Study By American Museum Of Natural History Paleontologists Reveals Timing Of Physical And Sexual Maturity Among Dinosaur Ancestors To Birds

Growth rings in fossilized bones of brooding dinosaurs enabled paleontologists to compare the animals' physical and sexual maturity.

Credit: Greg Erickson and Mick Ellison/AMNH

Paleontologists examining the growth rings in fossilized dinosaur bones have discovered that sexual maturity among bird-like dinosaurs, unlike modern birds, likely occurred well before the animals had grown to full size. The findings appear online in the journal Royal Society Biological Letters and may help reveal the reproductive lifetime and other traits of the long-extinct animals.

The scientists from the American Museum of Natural History, Florida State University, Science Museum of Minnesota, Montana State University and Chinese Academy of Sciences studied the annual growth rings in the bones of all seven non-avian dinosaurs whose fossilized remains have been found in brooding position on or near a nest. By counting the growth rings, which are similar to the rings in trees, paleontologists can determine how old a dinosaur was when it died. Non-avian dinosaurs like the ones the group studied are the closest extinct relative to modern birds, which are considered to be living dinosaurs.

Most birds grow rapidly after they hatch, attaining their full body size within their first year and giving them the ability to evade predators from an early age, but are generally not capable of reproducing until much later in life. In other animals closely associated with dinosaurs, such as Crocodylia (alligators and crocodiles) and Squamata (lizards and snakes), individuals are capable of reproduction well before they attain their maximum size.

The dinosaur specimens the group analyzed ranged in age from six to 18 years when they died and, in addition to exhibiting nesting behavior, five out of seven had not stopped growing. 

"This tells us that dinosaurs in the Cretaceous period had not yet evolved to the point that they exhibited bird-like reproductive characteristics," said Greg Erickson, a Research Associate at the American Museum of Natural History and associate professor at Florida State University, as well as lead author on the paper. "They had many other qualities in common with modern-day birds, but they obviously still had a long way to go."

The findings enabled the team to calculate several other characteristics of dinosaur life. For example, they estimate that non-avian dinosaurs from the genus Citpati had a reproductive lifespan of seven to nine years and may have laid 140 to 180 eggs during that time. They also conclude that, despite the appearance of many modern bird-like features such as hollow bones and feathers, the life history of non-avian dinosaurs still differed substantially from that of modern birds and, they write, "late avian maturation evolved no earlier than the genesis of the first bird, Archaeopteryx in the Jurassic period."

The study also included work by Mark Norell at the American Museum of Natural History and was supported by the National Science Foundation. 

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