Samplings: April 2008

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Cross-sections through the fossilized tibia, or shinbone, of a 120-million-year-old female Tenontosaurus skeleton, showing growth rings and medullary bone laid down in the marrow cavity just prior to egg laying. This individual died at the age of eight, shortly before she would have laid her eggs. 

Sarah Werning/UC Berkeley & Andrew Lee/Ohio University; fossil thin section courtesy of the Oklahoma Museum of Natural History

 As modern birds and reptiles do, dinosaurs laid eggs. But in the timing—if not the packaging—of their reproduction, they seem to have been more like large mammals: a new study suggests they grew fast and bred early. Andrew H. Lee and Sarah Werning, at the time both graduate students at the University of California, Berkeley, were counting growth lines in the fossil leg bones of a Tenontosaurus (a plant-eater) and an Allosaurus (a meat-eater), when they found medullary bone. A calcium-rich tissue that in female birds develops before ovulation to supply minerals for eggshells, medullary bone had been found in dinosaurs only once before—in a Tyrannosaurus rex. The growth lines of the Tenontosaurus, Allosaurus, and Tyrannosaurus females revealed that at death they were just eight, ten, and eighteen years old, respectively, and at half their full size, they were still growing rapidly. Yet, as the medullary bone showed, they were sexually mature. That made them mere adolescents, considering their species lived to about age thirty. Modern reptiles grow slowly and mature sexually before they reach full size; birds grow fast and reproduce only when they’re done growing. Dinosaurs, it now appears, took neither approach. Rather, they resembled humans and other large mammals in their rapid growth and precocious sexual maturity—a strategy that indicates high death rates in babyhood and adulthood. (PNAS)