SUE Lends a Hand: Field Museum Scientists Remove T. rex’s Arm for Argonne Study

Fossil skeleton of SUE the T. rex

Two Field Museum scientists are leaving their labs and going face-to-face with SUE, the biggest Tyrannosaurus rex ever discovered, armed only with a wrench. And they’re going to take her arm off.

SUE’s not just the world’s biggest T. rex—she’s also the best-preserved and most complete one. And that means that she contains a treasure trove of information for paleontologists to learn from.

“One of the big mysteries about T. rex is its tiny forelimbs,” says Pete Makovicky, Associate Curator of Dinosaurs. “We don’t know how it used them. But there could be clues in the fossils. When a bone is used a lot, the wear and tear cause tiny fractures that heal over time. With the right tools, we can see microscopic changes in the bones caused by that healing process. You also see things like a wider bone marrow cavity. When we remove SUE’s arm, we’re going to take it to the Argonne National Laboratory to try to look for these characteristics that will tell us how much it was used.” 

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Man standing under T. rex skeleton, examining its arm

Makovicky and McCarter Collections Manager of Fossil Vertebrates Bill Simpson are removing the arm for research. “SUE’s fossil is mounted with set screws, it’s like an Erector Set,” says Simpson. “We’ll carefully disassemble SUE’s right forelimb, and she’ll be a one-armed bandit for a few days.” (That said, Simpson is a trained professional; we don’t recommend hopping the barrier and trying to take apart SUE yourself.)

While the bulk of SUE’s skeleton will still be at The Field, albeit a little lopsided, her right arm will be at Argonne National Laboratory in Lemont, Illinois, about 30 miles southwest of Chicago. There, Argonne paleontologist and imaging specialist Carmen Soriano will take micro-CT scans of the arm to produce high-resolution images of its interior.

CT (computed tomography) scanners are based on the same principle the X-ray scanners doctors use when you break a bone. An X-ray machine shoots high-energy photons (X-rays) at the object being scanned. The denser the material the object’s made of, the more X-rays it absorbs. The machine creates an image of the pattern of X-ray absorption; it gives us a picture of which areas are denser than others, which can be useful for telling what’s inside of an object. A CT scanner goes a step further: instead of just producing a two-dimensional picture of the insides of an object, a CT scanner takes a whole bunch of images of different “slices” of the object. When all these slices are stacked together, they form a three-dimensional image of what’s inside.

To see the fine details in SUE’s arm bones that would tell us how much she used them, a regular CT scanner like what you’d find in a hospital won’t cut it—that’s why SUE’s arm is going to the Argonne National Laboratory. They have a synchrotron, the U.S Department of Energy’s Advanced Photon Source, which is a particle accelerator that excites electrons and causes them to release their built-up energy in the form of X-rays. The additional energy that the synchrotron provides enables their instruments to take incredibly fine slices, allowing scientists to see details at a cellular level. 

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The skeleton of a T. rex arm, with two sharp claws

“These X-rays will give us a map of the blood vessels and muscle attachments in the bone, which have never been seen before,” said Carmen Soriano, paleontologist and beamline scientist with the Advanced Photon Source at Argonne. “Thanks to these extreme brightness X-rays, we will be able to better understand the fine internal morphology of the skeleton, which will give us clues about how the arm could move and what it was used for.”

“We’re looking forward to working with Argonne on this project,” says Makovicky. “We’ll be able to get incredible details about the structure of SUE’s bones and how she used her arms, without damaging the fossil itself.”

“And it’s not just the use of her arm that we’ll learn about—there’s a spike on her upper arm bone where her triceps muscle was torn, and there are some pits on her knuckles that we might be able to learn more about. The possibilities are endless,” adds Simpson. “SUE’s the gift that keeps on giving.”