In Thursday's issue of the journal Nature, paleontologists estimate that the giant apatosaurus, formerly known as brontosaurus, put on more than 30 pounds a day at the height of its adolescent growth spurt.
Argentinasaurus, perhaps the heaviest dinosaur that ever lived, would have gained more than 100 pounds a day during its most rapid period of growth.
The discovery further discredits the old image of dinosaurs as little more than overgrown reptiles. It suggests that the animals developed some advanced metabolic tricks, such as warm-bloodedness, to maintain such rapid growth rates.
"I think it says a lot about the biology of these animals," said Gregory Erickson, one of the paleontologists who contributed to the research paper. "The bones are coming to life."
Before the 1960s, many scientists assumed that dinosaurs grew as slowly as modern-day reptiles. If that had been true, apatosaurus, weighing in at around 25 tons, would have taken more than a century to reach full size. Erickson instead estimates that an apatosaurus would have grown to adulthood in a much more realistic 12 to 20 years.
Erickson and two colleagues studied dinosaur leg bones from various fossil collections. They were able to estimate the body weights of the living animals by measuring the bones thickness. The ages of the animals came from counting annual growth rings in the bones, a common feature in fish, reptiles and dinosaurs.
The researchers then drew growth curves for the six dinosaur species they studied, plotting body mass against age. They found that some of the dinosaurs, especially the largest ones, grew as fast as almost any animal that ever lived.
The only animal that can outgrow a giant sauropod dinosaur is the blue whale, the fastest-growing -- and largest -- animal ever.
"Their numbers are probably pretty reasonable," said James Farlow, a dinosaur expert at Purdue University in Fort Wayne, Ind.
Generally, researchers find that within animal groups, growth rates are fairly uniform. Birds grow fastest, followed by mammals, marsupials, reptiles and then fish.
But Erickson and his colleagues were surprised to find that in dinosaurs, small species grew more slowly. The smallest dinosaur studied, the three-pound Shuvuuia deserti, grew only twice as fast as a reptile of comparable size.
"It suggests that something different is going on, but what that difference is is a tough call," Farlow said.
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