T. rex Grew Fast as a Teen

Tyrannosaurus rex endured an enormous teenage growth spurt, according to a study published Thursday in the journal Nature that provided the first explanation of how the giant dinosaur evolved to be so huge.

Based on yearly growth rings in 67-million-year-old fossil ribs and pubic bones, the researchers determined that T. rex gained 70% of its 6-ton adult weight from age 14 to 18, putting on more than 4.5 pounds per day. The huge carnivore usually died before it reached 30.

“We now know that T. rex lived fast and died young,” said Gregory Erickson of Florida State University, lead author of the study.

T. rex was at least three times larger than its closest relatives. Scientists reasoned that it could have evolved its great size in two ways: by prolonging the slow growth of its smaller ancestors, like a modern lizard, or by accelerating their growth spurts, like a modern bird.

Erickson knew the answer could be found in the annual rings that mark bones, like growth rings in trees. Unfortunately, most meat-eating dinosaurs’ bones are missing some of their growth rings because the bones hollowed out as the animals grew. Erickson discovered that ribs, pubic bones and one leg bone stayed solid.

Based on ring size in bones from 20 specimens from T. rex and three related species, he found that the T. rex evolved more like a bird than a lizard, growing about four times faster than its relatives but for the same length of time.