A new analysis of tyrannosaur fossils suggests T. rex took about 40 years to reach full size, far longer than earlier estimates. The research also hints that some well-known fossils may belong to entirely different species. Credit: Shutterstock For many years, paleontologists have studied annual growth rings preserved inside the fossilized leg bones of Tyrannosaurus rex . Much like the rings inside a tree trunk, these marks help scientists estimate how old the dinosaurs were when they died and