 By tabulating the number of specimens of each age group , Erickson and his colleagues were able to draw conclusions about life history in tyranosauridae populations . Their analysis showed that while juveniles were rare in the fossil record , subadults in the rapid growth phase and adults were far more common . Over half of the known T. rex specimens appear to have died within six years of reaching sexual maturity , a pattern that is also seen in other tyrannosaurs and in some large , long @-@ lived birds and mammals today . These species are characterized by high infant mortality rates , followed by relatively low mortality among juveniles . Mortality increases again following sexual maturity , partly due to the stresses of reproduction . While this could be due to preservation or collection biases , Erickson hypothesized that the difference was due to low mortality among juveniles over a certain size , which is also seen in some modern large mammals , like elephants . This low mortality may have resulted from a lack of predation , since tyrannosaurs surpassed all contemporaneous predators in size by the age of two . Paleontologists have not found enough Daspletosaurus remains for a similar analysis , but Erickson notes that the same general trend seems to apply . 
