Herds of large horned dinosaurs flourished but this period saw the rapid decline of the dinosaurs and the evolution of mammals. The three periods above comprise the Mesozoic Era which was ruled by reptiles including dinosaurs.
Dinosaur bones look a good deal like the bones of modern reptiles and mammals.
Scientists need to know about modern skeletons so that they can make informed guesses when they are assembling a dinosaur skeleton. Sometimes they have to guess more than usual if bones are broken or missing. At other times they have to make a more uncertain guess when a bone looks different from what they have seen before. Such guesses are not always correct. Scientists have calculated the ages of the layers of rock that make up the crust of Earth.