SUBPHYLUM VERTEBRATA

Class Amphibia

Origin and Evolution of Amphibians Introduction

About 400 million years ago, the first amphibians evolved from lobe-finned bony fishes and become first vertebrates to live on land.

Coelacanth

Origin and Evolution of Amphibians Introduction

The name amphibian comes from the Greek words meaning “double” and “life” and reflects the fact that many amphibians spend part of their life on land and part in water.

Origin and Evolution of Amphibians WHY THE MOVE TO LAND?

It is likely that the ancestors of amphibians left the water to escape predation and overpopulation and to gain access to the resources that were becoming abundant on land.

Origin and Evolution of Amphibians At that time, oceans, lakes, rivers, and

ponds supported a tremendous number and variety of fishes. Food and space were limited, and the numerous species of fishes competed intensely for them.

I have to get out of

here!!!

Origin and Evolution of Amphibians On land, however, there were no

predators, and terrestrial invertebrates such as arthropods, a promising food source, were beginning to diversity.

Characteristics of Early Amphibians The oldest known amphibian fossils date from

about 370 million years ago. Ichthyostega is the best known early amphibian. It had well-developed limbs and is thought to have been a crawler on land, but it still had an overall fish-like body

Humerus

Characteristics of Early Amphibians Zoologists believe that the limbs of the early amphibian

evolved from the fins of their fish ancestors. The forelimbs of amphibians (and all other terrestrial vertebrates) are homologous to the pectoral fins of fishes, and the hind limbs are homologous to the pelvic fins.

What are homologous structures?

Characteristics of Early Amphibians

The early amphibians also breathed air with lungs. As you may recall, lungs arose with the evolution of lungfishes, a group of lobe-finned fishes.

Characteristics of Early Amphibians Although early amphibians showed several

adaptations for life on land, they probably spent most of their time in the water. For example, Ichthyostega had a large caudal fin and lateral-line canals on its head. Its teeth were large and sharp, indicating a diet of fish, not insects.

Diversification of Amphibians During the late Devonian period and the

Carboniferous period (360 million to 286 million years ago), amphibians split into 2 main evolutionary lines. One line included the ancestors of modern amphibians and the other line included the ancestors of reptiles.

Amphibians have been a diverse, widespread, and abundant group since this early diversification.

Diversification of Amphibians Today there are about 2,800 species of

amphibians, belonging to three orders. The largest order, with more than 2,000

species, is Anura, which comprises the frogs and toads.

FrogToad

Diversification of Amphibians The order Urodela contains about 400

species of newts. And the third order, Apoda (or Gymnophiona), consists of about 160 species of caecilians, which are legless, tailless, tropical amphibians.

Newt Caecilians