Why did some they grow so big?
Paleontologists do not know for sure, but a large body size may have protected them from their predators, helped to regulate internal body temperature, or allowed them to reach new food sources (and probably some browsed treetops, as giraffes do today).
Except for whales, no modern animals are even close in size to the biggest dinosaurs; therefore, experts believe that the dinosaurs’ world was very different from ours and that food supplies and climate must have been favorable for reaching great size.
