How Did The Dinosaurs Evolve?

     Dinosaurs are one of the most famous groups of living organism ever to inhabit the Earth, but an interesting question is, where did they come from?
     Dinosaurs arose from a family of prehistoric reptiles called the archosaurs, who evolved from a common ancestor between fish and reptiles, one of the first creatures to make the transition from water to land.  From that common ancestor, three branches formed, the pelycosaurs, the therapsids, and the archosaurs.  The therapsids gave rise to the first mammals, who existed somewhat in secrecy along with the dinosaurs, but were unable to flourish with them.
     The archosaurs rose up after the Permian/Triassic Extinction Event, which was a massive extinction that wiped out three quarters of the world's terrestrial animals. The archosaurs, who thrived for thousands of years evolved and gave rise to the dinosaurs, who thrived for millions of years.
     The dinosaurs 'ruled' the Earth for an extremely long time, up until the K/T Extinction , that wiped out 70% of Earth's land animals, including the dinosaurs.  The most accepted theory behind this extinction is that a massive meteorite struck the Earth, clouding the sky for years killing many plants and many animals.  Without any plants, the herbivores starved and died out, and without herbivores, the carnivores had nothing to eat and also died out.  Also, the impact caused earthquakes and volcanic eruptions, destroying much of Earth as it was known 65 million years ago.
     Very few animals survived, but the ones that did consisted of some avian-like dinosaurs, that gave rise to birds, many mammals, and tortoises. With the extinction of dinosaurs, animals such as mammals were able to rise and evolve.

http://dinosaurs.about.com/od/dinosaurevolution/a/dinevolution.htm