Adaptation To Land

The first amphibians to spend a significant part of their life on land most likely evolved from lobe-finned fishes. Lobe-finned fishes had several preadaptations that allowed them to transition to life on land. Preadaptations are adaptations in an ancestral group that allow a shift to new functions which are later favored by natural selection.

Lobe-finned fishes ancestral to amphibians had a bone structure within their fins that worked as legs that could walk on land. Ancient lobe-finned fishes also had modified pouches in their digestive tracts, which evolved into the lungs of lungfish and swim bladders in most modern fishes. Some groups of ancient lobe-finned fishes also had internal nostrils that functioned in air breathing. In lobe-finned fishes ancestral to amphibians, these structures worked as a means of breathing on land.

Early amphibians required more oxygen than their fish ancestors. Because gravity makes movement on land more difficult than movement in water, early amphibians were likely to have had a higher metabolism than their fish ancestors. As a result, efficient hearts were an important adaptation that allowed oxygen to be delivered to the body more efficiently.

Characteristics of Early Amphibians

Amphibians and lobe-finned fishes share many anatomical similarities, including features of the skull and vertebral column. Also, the bones in the fin of a lobe-finned fish are similar in shape and position to the bones in the limb of an amphibian. Figure 40-1 shows a sarcopterygian (sar-KOP-te-RIJ-ee-uhn), an extinct lobe-finned fish that is thought to be closely related to amphibians. This fish probably lived in shallow water and used its sturdy pelvic and pectoral fins to move along the bottom and to support its body while resting.

objectives

• Describe three preadaptations involved in the transition from aquatic to terrestrial life.

• Describe two similarities between amphibians and lobe-finned fishes.

• List five characteristics of living amphibians.

• Name the three orders of living amphibians, and give an example of each.

vocabulary preadaptation tadpole figure 40-1

Early lobe-finned fishes, such as this sarcopterygian, are thought to be ancestors of the first land vertebrates.

figure 40-1

Early lobe-finned fishes, such as this sarcopterygian, are thought to be ancestors of the first land vertebrates.

Amphibian Skin Structure

Amphibians have similar bone structure to primitive lobe finned fishes. The colored bones in the figure are homologous structures between lobe-finned fish and amphibians. Homologous structures are anatomical structures that share a common ancestry.

Figure Lobed Fin Fishes

Comparing Fish and Amphibian Skin

Materials disposable gloves, lab apron, safety goggles, paper, colored pencils, living or preserved specimens of a fish and a frog

Procedure

1. Put on your disposable gloves, lab apron, and safety goggles.

2. Handle living animals gently. Touch and examine the skin of the specimens provided by your teacher. Record your observations.

3. When you are finished with your observations, remove your disposable gloves, lab apron, and safety goggles. Wash your hands with soap and water.

Analysis Why can a frog use its skin as a respiratory membrane, while a fish cannot? What behaviors in amphibians enable them to maintain moist skin?

The oldest known amphibian fossils date from about 360 million years ago. All of the early amphibians had four strong limbs, which developed from the fins of their fish ancestors, as shown in Figure 40-2. 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. The early amphibians also breathed air with lungs. Lungs arose early in the history of fishes and are found in the descendants of these early fishes—including terrestrial vertebrates.

Although the early amphibians showed several adaptations for life on land, such as sense organs for detecting airborne scents and sounds, they probably spent most of their time in the water. For example, some of the first amphibians had a large tail fin and lateral-line canals on their head. Their teeth were large and sharp, indicating a diet of fish, not insects. In addition, some of the early amphibians appear to have had gills like those of fishes.

Diversification of Amphibians

During the late Devonian period and the Carboniferous period (359 million to 299 million years ago), amphibians split into two 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.

Today there are about 4,500 species of amphibians, belonging to three orders. The largest order, with more than 3,900 species, is Anura, which includes the frogs and toads. The order Caudata contains about 400 species of salamanders. And the third order, Gymnophiona, consists of about 160 species of caecilians, which are legless tropical amphibians. Figure 40-3 on the following page shows hypotheses for the phylogenetic relationships between these three groups.

Class Amphibia

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