Evolution

Abstract

Life, almost certainly, began in the sea, and the first terrestrial vertebrates were amphibians. During the Upper Devonian period, the descendants of certain crossopterygian fishes ventured out of the water and began crawling from one pool to another. The Late Devonian was a time of increasing aridity, so that a selective advantage would have been obtained from being able to remain for a while on dry land. These first amphibians, or Ichtyostegalia, evolved in directions quite different from those followed by their ancestors. Although they had many characters in common, important differences appeared between the lung fishes and their allies and these early amphibians. This resulted from the specialization of the former for aquatic and, of the latter class, for terrestrial life. The Ichthyostegalia were four-limbed tetrapods and, while some were scaly, others had dry, leathery skins. Moreover, the ventral sides of the body were occasionally protected by bony plates. The skull of Ichthyostega shows the characteristic features of an amphibian whilst simultaneously retaining traces of its fish ancestry both in its shape and by the presence of a preopercular bone. In addition to the Ichthyostegalia (fish vertebrae), there were two other orders of Palaeozoic amphibians (Labyrinthodontia), viz. the Temnospondyli (divided vertebrae) and Anthracosauria (coal lizards). The reptiles evolved from Anthracosauria.