Dinosaurs matured sexually while still growing

From Austin, Texas, at a meeting of the Society of Vertebrate Paleontology

A distinct type of bone tissue preserved in the fossils of several species of dinosaurs suggests that the ancient reptiles were sexually mature long before they’d gained adult size.

In modern-day female birds, a thin layer of bone that’s particularly rich in blood vessels—known as medullary bone—serves as a reservoir of calcium for eggshell production. The presence of medullary bone is a definitive sign of both female gender and sexual maturity, says Sarah Werning, a paleontologist at the University of California, Berkeley.

Scientists first reported finding this type of bone in a Tyrannosaurus rex fossil in 2005. Since then, says Werning, medullary bone has been identified in the fossils of several dinosaurs, including the meat-eating Allosaurus and the herbivorous Tenontosaurus.

Several teams of researchers have analyzed features akin to growth rings in the fossils of various dinosaur species in order to estimate how quickly those creatures grew and gained weight (SN: 8/14/04, p. 99). From such analyses, Werning and Berkeley colleague Andrew Lee conclude that the Allosaurus and Tenontosaurus specimens that include medullary bone—a sign they were sexually mature females—were 10 and 8 years old, respectively. Both dinosaurs were far short of adult size, she notes. In fact, each creature had died at an age when the typical member of its species was growing at its fastest rate.

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