The dinosaurs were not a group of reptiles which lived together in a certain period of time; they were different animals with different qualities and body structures. The definition of a dinosaur is that they all had a hole in their hipbone, but that was also the only thing that they had in common. Some of the dinosaurs were big and large, such as the Brachiosaurus and the Tyrannosaurus, while some of them were tiny little dinosaurs. Because of their hipbone they all got placed under the same category, but there are two subcategories and under them there are another two categories and so a chart over all of the dinosaur groups can be done.