How fast did dinosaurs grow, and how long did they live?
Until recently, paleontologists could only guess about these questions. Because many extinct, non-avian dinosaurs, such as Tyrannosaurus and Apatosaurus, are so immense, it was generally assumed that they grew slowly and required 50-100 years to attain their great size.
But recent research involving microscopic analyses of the cellular structure of dinosaur bone has revolutionized our answers to these mysteries. Dinosaur bones contain growth rings, somewhat like the rings in tree trunks, which reveal yearly periods of rapid and slow growth. By studying growth rings in Tyrannosaurus bones, scientists can estimate how fast the animal was growing at different times in its life and how old individual animals were when they died.
These studies, done in conjunction with paleontologists at AMNH, document that that Tyrannosaurus, which attained a weight of more than 10,000 pounds as an adult, reached sexual maturity at about 20 years of age and lived for up to 28 years. In order to obtain its immense size and weight, it could put on almost five pounds a day during its phase of maximum growth. As one might expect, these rates of relative growth are fairly similar to those found in birds, which are the closest living relatives of Tyrannosaurus. Microscopic studies of these growth rings in Apatosaurus bones suggest that this sauropod's period of fastest growth occurred between about 7 and 13 years of age, when the animal gained about 32 pounds a day - or about 6 tons a year! Although paleontologists used to think that gigantic dinosaurs lived a century or so, based on studies of living reptiles like giant tortoises, growth rings show that their life span was probably a lot shorter, perhaps only 30-40 years.