Dino sex: Mrs. T. rex you've got a lovely daughter  

Teen sex, it turns out, is an issue at least as old as the dinosaurs.

Scientists have found inside the fossilized bones of three different types of dinosaurs the remains of a special type of calcium-rich bone tissue that forms just before egg-laying to enable pregnant female 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, according to the scientists.

"It's nice to know when dinosaurs started having sex," University of California at Berkeley paleontologist Sarah Werning, one of the researchers, said in a phone interview.

"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," Werning added.

Only dinosaurs and birds are known to have produced medullary bone, laid done in the marrow cavity shortly before egg laying. Paleontologists consider birds to be the living descendants of dinosaurs.

"It's actually really common to start reproducing before you're done growing. Most animals do that. Reptiles do that. Medium- and large-sized mammals do that. Whether our parents like it or not, humans do that," Werning said.

But birds do not reproduce before adulthood, Werning said.

"They 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 added.

The scientists found the medullary bone while making thin slices inside the bones of various dinosaurs.

Allosaurus, a bipedal carnivore about 39 feet long, lived about 150 million years ago. Tenontosaurus, a common plant eater about 26 feet long, lived about 115 million years ago. T. rex, a ferocious bipedal predator about 43 feet long, lived at the end of the age of dinosaurs, about 65 million years ago.

The three dinosaurs were all denizens of North America.

Based on growth rings in their bones, the scientists judged that the Tenontosaurus was 8 years old and the Allosaurus was 10 -- both qualifying as early adolescents. The T. rex was 18, still not fully grown, they reported in the Proceedings of the National Academy of Sciences.

Werning said 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 said, but they did not achieve full adult stature 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 added.