Dinosaurs are animals that dominated the terrestrial ecosystem for over 100-million years. Non-avian dinosaurs became extinct 65 million years ago (Mya) at the end of the Cretaceous period. Dinosaurs are known from both fossils and nonfossils including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin and internal organs, and "soft tissues, including blood vessels and cells lining them [1]".
Since the first dinosaur was recognized in the 19th century, their mounted skeletons have become a major attraction at museums around the world. Dinosaurs have become a part of world culture, and have remained consistently popular. They have been featured in bestselling books and blockbuster films like Jurassic Park, and new discoveries are regularly covered by the media. The term is also used informally to describe any prehistoric reptile, like the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs, and mosasaurs.
The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs, which are still known from a spotty fossil record.