[Photo] Plant-eating Tenontosaurus was vulnerable to attack.

Dinosaurs bred as early as age eight, long before they reached adult size, fossil evidence suggests.

Although they were descended from reptiles, and evolved into birds, dinosaurs grew fast and bred young, much like the mammals of today.

Researchers at the University of California found hallmark "egg-making" tissue in two juvenile females.

They say early sexual maturity was needed for survival, so females could lay eggs before becoming prey.

Calcium-rich medullary bone, which, in birds, is used to produce egg shells, was found inside the fossilised shin-bones of two specimens: the meat-eating Allosaurus and the plant-eater Tenontosaurus.

Sarah Werning and Andrew Lee of the University of California (UC), Berkeley, deduced from growth rings inside the bone that the two females were aged eight and 10, very young for dinosaurs, which lived to about 30.

Medullary bone has previously been found in a female Tyrannosaurus rex, and the scientists confirmed this finding, putting her age at 18.

"We were lucky to find these female fossils," said Sarah Werning. "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."

Growth clues

Studies of the tell-tale growth rings in dinosaur bones have revealed much about the way they grew.

Dinosaurs grew faster than present-day reptiles and had only a limited lifespan as adults before they fell victim to predator attack.

Like many groups, Tenontosaurus, which lived in North America during the Early Cretaceous period, 125 to 105 million years ago, would have had to reproduce young to ensure survival of the species.

"These were prey dinosaurs, so they were probably taken out when really young and small, or when old," Sarah Werning explained. "So, if you don't reproduce early, you lose your chance."

The discovery adds weight to the idea that dinosaurs were more like birds than reptiles. It also suggests that the reproductive strategy of modern birds is an ancient one, dating back some 200 million years, to when dinosaurs first evolved.

"This shows us beyond any doubt how fast dinosaurs grow," said Kevin Padian, a professor at UC Berkeley's Museum of Palaeontology, who was the students' advisor.

"They're growing as fast as big birds and big mammals," he told the BBC.

"To do this you can't have the metabolism of a crocodile; you need to have the metabolism more of a bird or a mammal."