CALGARY -- The T-Rex was the apex predator millions of years ago but their teeth suggest that, while still deadly, they many not have been as menacing at a younger age.

For the first time, tyrannosaur teeth are the focus of a study at the Royal Tyrrell Museum of Palaeontology in Drumheller, Alta.

Francois Therrien, the museum's curator of dinosaur paleoecology, noticed dramatic changes in tyrannosaur teeth as the animal matured. The younger tyrannosaurs had teeth similar to Komodo dragons that would allow them to slash with their teeth but not grab and hold on while a T-Rex over 11 years of age developed stronger jaws with thicker teeth giving them bone crushing abilities.

"This is the first time where we have a very detailed study about the changes that are happening in the feeding behavior and bite force during the life span of the animal," explained Therrien. "Because we have an amazing fossil record of tyrannosaurus here."

Southern Alberta has produced an abundance of tyrannosaur fossils of five different species on record.

Therrien is the lead author of new research being published in the Canadian Journal of Earth Sciences. Darla Zelenitsky is second author and professor of dinosaur paleontology at the University of Calgary.

"The tyrannosaur jaw was designed for basically holding or grasping struggling prey," said Zelenitsky. "So that didn't change all the way from the small juvenile forms to the adult forms (but) they switched over to much larger prey like large duck billed dinosaurs or horned dinosaurs that are very common here in Alberta."

The study compares over two dozen specimens from four-year-olds to fully-grown adults. The younger, more agile tyrannosaurs had razor sharp teeth and likely hunted faster moving prey.

"When the animal was 60 per cent grown, the jaws changed and they became more massive and stronger," said Zelenitsky. "So the strength of the bite actually increased significantly in the more adult growth stages."

The study looked at specimens of Albertosaurus and Gorgosaurus.

"We looked at 26 individuals of one species and 13 of another species that covered a full spectrum of growth," said Therrien. "That's how we're able to determine how bite force and feeding behavior changed during the life span of those tyrannosaurs."

Therrien says as the animals matured they became the apex predators of the late Cretaceous ecosystems in both Asia and North America. They needed strong teeth and jaws as their upper extremities wouldn't allow them to hold onto their prey.

"Tyrannosaurs throughout their lives have very short fore limbs," said Therrien. "They're incapable of grasping their prey, so it's like you trying to bite an apple with your hands behind your back, if the apple decides to fight back your jaw will have to do all the work to seize and maintain the prey."

Many of the specimens used in the study are on display in the gallery of the Royal Tyrrell Museum that is open to the public seven days a week.