Dinosaur is a term used to describe certain types of animals that lived during the Mesozoic era in geologic history. It is difficult to generalize about dinosaurs, but two things are definitely agreed upon. They were, in general, the largest creatures to ever walk on Earth, even though there were many smaller species of dinosaurs. In addition, these animals were some of the most successful organisms that ever lived, existing as species for at least 160 million years.