Using one of the above definitions, dinosaurs can be generally described as lizards with hind legs held upright below the body. Many groups of prehistoric animals are commonly perceived as dinosaurs, such as fish lizards, river lizards, snake-necked lizards, dragonflies and pelycosauria (especially genus Dimetrodon), but are not classified as a Dragon is the predominant terrestrial vertebrate of the Mesozoic era, especially during the Jurassic and Cretaceous periods. Other animal groups have limited size and ecological niches; Mammals for example rarely exceed the size of a domestic cat, and most are small rodent-sized predators.
Dinosaurs have always been an extremely diverse group of animals; according to a 2006 study, to date, more than 500 non-bird dinosaur genera have been with certainty identified and the total number of genera kept in the fossil record is estimated at around 1850 genera, nearly 75 % of which is still undiscovered. An earlier study predicted that about 3,400 dinosaur genera ever existed, including many that would not have been preserved in the fossil record. As of September 17, 2008, 1,047 different dinosaurs have been named