Earliest Ancestors of Dinosaurs

Introduction

Before you start talking about dinosaurs, how they lived, what they ate, etc., believe it is necessary a previous location of the group in systematic zoology.This is the end of this chapter. We'll see what are the reptiles, how they came about and how it later evolved, resulting in different groups, until the dinosaurs, while making pit stops at major groups.

The origin of reptiles

Makes about 310 million years ago, in the Devonian, the first vertebrates appeared on land. It was theIchthyostega (bottom) the first amphibian, which had fallen from the crosopterigios, some fish possessing functional lungs and two pairs of muscle flaps with bone deposition similar to the bones (tetrapods) that could be used to move your body and hold his own weight without relying on the buoyancy water. Existing theories on why they left these things aquatic life suggest a drying of the medium in which they lived, which managed to survive thanks to his lungs and also preformed an escape marine predators to colonize new environments and expand its ecological niche, the colonization of terrestrial environments meant different evolutionary radiation of amphibians. However, its colonization of the terrestrial environment was not complete, since, as with current amphibians, maintained a profound dependence on water for reproduction.

In the Upper Carboniferous, about 325 million years, a small group of amphibians, Antracosaurios gave rise to reptiles, in particular small and light Captorrinomorfos. The emergence of the reptiles and was a genuine settlement of the land environment, as it showed many adaptations to avoid dehydration, which meant living in an environment out of the water and survived forms scaly reptilian skin that decrease body perspiration , ventricle partially separated, excretory and reproductive ducts independent of each other and above all, the formation of an egg protected from desiccation by a shell: the amniote egg.

This egg (right figure) is formed by a protective membrane calcification within which a series of membranes nourish and protect the embryo, it is connected to a yolk sac, which is nothing more than a bag of reserve substances from which fed during their stay inside the egg and allantoic cavity, which is where waste products accumulate as precipitates nitrogen, in turn, the amniotic membrane covering the amniotic cavity, inside which the embryo lies This covers and protects the contact with the skin. The living beings that have this egg are called amniotes. The complexity of the egg suggests that the transition between amphibians and reptiles did not occur more than once, and therefore the degree of evolution of reptiles is monophyletic and that was not formed as a result of homologies between different groups.

The egg represented the total independence of the reptiles of the aquatic environment as the amniotic cavity filled with liquid, resembling the aquatic environment needed by the embryo to develop, could be interpreted as a small isolated pond dry external environment. Because of this, reptiles rapidly colonized the terrestrial environment, undergoing radiation originated different storylines. For the classification of new lines of amniotes ...