The evolution of the dinosaurs begins with the DevonianPeriod, close to four hundred million years ago. The occurrence of the animals migrating form water to land occurred during the Devonian Period. Animals did this in search of food sources which have yet to be exploited. It is a theory that perhaps early amphibians were the ancestor of all land-living animals.
Following the Devonian Period was the Carboniferous Period. This period was the main phase of evolution for Tetra-pods. Tetra-pods are amphibians, reptiles, birds, and mammals processing four legs and bearing toes, The loss or modification of some limbs occurred in some species during this period. The most major change in this period was the development of the amniotes. Amniotes were animals able to lay shelled eggs. this allowed for animals to reproduce out of the water for the first time. Reptiles and amphibians were able to evolve separately sharing the same territory. This explains why reptiles and amphibians are able to co-exist together today.
Reptiles first began to dominate the land during the Permian Period, Mammal-like reptiles such as the pelycasaurs and the therapsids were the major groups living in this period. It is important to know that the diapsids eventually led to the dinosaurs, Diapsids were rare in the Carboniferous and Permian Periods. Diapsids were a group of reptiles with two openings on each side of the skull directly behind the eyes. The extra openings allowed for an increase in muscle mass, giving these creatures he ability to have a strong bite. At the end of this period occurred a mass extinction of the tetrepods. the cause of the extinction is not known, but is said to hae led to the evolution of the dinosaurs. This extinction marked the end of the palaeazaic period. The mesozic period followed but was the most important to the evolution of the dinosaurs.
The Mesozoic Era divided into three major periods in which dinosaurs lived. These periods are known as Triassic, Jurassic, and Cretaceous. These periods were the periods of the dinosaurs. Dinosaurs ruled the planet in these times. These periods talked about in this article all led up to the Triassic period (about 250 million years ago). In this period, the evolution of Archosaurs and early dinosaurs took place.
For more information and a detailed walk through of the lives of dinosaurs including the Triassic period, Jurassic period, Cretaceous period, and the extinction of dinosaurs please visit (http://palaeo.gly.bris.ac.uk/communication/boulton/evolution.html)