Being the world’s “most complete” T. rex is one of the claims to fame of the Field Museum’s apex fossil Sue. Thursday morning, Sue became a little less complete.
With a few twists of an Allen wrench to remove a couple of set screws, museum collections manager Bill Simpson loosened the famous dinosaur’s tiny right forelimb, about the length of an adult human arm.
“Last one and it’s going to be yours,” he said to Pete Makovicky, curator of dinosaurs.
The men were standing under Sue’s rib cage on the land form that supports the late predator in its enclosure in the museum’s central Stanley Field Hall. A gaggle of media cameras peered in at them. A rolling cart labeled “GEOLOGY” and topped with soft packing material waited to receive the precious cargo.
“All yours,” Simpson said as the second bolt came loose. “Lift it up.”
Makovicky did as instructed, the skeleton’s much more massive upper arm bones wiggled a little, and then the forelimb of the most expensive fossil ever purchased was in his hands, headed for a weekend date with a cutting-edge microscope at Argonne National Laboratory.
Through Tuesday, Sue will be on display without the forelimb. That means no high fives. No glad-handing. No waving to the adoring crowds, Queen Elizabeth-style. No whatever it is T. rexes did with those bizarrely disproportionate appendages.
That question — the long-debated purpose of the short Tyrannosaurus forelimb — is the main reason for Thursday’s temporary amputation.
“It’s been one of the enduring mysteries in dinosaur paleontology,” Simpson said a little later, as the bone structure, held together by a black metal framework, rested on the geology cart, its two claws, or fingers, looking more fearsome there than they did when mounted on the 40 1/2-foot animal remains.
“What we’re looking to discover is whether the forelimb was used at all,” he explained. “Maybe it’s just a vestigial appendage. We don’t know.”
Beginning Saturday morning at Argonne, Carmen Soriano, the laboratory’s paleontologist, will put Sue’s radius and ulna, the two main forelimb bones, before a scanner that more typically is used to look at barely visible things, most frequently from materials and energy sciences.
The microtomography station is one of more than 60 instruments using the energy generated by Argonne’s massive photo electron ring to study the basic elements of matter. In this case, Soriano explained, the X-ray will function like an ultra-high-powered version of the CT scanner you might find at a hospital.
Working essentially full time between Saturday morning and the bones’ return to the Field on Tuesday, it will generate a 3-D image of the arm bones, inside and out, down to the cellular level.
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“It is 1 billion — with a ‘b’ — times more powerful than a CAT scan,” Soriano said, and this will be its first use on big dinosaur bones. Soriano has used another Argonne instrument on a smaller specimen, helping to identify the once-mysterious Tully monster, the Illinois state fossil, for the first time as a vertebrate probably related to the modern lamprey.
Like a deli slicer but using X-rays instead of a spinning blade, Makovicky explained, the device examining Sue’s bones will take thousands of cross-section images and then “restack” them into the 3-D model, tomography. Those slices are about 100 microns thick, he said, compared with the 2-millimeter ones produced by a typical medical scanner.
The goal is to look for signs of stress in the cellular structure that would indicate to what degree the animal had used those bones. There’s more to the comparative anatomy study he and scientific colleagues are conducting, he said, but a “key question … is, Did T. rex use its arms or not?”
The sex of Sue is unknown; it was named for Sue Hendrickson, who discovered the skeleton on a South Dakota ranch in 1990. The Field bought the fossil at auction for $8.36 million and has had it on a display since 2000 in its central hall, where it has become one of the world’s iconic museum pieces.
But the Field likes to display actual fossils, not replicas, and so Sue’s bones have been taken off for study before, Simpson said. Most recently, for instance, a University of Chicago graduate student borrowed a shoulder bone to study signs of muscle attachment, he said. (A whole suite of injuries Sue suffered in its 28-year life are visible on the animal’s bones.) National Geographic borrowed a forelimb a few years back.
“Our exhibits are not just exhibits,” said Simpson. “They are genuine research objects.”
Before the newest Sue research could begin, the bones had to leave the museum’s main floor. The instrument of disassembly went back into a little Ziploc labeled “Allen key for SUE set-screws” and back into a drawer in the cart. Among other photos taken with the forelimb, Makovicky and Soriano knelt down to pose at the tip of the fingers. Soriano shaped her hand into a claw, laughing.
Simpson and a colleague in collections wheeled the cart into an elevator and then to their department, behind the exhibit space on the east side of the second floor.
There, in the presence of thousands of other fossils, Sue’s forelimb would be disassembled, the radius and ulna packed into a box for Makovicky to drive out to Argonne.
The bones have only a limited time on the Argonne instrument, so he is hoping, he said, that the radius and ulna scan will go quickly enough to enable them to get a look at some of the other forelimb bones.
“It might be interesting,” he said, “to get some high-resolution data on the knuckles. There’s an indication there of what might be some gout.”
And then Simpson and Makovicky will return Sue to the state of near-completion that, along with its girth and level of preservation, makes it so highly valued. Simpson isn’t that worried about the procedure.
“It’s an erector set, and if you have the right tool, you can get it off quickly,” he said. “Getting it back on will be more of a challenge.”
Twitter @StevenKJohnson
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