TULSA, Okla. - No one knows exactly how big a Tyrannosaurus rex could get, but an Oklahoma researcher is looking for the answer in 65 million-year-old fossilized bones.
Lately, museums all over the U.S. and Canada have sent pieces of their T. rex skeletons to Dr. Holly Ballard’s lab at Oklahoma State University’s Health Sciences Center in Tulsa. When Ballard returns the fossils, they’re missing small sections, which she slices out of the bone to mount on microscope slides, then replaces with plastic casts. But the fossils go home with stories to tell, because from those thin slices of bone, Ballard learns how old each dinosaur was when it died, how quickly it grew, and maybe even a few tidbits about its health — exactly the kind of things you wonder when you stand in a museum and look up at a dinosaur skeleton.
That’s because animal bones, whether they belong to you, a deer, or a T. rex, grow in a series of layers like the rings in a tree trunk. When you look at a slice of bone, ring represents a year of growth, and thicker rings represent years when the animal grew more.
“The spacing will tell you have much active growing there was from year to year,” Ballard says. “The rings are starting to get more closely spaced near the edge. This is typical of larger animals, because when you're young, you're going to be growing a lot from year to year, but as you get older, you're not growing as much from year to year. Think of us: When you're in your 20s, you're pretty much done.”
The largest T. rex skeleton ever found, Sue (or FMNH PR 2081) at Chicago’s Field Museum, is 40 feet tall and probably weighed somewhere between 9 tons and 15 tons in life. A study of Sue’s rib bones revealed that the dinosaur was about 28 years old when it died. It would be easy to take Sue's dimensions for an answer to the question “How big could T. rex grow?” but one or two individuals just can’t tell scientists enough about the normal size range, or growth patterns, of a whole species.
“It’s like people: No one is going to grow exactly the same from person to person, so if you have a single dinosaur specimen, you can't infer that it's growing like the entire species would have been growing,” Ballard says. “We really still don't know how big T. rex could have gotten. Sue is definitely a large specimen, but is it as large as T. rex could have gotten?”
But if Ballard can look at slices of enough T. rex leg bones, she can bring us close to an answer. By looking at the growth rings, she can measure how T. rex grew from hatching, through childhood and adolescence. She’ll plot that growth on a graph. For most animals, that graph is a curved line, steepest at the bottom left where the young animal is growing fast, then slowly flattening as it nears the upper right side of the graph, where the adolescent animal’s growth slows and eventually stops. If she can plot enough of those curves, Ballard can trace them upward and predict where they would probably have flattened out: T. rex adulthood. By comparing the growth curves from many dinosaurs, Ballard hopes to define the normal growth and adult size for T. rex.
Ballard is also studying some close T. rex relatives, somewhat smaller — but still fearsome — dinosaurs called Gorgosaurus and Albertosaurus. By filling in some gaps in what scientists know about these dinosaurs, Ballard’s work may shed some light on why T. rex grew so much larger than its cousins.
“They didn't get as big as T. rex, but how was their growth different or similar to T. rex? Were they stopping their growth earlier (than T. rex) or just growing longer more slowly?” Ballard says. Other researchers have suggested that possibility, and Ballard hopes to test the idea with more data.
So far, Ballard has taken samples from about 15 T. rex specimens, and she says she needs at least 15 more to get results that are reliable enough for scientific standards. When Ballard is finished, this will be the largest study of T. rex bone tissue ever carried out. It will also be among the only studies to use the leg bones, which reveal more information about growth and size than the rib bones and vertebrae that other studies have used.
“I'm still trying to find a T. rex in my sample set that is actually adult, in that it has stopped growing. I haven't found that endpoint yet, which is pretty amazing,” Ballard says. “I'm getting close. I've got some that have very closely spaced rings, but nothing that I would call an endpoint.”