Besides gestation, a larger body typically takes longer to reach sexual maturity – another reason dinosaurs didn"t make it, especially the larger ones. "It took them quite a while to grow into an adult. For something like T. rex it took about 20 years," says Brusatte. "It"s not that they didn"t grow fast, it"s just that so many of them were so big that it took them a long time to go from a tiny little hatchling into an adult."