Tyrannosaur life tables: an example of nonavian dinosaur population biology.

The size and age structures for four assemblages of North American tyrannosaurs-Albertosaurus, Tyrannosaurus, Gorgosaurus, and Daspletosaurus-reveal a pronounced, bootstrap-supported pattern of age-specific mortality characterized by relatively high juvenile survivorship and increased mortality at midlife and near the maximum life span. Such patterns are common today in wild populations of long-lived birds and mammals. Factors such as predation and entrance into the breeding population may have influenced tyrannosaur survivorship. This survivorship pattern can explain the rarity of juvenile specimens in museum collections.