Bony tissue reveals dinosaur was pregnant
Bony tissue reveals dinosaur was pregnant

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By David Zizzo

Published: January 22, 2008

One day in a place that would someday be known as Montana, a juvenile Tenontosaurus was attacked and killed by a pack of raptors. More than 100 million years later, Sarah Werning learned something personal about the doomed animal: It was pregnant.

"It was totally unexpected,” Werning said.

Werning made the discovery in summer 2005 while she was working on her master's thesis at the University of Oklahoma. The find made a "Tenonto” specimen at the Sam Noble Oklahoma Museum of Natural History one of only three dinosaur fossils in the world for which the gender is known.

The evidence was there all along, of course, but it wasn't until new techniques came along that Werning was able to learn intimate details of the life of a creature long disappeared. Actually, Werning found the evidence before she even knew what it meant.

Using rock saws and polishers, Werning had sliced paper-thin wafers from the shin and thigh bones of a Tenontosaurus. The Noble Museum has one of the largest collections of Tenonto fossils. The specimen Werning was working with was found in Montana.

Werning was looking at samples under a microscope when she noticed "it had this weird tissue on the inside of the bone, lining the marrow cavity. I had never seen anything like that before.” She set it aside.

As Werning was finishing her study, Mary Schweitzer, a researcher from North Carolina State University, published a paper on medullary bone tissue she found in bones of a Tyrannosaurus rex. Like modern birds, Schweitzer found, dinosaurs built up calcium-rich medullary tissue as they were preparing to lay eggs. Birds, presumably just as dinosaurs, liquefy the material and use it to construct egg shells.

"I said, ‘Oh my God, that's exactly what this is,'” Werning said of the mystery material she had spotted in Tenonto bones. Another researcher reading the same paper, Andrew Lee, found the same material in Allosaurus bone.

"Once we knew what it was, it was really exciting,” Werning said.

Werning, now working on a doctorate at the University of California at Berkeley, and Lee, at Ohio University, collaborated on a scientific paper published last week. The paper in the Proceedings of the Natural Academy of Sciences concluded reproductive strategy of dinosaurs differed from that of the modern birds they are said to resemble in many ways. Birds reach adulthood before reaching reproductive maturity, Werning said. "It had been thought that maybe dinosaurs did that, too.”

But the latest finds suggest dinosaurs, like modern reptiles and large mammals, apparently reached reproductive maturity well before becoming adults, a good survival technique in the challenging environment faced by dinosaurs.

Because researchers rarely find fossils of adult dinosaurs, some have speculated that the ancient beasts never stopped developing, Lee said. The new study suggests another explanation: Dinosaurs grew fast but only lived three to four years in adulthood.

By counting growth rings on bone, the researchers were able to determine that the plant-eating Tenonto, which was probably 5 feet tall at the shoulders and 20 feet long at the time, died at age 8, while the carnivores Allosaurus and T. rex were 10 and 18, respectively. Since egg-laying season is so short, finding medullary tissue buildup in fossils is extremely rare.

Bones from about 20 T. rex specimens have been examined, and only one has shown medullary tissue, Werning said.

The find shows that specimens in museums — although they already have been examined and, in many cases, stored for years without being used — remain a rich source of knowledge, said Rich Cifelli, curator of vetebrate paleontology at the Noble Museum. "Somebody else will come along with a different kind of question, new tools and techniques.”

As technology and techniques advance, such as CAT scans, electron microscopes and various methods of chemical analysis, specimens can