C H A P T E R 6 Amphibians Amphibians are the first quadrupedal vertebrates that can support themselves and move about on land. They have a strong, mostly bony, skeleton and usually four limbs (tetrapod), although some are legless. Webbed feet are often present, and no claws or true nails are present. | Linzey Vertebrate Biology 6. Amphibians I Text The McGraw-Hill Companies 2003 CHAPTER 6 Amphibians INTRODUCTION Amphibians are the first quadrupedal vertebrates that can support themselves and move about on land. They have a strong mostly bony skeleton and usually four limbs tetrapod although some are legless. Webbed feet are often present and no claws or true nails are present. The glandular skin is smooth and moist. Scales are absent except in some caecilians that possess concealed dermal scales. Gas exchange is accomplished either through lungs absent in some salamanders gills or directly through the skin. Amphibians have a double circulation consisting of separate pulmonary and systemic circuits with blood being pumped through the body by a three-chambered heart two atria one ventricle . They are able to pick up airborne sounds because of their tympanum and columella and to detect odors because of their well-developed olfactory epithelium. The emergence of a vertebrate form onto land was a dramatic development in the evolution of vertebrates. Some ancestral vertebrate evolved a radically different type of limb skeleton with a strong central axis perpendicular to the body and numerous lateral branches radiating from this common focus. This transition had its beginnings during the early to middle Devonian period and took place over many millions of years Fig. . It involved significant morphological physiological and behavioral modifications. A cladogram showing presumed relationships of early amphibians with their aquatic ancestors as well as with those amphibians that arose later is shown in Fig. . Phylogenetic relationships depicted in such diagrams are controversial and subject to a wide range of interpretations. EVOLUTION Controversy surrounds the ancestor of the amphibians. Was it a lungfish a lobe-finned rhipidistian or a lobe-finned coelacanth Rhipidistians which are now extinct were dom inant freshwater predators among bony fishes. Did amphibians .
