Dinosaurs were a widely distributed and diverse group of reptiles that lived from approximately 215 to 65 million years ago - though modern birds are likely descended from some of these ancient creatures, evolving from about 150 million years ago. Dinosaurs are distinct from today's reptiles in several ways, not least their generally rather larger size.
Dinosaurs thrived from the Late Triassic Period, but went extinct around the end of the Cretaceous Period, a time known as the Cretaceous–Tertiary boundary or K–T boundary. A widespread myth exists that all the dinosaurs were wiped out at the same time, though fossil evidence shows that many had died out or were disappearing already by the end of the Cretaceous. Furthermore, other kinds of animals also disappeared at the time, along with many plants, though many did not seem to be as critically affected as the dinosaurs (early mammals for example, continued).
The most popular explanation for the extinction of the dinosaurs, and indeed many other forms of life, is the 'asteroid theory' - i.e. a large object from outer space crashed onto the dinosaurian Earth, causing a catastrophic extinction from which only the hardiest creatures would emerge. This is by no means the only theory, but it has led to more fruitful debate than other ideas, such as the claim that the dinosaurs succumbed to disease.
The best-known dinosaur is Tyrannosaurus rex, which in its day would have been a fearsome species, with adults often over 20 feet tall and moving using powerful hind limbs as they searched for prey. 'T. rex' is, however, just one of over a thousand species so far identified using the fossil record. Dinosaurs were first recognised as a category new to science in the nineteenth century, and were named by Richard Owen, an English anatomist. Prior to that time, and indeed in the decades to come, dinosaur bones were often mistaken for other species, especially modern reptiles. The age of dinosaur remains, their diversity, their location in different geological strata and the obvious evolutionary adaptations and homologies in their structure all point to their being long-dominant on the planet many millions of years ago.