Dinosaurs are an extinctsuperorder of reptiles that first appeared approximately 210 million years ago. A few lines of primitive dinosaurs diversified rapidly after the Triassic; the reign of dinosaurs encompassed the ensuing Jurassic, and Cretaceous periods. At the end of the Cretaceous, 65 million years ago, all species of dinosaur became extinct (the Cretaceous-Tertiary extinction event), except for the closely-related line that had already led to the first birds. There is now sufficient evidence to demonstrate that the birds are the descendants of early dinosaurs.
The formal name Dinosauria was first proposed by the English scientist Richard Owen in 1842. The term is a combination of the Greek words deinos ("terrible" or "fearfully great" or "formidable") and sauros ("lizard" or "reptile").
Many other types of reptiles lived at the same time as the dinosaurs. Some of these are commonly, but incorrectly, thought of as dinosaurs: these include plesiosaurs (which are not closely related to the dinosaurs), and Pterosaurs, which developed separately from reptile ancestors in the late Triassic.
Dinosaurs are archosaurs, like modern crocodylians. These are set apart by having diapsid skulls with teeth that grow from sockets, rather than as direct extensions of the jaw bones, as well as various other characteristics. Within this group, the dinosaurs are set apart most noticeably by their gait. Instead of legs that sprawl out to the side, as found in lizards and crocodylians, they have legs held directly under their body.
There has been a constant and vigorous debate over the method of regulation of temperature of dinosaurian blood, a debate first popularized by Robert T. Bakker. Since the first discovery of dinosaurs, the idea that they were ectothermic creatures was considered most likely. This would have meant that the animals were mostly slow, dormant organisms, comparable to modern reptiles that need the sun to heat their bodies. However, new evidence of dinosaurs in chilly temperate climates and scientific analysis of the blood-vessel structures within dinosaur bone have opened the possibility that some dinosaurs were endothermic, regulating their body temperature by internal biological methods. Skeletal structures suggest active lifestyles for theropods and other creatures. This means that an endothermic cardiovascular system would have been more suitable for dinosaurs. This doesn't mean all dinosaurs were endothermic, though. The debate still continues to this day (although many paleontologists would agree that endothermic systems are more likely now).
The behavior of dinosaurs will always be a mystery simply because none exist today. The only evidence paleontologists have to go on are fossil tracks, skeletons locked in battle (Velociraptor and Protoceratops), and fossilized nests. All the evidence varies, depicting several different behaviors. Herbivores may have been much more social, migrating in huge herds much like modern day mammals (i.e. African species). This could have been a successful predator warning system, depending on the predator. The carnivorous dinosaurs possibly exhibited social characteristics as well, like wolves and large cats. Families may have traveled together for a very long time in order to maximize survivability. But all of this is speculation, and a more accurate description is likely distant in the future. Considering the dinosaurs were successfully the dominating creatures for millions of years, some type of social order seems likely to have been present.
Although the speed of extinction cannot be deduced from the fossil record alone, the latest models suggest the extinction was extremely rapid. It appears to have been caused by heat caused by the meteorite impact and the matter ejected from the crater re-entering the atmosphere around the world. Other theories link the extinction with increased volcanic activity, decreasing oxygen level in the atmosphere and dropping temperatures.
The survivors of this mass extinction appear to have been two things: small animals hiding underground and/or aquatic creatures relatively safe underwater. Neither of this applies to dinosaurs, most of whom were probably incinerated in the heat blast. Other groups as well as the dinosaurs, including ammonites, mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most kinds of bird, and many groups of mammals, became extinct.
