Cross-sections through the fossilized tibia of a 120 million-year-old female Tenontosaurus, showing growth rings and medullary bone laid done in the marrow cavity just prior to egg laying. This individual died at the age of eight, shortly before she would have laid her eggs. Credit:Sarah Werning/UC Berkeley & Andrew Lee/Ohio U.
Dinosaurs had pregnancies as early as age 8, far before they reached their maximum adult size, a new study finds.From the press release:
Researchers 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 previuosly 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 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.
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.