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Bones belonging to two "teenage" Tyrannosaurus rex (T. rex) dinosaurs have given scientists a new understanding of how these predators became the huge creatures we know today.

Nicknamed "Jane" and "Petey" the fossil skeletons show that young T. rex dinosaurs were slim, fast and had knife-like teeth for cutting food, unlike the adult T. rex.

Scientists had thought that the bones had belonged to a different dinosaur species, a smaller relative known as Nanotyrannus, but after more tests they discovered that the bones were part of the T. rex family.

Dr Holly Woodward, associate professor at Oklahoma State University said: "Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others... The problem is that those smaller fossils may be from younger animals... So, for a long while we've had large gaps in our understanding of how dinosaurs grew up, and T. rex is no exception."

According to researchers, the young T. rex would have been "slightly taller than a large horse and twice as long". Adult T. rex were one of the largest meat-eating dinosaurs that ever lived coming in at around 12 meters long and six meters tall, so how did they go from being so small to so huge?

Scott Williams from the Museum of the Rockies in Montana, US, said their findings show these dinosaurs "go through a drastic change" a kind of growth spurt.

To find out how old the dinosaurs were and how much they grew, researchers removed thin slices from the leg bones and had a look at them in detail.

They found that by counting the rings within the bone, like counting tree rings, Jane and Petey were around 13 and 15 when they died.

After all the tests researchers believe it took the T. rex up to 20 years to reach adult size, undergoing drastic changes as it matured.