Dinosaurs had teen sex
Dinosaurs reached sexual maturity long before they were fully grown, becoming pregnant in their early adolescence, say US researchers.
The researchers identified tell-tale signs of pregnancy in fossilised bones from three young female dinosaurs, aged 8, 10 and 18.
They found the remains of a special type of calcium-rich bone tissue that forms just before egg-laying to enable pregnant females to produce eggshells.
All three pregnant dinosaurs - the meat-eaters Tyrannosaurus rex and Allosaurus, and the plant-eater Tenontosaurus - were adolescents and had not yet achieved their maximum adult size, say the researchers.
"It's nice to know when dinosaurs started having sex," says Werning.
"It's also nice to be able to compare them to living animals in terms of understanding the timing of major events in their life."
Only dinosaurs and birds are known to have produced medullary bone, laid down in the marrow cavity shortly before egg laying.
Paleontologists consider birds to be the living descendants of dinosaurs.
But, birds do not reproduce before adulthood, say the researchers.
"[Birds] finish growing completely. And they all do it within about a year. And then they wait months or years before starting to have sex," Werning says.
Dinosaurs, on the other hand, mature sexually more like certain medium to large mammals, including humans, which are also able to reproduce before they finish growing, the researchers suggest.
"It's actually really common to start reproducing before you're done growing," says Werning. "Whether our parents like it or not, humans do that."
Slices of bone
The researchers found the medullary bone while cutting thin slices inside the bones of various dinosaurs.
They estimated the age of the dinosaurs by examining growth rings in their bones.
Werning says it is likely that starting reproduction in adolescence made a lot of sense. The life span of these types of dinosaurs was about 25 to 30 years, she says, but they did not achieve full adult maturity until about age 20, and there was a lot of adult mortality in this dino-eat-dino world.
That would leave just a handful of years to reproduce and ensure species survival. "If you start reproducing earlier, you have more time and more chances to reproduce," Werning says.
Allosaurus, a bipedal carnivore about 12 metres long, lived about 150 million years ago. Tenontosaurus, a common plant eater about 8 metres long, lived about 120 million years ago. T. rex, a ferocious bipedal predator about 13 metres long, lived at the end of the age of dinosaurs, about 65 million years ago.