Growth Spurt: Teenage tyrannosaurs packed on the pounds

Detailed analyses of fossils of Tyrannosaurus rex and some of its more ancient kin suggest that the creatures experienced an extended surge in growth during adolescence, putting on as much as half their adult weight in a mere 4 years.

GRAND OLD SUE. Growth rings in a rib (inset) from the large tyrannosaur named Sue suggest that the creature stopped growing and then lived another 9 years. Field Museum

T. rex, one of the most fearsome meat eaters ever to stroll the planet, weighed more than 5,000 kilograms as an adult. Scientists have long debated whether such large dinosaurs achieved their bulk through rapid growth when young, delayed adulthood that enabled a longer growth period, or both. After scrutinizing the remains of four tyrannosaur species, researchers suggest that the beasts went through a substantial teenage growth spurt.

Many dinosaur bones show annual growth rings, as trees do, says Gregory M. Erickson, a paleobiologist at Florida State University in Tallahassee. In large, weight-bearing bones, some of the rings deposited early in life are erased as the bones’ marrow cavities expand. However, some other bones don’t have such cavities and so preserve all their growth rings.

Erickson and his colleagues counted the growth rings in fossil remains of seven specimens of T. rex, a species that lived in North America between 68 million and 65 million years ago. The smallest animal, which was 2 years old when it died, weighed about 30 kg. The largest—the specimen dubbed Sue, which is now housed at the Field Museum in Chicago—was also the oldest. Sue weighed 5,650 kg when she died at age 28.

Data from other specimens filled in T. rex‘s growth curve, says Erickson. When members of the species reached about 14 years of age, they entered a 4-year period during which they gained, on average, more than 2 kg per day. The spacing of growth rings in Sue and another T. rex suggests that the animals stopped growing when they reached about age 18. The researchers report their findings in the Aug. 12 Nature.

Similar analyses of three smaller, more ancient tyrannosaur species indicate that those creatures—one of which grew to about 1,800 kg and two that reached only 1,000 kg as adults—also had 4-year-long growth spurts as teens. During that adolescence, however, these tyrannosaurs gained no more than 0.3 to 0.5 kg per day.

This general pattern of development—rapid weight gain in adolescence among larger dinosaur species and slower growth among their smaller relatives—matches what has recently been found for plant-eating sauropods and some other groups of dinosaurs, says Kristi Curry Rogers of the Science Museum of Minnesota in St. Paul.

Previous analyses suggested that an adult T. rex couldn’t run very well, if at all (SN: 3/2/02, p. 131: No Olympian: Analysis hints T. rex ran slowly, if at all). A young T. rex might have had a feeding strategy different from that of an older one, says Thomas R. Holtz Jr. of the University of Maryland in College Park. Some scientists have suggested that the meat eater was actually a scavenger, not a predator (SN: 3/22/03, p. 190: Available to subscribers at Was T. rex just a big freeloader?).

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