).

Beside helping to identify the bone fragments, Erickson applied his knowledge of the structure and power of the animalís jaw, to explain how Tyrannosaurus rex smashed the bones of its prey. As he put it, "The beasts' teeth were not equipped to chew bones, but their enormous bite force left jumbled masses of bone ranging in size from crumbles to large chunks in their dung."

He then went onto say that, "T. rex couldn't chew as people do because its upper and lower teeth didn't meet each other. But, those powerful teeth might have still pulverized bone as they sheared past each other."

It is hard to believe that only twenty-five years ago many scientists saw dinosaurs as dumb, slow animals just waiting to become extinct. Now, because of the work of Chin and Erickson, among others, paleontological thinking has greatly changed. As a result, we have a much clearer understanding of how the dinosaurs lived and how they were able to dominate the earth for so long.

In the end, perhaps, it is not what you do in life that matters, but what you leave behind. Tyrannosaurus rex, left behind a great piece of paleontological evidence.

Reference

(1) Chin, T.T. Tokaryk, G.M. Erickson and L.C. Calk. 1998. Nature 393:680-682.

About the Author

1 Steve Brusatte, 15, of Ottawa, IL, has been researching the lives of famous paleontologists in hopes that he can become the first teenager to write a book on the subject. The book, which includes such famous names as John Horner, Mark Norell, Tom Holtz, Peter Dodson, and both Chin and Erickson, will focus on the contributions of modern paleontologists (including amateurs, collectors, and authors). For more information, or to make a publishing offer, please contact Steve at http://www.geocities.com/CapeCanaveral/Galaxy/8152**


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