( ida State University to produce the fir...)Tyrannosaur survivorship -- tough times for teens

They found that the complex survivorship pattern is remarkably similar to that seen in living large mammals. High newborn mortality rates due to predation alone subside once a threshold size is reached and it appears that such a threshold was reached by age two in Albertosaurus sarcophagus, say the research team. The mortality rates remained low until about the 13th year of life, at which point they reached total lengths of six metres or 60 per cent of their maximum recorded size. At that point, mortality rates escalated to more than 23 per cent a year.

70% of the animals surviving to two years of age were still alive at age 13, which would help explain why so few bones of young adults have been found. "One implication of these findings is that the previously mysterious rarity of sub-adult tyrannosaur specimens is due to their exceptionally low mortality rates," said Currie.

Some people have speculated that tyrannosaurs must have rocketed to adult size in a few years or less, leaving only a small fraction of development that juveniles could have contributed to the fossil record. "However, this notion is inconsistent with our growth curve," say the researchers in the paper. "Instead, we suggest that these young animals simply had low mortality, just like older juveniles and subadults of most large terrestrial mammals today."

The estimated survivorship curve also provides a possible explanation for the rarity of individual giants--just two per cent of the population lived long enough to attain maximal size and age for the species.