Dinosaur origins are uncertain, although most authorities agree that both orders arose from an ancestor classified among the Pseudosuchia in the extinct group of archosaurian reptiles, the thecodonts. (However, some authorities think that the pronounced anatomical differences between ornithischians and saurischians may be evidence of separate thecodont ancestry.) The pseudosuchian thecodonts Ornithosuchus and Euparkeria are often cited as likely anatomical types that could have given rise to saurischians and possibly to ornithischians, but the evidence is too fragmentary or controversial to be conclusive.

During the 140-million-year reign of the dinosaurs, many new varieties evolved and many primitive predecessors died out. Not all kinds became extinct at once. Rather, the fauna changed repeatedly with the passage of time. The last of the dinosaurs disappeared at the end of the Cretaceous, marking a major turnover in the history of life on Earth. Their extinction has been attributed to causes ranging from increases in cosmic radiation to changes in climate, to volcanic activity, and to continental drift. In fact, evidence indicates that sea levels did fall and climatic temperatures did drop at the end of the Mesozoic Era.

A controversial but popular extinction theory was proposed in the 1980s by physicist Luis Alvarez and his geologist son Walter. They found a larger than normal amount of iridium in samples of sedimentary layers between rocks of Late Cretaceous and Early Tertiary time and concluded that the iridium came from outer space, most likely from an asteroid or meteorite that struck the Earth at that time. They theorized that the impact caused an enormous cloud of dust created by that event to circle the Earth, enshrouding it from sunlight and striking at the food chain by killing many or most plants. Research on this theory continues, and a location for the asteroid impact has been found in the region of the Yucatán Peninsula of Mexico and named Chicxulub meteorite crater. Some geologists, while accepting the iridium evidence as meaningful, have suggested that the iridium resulted from massive volcanic eruptions over a longer period of time, resulting in worldwide climatic changes. The impact theory, however, has gained increasing acceptance, and in 1998 at least one geologist was claiming to have identified a small fragment of the original dinosaur-killing asteroid.

Whatever the cause, the dinosaurs are now gone. As suggested above, however, in a way they may remain. Birdlike dinosaur fossils continue to be found, and many paleontologists consider birds almost certainly to have evolved from small bipedal dinosaurs during the Jurassic. If so, the children of the dinosaurs are still here today.