Nov. 24 (UPI) -- There's more than one way to get really big -- for theropod dinosaurs, anyways.
While Tyrannosaurus rex, one of the largest carnivores to walk the Earth, had massive teenage growth spurts to thank for its awesome size, new research suggests other theropod dinosaurs, some nearly as big as T. rex, grow more steadily.
From nose to tail, the biggest T. rex dinosaurs stretched some 42 feet. The Jurassic meat-eaters weighed more than 16,000 pounds. Fossils suggest some of its closest relatives grew nearly as large.
Scientists knew T. rex wasn't alone in its ability to reach gargantuan proportions, but researchers were less sure about how other theropods managed to get so big.
"We wanted to look at a wide swath of different theropods, two-legged, carnivorous dinosaurs, in order to understand broader patterns of growth and evolution in the group," study author Tom Cullen said in a news release.
Cullen, now a postdoctoral researcher at the North Carolina Museum of Natural Sciences, conducted the study -- published Tuesday in the journal Proceedings of the Royal Society B -- while studying at the Field Museum in Chicago.
"We particularly wanted to understand how some of them got so big -- is the way T. rex grew the only way to do it?" Cullen said.
Humans tend to go through a period of rapid growth during adolescence and remain the same size throughout adulthood, but other animals continue growing throughout their lifetime.
"Growth rate really varies, there's no one size fits all," said Cullen. "Birds have super growth spurts and reach adult size really fast, while reptiles like alligators and various lizards and snakes have extended growth. With them, a really, really big individual is probably really old."
Dinosaurs are related to both modern birds and reptiles, and theropods are the living descendants of small theropods from the Jurassic period. To find out whether theropods grew more like birds or reptiles, researchers analyzed the bones of dozens of theropod specimens.
Cullen and his research partners were able to estimate the growth rates of different theropod species by measuring the spacing of tree ring-like markings inside dinosaur bones.
Researchers drilled and extracted cores from the middle of several dozen bones belonging to a full spectrum of different-sized theropods -- small, medium and large species from all over the globe.
Back in the lab, researchers cut thin slices from their bone cores and examined them under the microscope.
"Most animals have a period every year when they stop growing, traditionally suggested to be in times like winter when food is more scarce. It shows up in the bones as a line, like a tree ring," says Cullen. "By analyzing these growth lines and examining the bones for new regions of growth, scientists can get a rough estimate of an animal's age and how much it grew every year. There are also clues in the bone structure."
In addition to noting growth rings, researchers were able to examine blood vessel structures passing through the different layers of bone.
"These vascular canals tell you roughly how fast the bone was growing," Cullen said. "If the canals are more organized, the bone was being laid down more slowly, and if the structure is chaotic, it grew more quickly."
The patterns among the bones of coelurosaurs, the family of theropods that includes T. rex, revealed the presence of a teenage growth spurt. The analysis also showed coelurosaurs stopped growing when they reached adulthood. The largest-known T. rex, a specimen at the Field Museum named SUE, stopped growing at age 20, but lived to be 33.
Conversely, the bones of allosauroids showed some theropods families grow more slowly, but continued to grow throughout adulthood.
Bone patterns showed one allosauroid member, a carcharodontosaur from Argentina, grew to be nearly as big as SUE, but not until between the ages of 30 and 40.
Researchers suggest its possible environmental pressures dictated the growth rates of different theropod families.
The plant-eaters that lived alongside coelurosaurs, such as Triceratops and duck-billed hadrosaurs, also enjoyed a teenage growth spurt. Allosauroids, on the other hand, lived alongside long-necked sauropods, which grew fast but took a long time to reach full maturity.
"We can't say for sure, but there could be some kind of a selection pressure for the coelurosaurs to grow quickly to keep up with their prey, or pressure for the allosauroids to keep growing in size since their prey were also increasing in size," says Cullen.
"But it's pretty speculative. It could be that even if the sauropods kept growing their whole lives, they had so many offspring that there was always something small to eat," Cullen said.