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T. Rex: Expert Q&A

For a week in April 2005, paleontologist Richard Kissel of Chicago's Field Museum answered questions about Tyrannosaurus rex.


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Q: The NOVA scienceNOW segment cited a dendrochronological assumption regarding interpretation of dinosaur bone rings as providing proof of annual age. Trees may experience annual seasonal variation with associated dormant seasonal growth. Why assume the same basis of age determination for a non-hibernating reptile? Thanks. Ed Kohn

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Richard Kissel: Hi, Ed! Yes, counting the growth lines in T. rex bones is the same basic idea as counting the rings of a tree. More importantly, however, it's also similar to counting the growth rings in the bones of modern crocodiles. In crocodiles, each ring reflects a change in the crocodile's growth rate, which usually occurs once a year between the dry and wet seasons. Thus, counting the growth rings in the bones of a crocodile can allow a scientist to know that crocodile's age when it died.

In their recent article on tyrannosaur growth, Greg Erickson and his team suggest that, since you can determine the age of crocodiles by counting these growth rings (the same technique works for lizards, too), the same age-determination method should also work for their reptilian brethren, dinosaurs.

When studying dinosaurs and other critters from bygone eras, scientists often look to modern animals for clues as to how they lived and, in this case, when they died.

Q: How could a species so robust that it could endure 160 million years disappear so quickly and completely? Was the cause(s) of their demise unique in the history of our planet's species? Rick Freeman

Kissel: Hi, Rick! No single dinosaur species (like T. rex) existed for as long as 160 million years, but as far as dinosaurs in general are concerned, many scientists agree that the reign of the dinosaurs was ended at the end of the Cretaceous Period by the impact of a large asteroid. Not only did this impact wipe out most of the dinosaurs, but it was probably responsible for the extinctions of many other types of animals at that time, too (such as the flying pterosaurs, the Nautilus-like ammonoids, and the giant marine reptiles). Whenever a large number of species go extinct worldwide over a relatively short span of time, scientists call it a "mass extinction." The mass extinction at the end of the Cretaceous was one of six mass extinctions that have occurred throughout Earth's history.

Also, notice that I say "most" dinosaurs went extinct at the end of the Cretaceous Period. Because birds evolved from small meat-eating dinosaurs, birds are considered dinosaurs! Thus, dinosaurs are still around, and that's putting it lightly: some 10,000 species of birds...oops, I mean "dinosaurs"...exist today! For comparison, the reported number of mammal species, including everything from rats to cats to humans, is around 5,000. Indeed, the common notion that we're currently living in the Age of Mammals is quite misleading.

To answer your second question, some scientists do think that other mass extinctions were similarly caused by a large impact, but the evidence for these events is not nearly as strong as that for the Cretaceous extinction. Like all ideas in science, the finding of new evidence will help confirm (or change) our understanding of these other mass extinctions. Science is a dynamic process: always changing as we learn more.

Q: Does anyone know the color and texture of the skin for T. rex or other dinosaurs? If so, how do they know? C. Lorson

Kissel: Because they are very hard and durable, bones and teeth are more easily preserved as fossils than skin and other soft tissues. However, luck occasionally shines down on us scientists, too, and we actually do find fossilized skin or skin impressions of dinosaurs. As far as I know, no T. rex fossil has been found that preserves details of the skin, but scientists have found the fossilized skin of "duck-billed" hadrosaurs, and this skin is thick and wrinkled, with bony knobs of various sizes embedded throughout. Unfortunately, due to the processes of fossilization, the color of the original skin is unknown, so artists usually rely on living animals for their inspiration when deciding the color of a dinosaur.

Q: My son Gavin loves dinos. He says T. rex may have had feathers. Is this true? Francisco

Kissel: Hi, Francisco! Your son Gavin is right. Although we've never found a T. rex fossil with feathers, it does make sense that T. rex may have had them. Why's that? Well, as you may have seen in the news, fossils of many types of feathered dinosaurs have been discovered in recent years. One of these dinosaurs, Sinosauropteryx, is actually more primitive than Tyrannosaurus. In other words, on the family tree of meat-eating dinosaurs, Sinosauropteryx sits on a branch lower than the one with Tyrannosaurus. It therefore makes sense that, since Sinosauropteryx had feathers, all of the meat-eating dinosaurs that are more derived than Sinosauropteryx (those on branches of the family tree above Sinosauropteryx, like T. rex) also had feathers.

Furthermore, a team of scientists led by Xing Xu reported a new type of feathered dinosaur just last year. This dinosaur, named Dilong paradoxus, is a tyrannosaur! Thus, the discovery of a feathered dinosaur that is very closely related to T. rex also supports the idea that T. rex may have had feathers, too.

Q: How widespread was T. rex around the globe? How did the break-up of Pangaea effect the species' dispersal? Anonymous

Kissel: Fossils of T. rexare known from sites located throughout western North America, and they've all been found in rocks that date back to the end of the Cretaceous Period, some 67 million years ago. The breakup of Pangaea, on the other hand, actually began in the Jurassic Period, millions of years before T. rex lived. By the Late Cretaceous, the continents were approaching their present positions, so a Late Cretaceous globe would not have looked too strange to us. However, North and South America were not yet connected, but North America and Asia were. This geography probably explains why meat-eaters like tyrannosaurs hunted in the north, but different types of meat-eaters, like the abelisaurs, ran amok in the south. On different landmasses, different types of animals evolve over time.

Q: What is the estimated top speed of a T. rex? Ray

Kissel: Hi, Ray! Yes, after seeing that T. rex chase the jeep in Jurassic Park, many people want to know just how fast T. rex could run. In the past, some scientists have thought that T. rex was a fast runner, capable of reaching speeds of up to 45 miles per hour. Recent studies suggest otherwise. Using a computer model to calculate the amount of leg muscles that T. rex would have needed to run fast, scientists John Hutchinson and Mariano Garcia determined that such high speeds were not possible. Instead, many scientists think that T. rex could possibly run at speeds of "only" 10 to 20 miles per hour. An important thing to remember when thinking about the speed of T. rex, however, is that it only had to run a bit faster than its prey, and I doubt that "duck-billed" hadrosaurs and horned ceratopsians were running at cheetah-type speeds!

Q: Who would win in a fight, T. rex or Allosaurus? (I like Allosaurus better.) Timmy

Kissel: Hi, Timmy! That's a very common question, but I'm sorry to say that such a fight would never have happened. Allosaurus lived around 150 million years ago (during the Late Jurassic Period), whereas Tyrannosaurus lived around 67 million years ago (during the Late Cretaceous Period). In other words, Tyrannosaurus actually lived closer in time to you than it did to Allosaurus! Allosaurus and Tyrannosaurus never met.

However, now that I've performed my scientific obligation with that answer, I'll give you my "real" answer: I'm sorry to say that I think Allosaurus wouldn't stand a chance. Tyrannosaurus was much larger and heavier, and its jaws were simply enormous (and extremely powerful). If that mouth got hold of an Allosaurus, it wouldn't be long before you started hearing the crunching of bones.

Q: What did T. rex es eat? Anonymous

Kissel: Hello! For most dinosaurs, the best that we can say about their diet is based on the type of teeth that they have. In general, the meat-eaters (like T. rex ) have long, pointed teeth that have edges like steak knives; these types of teeth are good for puncturing and slicing through flesh. Plant-eaters, on the other hand, usually have teeth that are good for snipping and cutting or crushing plants.

However, in a bit of luck for us scientists, we actually have some fossil evidence that tells us the types of animals that T. rex was eating. Tooth marks found in Triceratops and Edmontosaurus bones are perfect matches with T. rex teeth. Thus, we have good evidence that these two dinosaurs were part of the T. rex diet, but T. rex probably would've eaten any type of meat that it could find: dinosaurs, mammals, lizards, you name it!

Q: Were T. rex es hunters or scavengers? Fredis Arias, Los Angeles, California

Kissel: Hi, Fredis! Your question is a common one these days. In the 1990s, paleontologist Jack Horner argued that T. rex was a scavenger, only eating animals that had already died, rather than actively hunting and killing them first. Because this claim contradicted the common notion that T. rex was a vicious predator, the media—from newspapers to radio to television—quickly picked up on the story, leading many people to think that science favored the "T. rex was a scavenger" theory.

However, of the paleontologists that I have met, it seems that only Jack subscribes to the theory. I, too, disagree with Horner; the arguments that I've seen him put forward for a scavenging T. rex are not strong. For example, he states that the short arms of T. rex were useless for hunting. True, but don't forget about the massive skull that's armed with a mouth full of teeth. The impact and bite from such jaws were no doubt quite effective in taking down prey, so who needs arms? Another argument that Horner commonly presents is that T. rex was not a fast runner, so it couldn't have successfully chased and captured its prey. Indeed, T. rex was probably no speed demon (see my answer to one of the previous questions), but neither were its potential prey items, the "duck-billed" hadrosaurs and horned ceratopsians, so T. rex did not need to be a fast runner—only fast enough.

The above arguments, as you've probably noticed, are quite hypothetical, so do we have any hard evidence that favors one idea over the other? At the Denver Museum of Nature & Science, within their wonderful Prehistoric Journey exhibition, one can find a very nice skeleton of the hadrosaur Edmontosaurus. If you ever have a chance to see this skeleton, look closely at the tail, and you'll notice that a chunk of one of the vertebrae is missing. Ken Carpenter argues that this injury resulted from the bite of a T. rex , and since the injury healed after the bite was inflicted, it's quite clear that the failed attack happened while the hadrosaur was alive.

Ultimately, my conclusion regarding the whole matter, and it's a conclusion shared by many paleontologists, is that the answer to your question is "Yes." T. rex was probably an active hunter, but—at the same time—it wouldn't pass up a free meal if it happened upon an already dead dinosaur.

Q: How do experts know that an infection caused the holes in Sue's jaw? Anonymous, Greensboro, North Carolina

Kissel: When these holes were first discovered, it was thought by some that they were tooth marks inflicted by another T. rex . However, as pointed out by Chris Brochu in his 2003 study of Sue, none of these holes line up in any type of bite line; instead, their placement is quite random, so we can confidently say that these holes probably did not result from the bite of another T. rex . An alternative explanation is that some sort of infection caused the holes. However, we don't know for sure that an infection produced these holes; rather, it's a plausible explanation.

Q: Hi, Richard. Do you like working at the Field Museum? What is the best part? My family likes to visit! Shari Toth, Chicago, Illinois

Kissel: Hi, Shari! I'm glad that you and your family like to visit The Field. Yes, I'm happy to say that I do enjoy working at the museum. It's a world-renowned institution, and the people that work here possess an incredible assortment of unique talents, from the writers and artists that develop and design exhibits to the scientists that study everything from DNA to dinosaur bones. I've never met a more interesting group of people in my life.

I'm also really enjoying the project that I'm currently working on, the complete renovation of our fossil exhibition. Communicating the four-billion-year story of life over time to the public is, as a paleontologist and an educator, quite rewarding and very satisfying. I hope that you and your family come and see it next year!