Dinosaurs had pregnancies as early as age 8, far before they reached their maximum adult size, a new study finds.

Researchers at Ohio University and University of California at Berkeley have found medullary bone – the same tissue that allows birds to develop eggshells – in two new dinosaur specimens: the meat-eater Allosaurus and the plant-eater Tenontosaurus. It’s also been found in Tyrannosaurus rex.

The discovery allowed researchers to pinpoint the age of these pregnant dinosaurs, which were 8, 10 and 18. That suggests that the creatures reached sexual maturity earlier than previously thought, according to the scientists.

The scientists originally studied the bones, which come from different geologic periods, to learn more about dinosaur growth rates. Because researchers rarely find fossils of adult dinosaurs, some have speculated that the ancient beasts never stopped developing, said Andrew Lee, a postdoctoral student at the Ohio University College of Osteopathic Medicine who conducted the work as a graduate student at University of California at Berkeley with scientist Sarah Werning.

The new study suggests another explanation: Dinosaurs grew fast but only lived three to four years in adulthood. Offspring were probably precocious, like calves or foals, Lee said.

“We were lucky to find these female fossils,” Werning said. “Medullary bone is only around for three to four weeks in females who are reproductively mature, so you’d have to cut up a lot of dinosaur bones to have a good chance of finding this.”

The research also offers more evidence that dinosaurs were less like reptiles and more like birds. Though dinosaurs had offspring before adulthood, their early sexual maturity was more a function of their tremendous size than any anatomical similarity to crocodiles.

When they factored in the size of the dinosaurs, Lee and Werning found that the reptile model for sexual maturity predicted that the ancient beasts would have had offspring as late as age 218. “That’s clearly ridiculous,” Lee said. Research shows that most dinosaurs only lived until age 30, though long-necked creatures such as Brontosaurus may have reached 60.

“We hope this is the last nail in the coffin, but some scientists still cling to the notion that dinosaurs weren’t like birds,” Lee said.

The discovery also sheds new light on the evolution of birds. The presence of medullary tissue in these dinosaurs, which lived as long as 200 million years ago, shows that the reproductive strategies of modern birds have ancient origins.

Because birds have evolved to be much smaller than dinosaurs, however, their reproductive strategies and growth spans now bear no resemblance to those of T. rex. Birds grow to adulthood in only 40 days, but may take one to 10 years to reach sexual maturity. That’s relatively similar in other tiny critters such as shrews, Lee said.

The work was funded by grants from the Geological Society of America, the Paleontological Society and the University of Oklahoma Graduate Student Senate to Werning; and grants from the Jurassic Foundation and UC Berkeley’s Department of Integrative Biology to Lee.

Published in the Jan. 15 Proceedings of the National Academy of Sciences.