That's a great question since no one was around at the time to write down a record of the dinosaurs that were on the Earth at that time. But here's what paleontologists do. They can determine the ages of the rocks where the fossils are found. And since the fossil is found in that mass of rock, scientists figure that the dinosaur must have been around at the same time that the rock started forming. So the ages of the rock the fossil is found in tell us the age on dinosaur. It would be pretty hard to be any newer than the rock, since it's hard to put something into rock without breaking it. -- Thor