How big were newly-hatched T. rexes and what was their life span?
from Kate, age 15, LA, CA, usa; July 18, 2001

TOM: No one has found T. rex eggs or babies yet. However, based on finds from other dinosaurs, baby T. rex were probably quite small: maybe 1 meter (about a yard) long or less, and half of that would be tail. They might have been even smaller than that: bones of embryos in the egg of a Portuguese Allosaurus show that that dinosaur was only about 1 foot long (30 cm) when it hatched. In fact, ALL dinosaurs seem to have been quite small when born: a major difference between dinosaurs and mammals. After all, a baby elephant is still a big animal, bigger than most species of antelope that live on the same plains. However, a baby titanosaur is a little animal, smaller than most breeds of dogs, while an adult titanosaur is as big as a herd of elephants!

No one knows for certain the life span of dinosaurs. We can estimate the time it took for them to reach adult (breeding) size. In the case of tyrannosaurs, this was probably about the same time it took similar-sized hadrosaurids (about 7 years or so). If dinosaurs were fully warm-blooded, their lifespan in the wild might be related to their body size. Since T. rex was about the size of an African elephant it might have had a similar life span (70 years or thereabouts). If dinosaurs were fully cold-blooded (unlikely based on many lines of evidence), their lifespans might have been much longer.