Alligators, crocodiles, monitors (e.g. Komodo dragon) all fit the description of dinosaurs as large lizards. There were many types of dinosaurs that were smaller than these reptiles today. Consider the Komodo dragon (Varanus komodoensis), one of the largest and most fierce land-living reptiles alive today. It is about 3 m long, can outrun a human in short distances, eats deer, hogs, and can be dangerous for even a man.

There are two major competing hypotheses regarding avian evolution, which differ primarily with respect to when the first true birds appeared. One hypothesis proposes that the first birds descended directly from ancestral reptiles about 230 million years ago in the early to middle Triassic. Here, we refer to this idea as the basal archosaur hypothesis; "archosaur" is the name for the ancestral reptiles from which birds, crocodiles, and dinosaurs evolved. The other hypothesis advocates a much later entry of birds, with derivation from the dinosaurs some 100 million years after the time proposed by the basal archosaur hypothesis. This idea we refer to as the theropod dinosaur hypothesis.

The Dinosaurs come from the superorder Archosauria, an assemblage of different forms, which originally diverged from the ancestral diapsids by the late Paleozoic. The first dinosaurs appeared by the Middle Triassic in South America. From the late Triassic to the end of the Cretaceous, dinosaurs showed extensive radiation into a variety of groups and ruled the land, sea, and aerial environments. Dinosaurs can be divided into two large groups based on the structure of the pelvis: the saurischians and the ornithischians. The saurischian dinosaurs split further into two distinct lineages, the herbivorous sauropods (e.g.,Brachiosaurus) and the carnivorous theropods (e.g., Tyrannosaurus), which appeared in the late Jurassic. All ornithischians were herbivores (e.g., Stegosaurus).

All dinosaurs disappeared suddenly (by geological time standards) by the end of the Cretaceous (65 million years ago). The exact cause of the extinction has been debated rigorously and extensively. At the end of the Mesozoic, did a catastrophic event (e.g., the impact of a large meteorite or asteroid) occur and kill plants and animals, as in the Alvarez hypothesis (Alvarez, 1987)? Or, were dinosaurs vulnerable to gradual geological changes (e.g., lowering of the temperature) occurring on the earth at this time? Whatever the probable cause, whereas dinosaurs (as well as numerous other plant and animal species) disappeared, the ancestors of mammals and birds survived.