"In carnivorous reptiles, such as crocodiles, for examplethrough their history, gigantism evolved numerous times," Erickson said. "Every time we see 30-to-40-foot [9-to-12-meter] crocs, it is related to the prey available. When there's giant prey available, giant crocs appear" and fill that niche.
Gigantism offers some advantages. As animals grow larger, they become less vulnerable to certain predators. Size can also provide access to new food sources, such as high leaves.
Erickson notes that giant animals also enjoy efficiency advantages in locomotion and energetics. "For instance, ectothermic [cold-blooded] animals like reptiles, if very large, probably wouldn't cool down overnight, so they wouldn't be as dependent on the sun to warm them back up."
Bones Yield "Growth Rings"
The key to T. rex's growth history was found in bones kept by some of the United States' major museums. Many dinosaur bones, like trees, feature seasonal growth rings.
"If you were to cut a bone in half and put a slice under a microscope, you can actually see these growth lines," Erickson said, noting that such research has become more popular in the last decade. "Counting the growth lines is exactly like counting the growth lines in a treeit's the entire record of the animal's growth."
Clustered growth lines near the end of T. rex's development show that the animals reached an adult size and stopped growing, rather than gaining size throughout their lifespan.
The team examined the growth rings of adults, juveniles, and subjuveniles for four different species of Tyrannosauridae. Dinosaurs related to T. rex the researchers studied for comparison were Albertosaurus sarcophagus, Gorgosaurus libratus, and Daspletosaurus torosus.
The scientists estimated specimens' ages using the growth-line method, and approximated body mass by leg-bone size. Body-mass estimates ranged from 29.9 to 5,654 kilograms (66 to 12,465 pounds). That age-mass data allowed the researchers to establish a growth curve.
The researchers verified their results by comparing the dinosaurs' growth rings to those of modern alligators and lizards. Those animals' growth rings are similarbut their growth patterns are not.
T. rex's growth rate is comparable to that of the modern elephant. But while elephants have a lifespan of more than 70 years, T. rex lived an average of 30 years.
Just as in the biological study of living species, establishing an extinct animal's growth curve is a key stepping-stone to broader understanding.
With the life-history parameters, we can better understand T. rex evolution, biology, locomotion, and population dynamics, said Peter Makovicky, dinosaur curator at the Field Museum and a co-author of the study.
Erickson said that future research will focus on the growth of other dinosaur species.
"We'll follow up by looking at other dinosaur groups and see how they became gigantic, [to see] if this is the pattern that dinosaurs followed. If that's the case, it would be a very interesting evolutionary pattern."
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