Bone study transforms understanding of dinosaur growth

Researchers examined rings called cortical growth marks laid down annually inside the bones as these animals grew

The dinosaur group called theropods included the largest land meat-eaters ever on Earth - up to the size of a school bus. But there also were theropods the size of a tabby cat and others of various dimensions, including beaked and toothless fruit eaters and some bizarre long-clawed species that redefine weird.

Scientists had long believed that these dinosaurs followed a uniform pattern in determining the body size of a species, whether gigantic or diminutive, with the rate of growth being the deciding factor - faster meaning bigger and slower meaning smaller. A new study adds to the evidence overturning that idea.

Researchers said on Thursday they examined rings called cortical growth marks laid down annually inside the bones as these animals grew - akin to growth rings inside tree trunks - of three dozen theropod species. Widely spaced rings indicate a speedier growth rate. Closely spaced rings indicate slower growth.

They found no common pattern underpinning body size. Some huge theropods grew quickly and stopped growing and some grew gradually over a longer period of time to reach the same size. Some small theropods grew rapidly and others slowly.

"Our paper overturns an axiom about dinosaur growth, suggesting that other groups should be studied systematically as well to see if our conclusions apply to other animals," said palaeontologist Mike D'Emic of Adelphi University in New York, lead author of the study published in the journal Science.

"It's equally likely that an animal evolved larger body size by growing at the same rate for longer, rather than just growing faster. It is surprising and important to know that animals are just as likely to evolve to gigantic sizes at slow rates. That means they are smaller for longer - and are thus susceptible to predators for longer - and have longer lifespans," D'Emic added.

Tyrannosaurus, topping 40 feet (12 meters) long, was an example of a big and fast-growing theropod, with a large growth spurt in its teenage years - in some years quadrupling its body mass. Acrocanthosaurus, a bit smaller than T. rex, was an example of a big but slow-growing theropod.