NCSU paleontologist identifies T. rex's sex

NCSU paleontologist identifies T. rex's sex

Staff WriterJune 3, 2005 

Scientists dream of shaking up their disciplines at least once. In three months, Mary Higby Schweitzer has rocked hers twice.

Schweitzer, a paleontologist at N.C. State University, in March unveiled what looks like soft tissue inside a thigh bone of a 68 million-year-old Tyrannosaurus rex. That revealed a whole new place to look for biological remains of extinct dinosaurs.

Now, Schweitzer is giving paleontology, for the first time, a way to identify females among dinosaur fossils.

In an article being published today in the journal Science, Schweitzer describes a thin layer of bone tissue inside both legs of that same T. rex. It resembles temporary bone tissue that primitive female birds produce before laying eggs. Calcium from that tissue creates eggshells and, ultimately, bones in baby birds.

"Think about it. Birds must get calcium from somewhere. If they drew it out of their bones, they'd get severe osteoporosis," Schweitzer said. "This way, they can quickly mobilize it."

The discovery delivers more than a new way to sort some she's from he's, said Hans-Dieter Sues, the Smithsonian Institution's associate director for research and collections. It also bolsters abundant evidence that birds descend from long-extinct meat-eating dinos.

"It's another piece of evidence that fits into this story," said Sues, a former president of the Society of Vertebrate Paleontology. "The evidence for the dinosaur-bird connection is overwhelming."

Well-preserved fossil

Schweitzer's success stems, in part, from her unusual anatomical and molecular focus and her collaboration with a former teacher, famed Montana paleontologist Jack Horner. Horner's team dug into 46 feet of remote rocks and sediment to recover the well-preserved T. rex remains near Fort Peck Reservoir in Montana in 2000.

Because a leg bone was too large to fit in the helicopter ferrying the fossils back to Montana State University's Museum of the Rockies, where Horner's crew is based, scientists cracked it in two. Horner wanted Schweitzer to look at its insides.

Schweitzer, a one-time high school substitute teacher who began paleontology studies in her 30s, scours fossils for clues to dinosaurs' living anatomy, especially at microscopic levels. She found what resemble blood cells and blood vessels in dinosaur fossils before her startling soft-tissue find this spring. That suggested more than bone survives in dinosaur remains, maybe proteins or even DNA fragments.

As soon as Schweitzer got a good look at what was coating the hollow interior of the Montana T. rex's leg bone, she said, she thought she saw medullary bone, bone tissue unique to female birds. She'd learned of it while researching anatomical features in warm- and cold-blooded creatures.

"We were pulling bones out of the boxes, and this lining tissue was all over it. I turned to my technician [NCSU's Jennifer Wittmeyer] and said, 'Oh my God. We have a female. It's pregnant!' " said Schweitzer, who also holds a research appointment at the N.C. Museum of Natural Sciences.

Schweitzer's microscopic studies show the bone layer is less dense and otherwise distinctive -- "swirly, wormy material" in Wittmeyer's words -- from the more permanent cortical bone in the T. rex remains. And it resembles the medullary bone that the women found in ostriches and emus. Such primitive birds have anatomies that scientists say resemble dinosaurs more closely than birds that evolved later.

Because this bone tissue appears in birds only as they prepare to produce eggs, Schweitzer and Sues said it would not identify all female dinosaur fossils. Where it survives, it may only pinpoint female dinosaurs of reproductive age that were preparing to make babies.

Previously, the only clues to sex that paleontologists had were the presence of eggs near fossils.

Sues said it may be difficult to find this special bone tissue in other dinosaurs since the T. rex that Schweitzer studied was not as badly smashed as some and was less infiltrated by minerals than others. Still, now that people know to look, who knows what they will find?

"Up to a few years ago, when it came to these molecular studies, some older colleagues said that's a load of crock, you know. People thought none of these things survived," Sues said. "Mary really occupied a unique niche. Since she is finding all this, people will look for other things. There's great excitement."

Staff writer Catherine Clabby can be reached at 956-2414 or cclabby@newsobserver.com.

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