Large meat-eating dinosaurs attained their great size through very different growth strategies, with some taking a slow and steady path and others experiencing an adolescent growth spurt, according to scientists who analyzed slices of fossilized bones.
The researchers examined the annual growth rings - akin to those in tree trunks - in bones from 11 species of theropods, a broad group spanning all the big carnivorous dinosaurs including Tyrannosaurus rex and even birds. The study provides insight into the lives of some of the most fearsome predators ever to walk the Earth.
The team looked at samples from museums in the United States, Canada, China and Argentina and even received clearance to cut into bones from one of the world's most famous T. rex fossils, known as Sue and housed at the Field Museum in Chicago, using a diamond-tipped saw and drill.
Sue's leg bones - a huge femur and fibula - helped illustrate that T. rex and its relatives - known as tyrannosaurs - experienced a period of extreme growth during adolescence and reached full adult size by around age 20. Sue, measuring about 42 feet (13 metres), lived around 33 years.
Sue inhabited South Dakota about a million years before dinosaurs and many other species were wiped out by an asteroid impact 66 million years ago.
Other groups of large theropods tended to have more steady rates of growth over a longer period of time. That growth strategy was detected in lineages that arose worldwide earlier in the dinosaur era and later were concentrated in the southern continents.
Examples included Allosaurus and Acrocanthosaurus from North America, Cryolophosaurus from Antarctica and a recently discovered as-yet-unnamed species from Argentina that rivaled T. rex in size. The Argentine dinosaur, from a group called carcharodontosaurs, did not reach its full adult size until its 40s and lived to about age 50.
Big theropods share the same general body design, walking on two legs and boasting large skulls, strong jaws and menacing teeth.
"Prior to our study, it was known that T. rex grew very quickly, but it was not clear if all theropod dinosaurs reached gigantic size in the same way, or if there were multiple ways it was done," said paleontologist and study lead author Tom Cullen of the North Carolina Museum of Natural Sciences and North Carolina State University, also affiliated with the Field Museum.
The research was published this week in the journal Proceedings of the Royal Society B.
"Theropod dinosaurs represent the largest bipedal animals to have ever lived and were also the dominant predators in terrestrial ecosystems for over 150 million years - more than twice as long as mammals have been dominant," added University of Minnesota paleontologist and study co-author Peter Makovicky.