Amphibians

Amphibians are tetrapods (four-limbed vertebrates) that lay eggs in water. They are the ancestral group to all of the other tetrapods, including reptiles, dinosaurs, and mammals, as well as birds.

The most likely ancestors of amphibians, and all other tetrapods, are a group of extinct lobefin fish known as rhipidis-tians. These fish have a similiar skull morphology to the earliest amphibians and the pattern of limb bones common to all subsequent tetrapods, including humans (Fig. 11.4). This is the pattern of one upper bone, two lower bones, and many peripheral bones in each limb. For most tetrapods there are five peripheral bones, for example human fingers, but some species have modified this number and the earliest amphibians tended to have more than this, typically seven or eight digits.

With their strong fins and ability to breathe air, lobefin fishes were excellently preadapted to a life lived partly on land. In addition, they were adapted to shallow water habitats, in both the seas and fresh water. This put them physically close to land, in an environment that could intermittently become problematic, during a dry season or when stagnation caused a lack of oxygen supply. Modern lungfish leave water to escape from such conditions and to find food. The ability to crawl over land might also have allowed lobefins to lay their eggs in isolated bodies of water for protection from scavengers.

The two best-known early amphibians are called Ichthyostega and Acanthostega. Both are found in late Devonian sediments from Greenland. They both appear to have been lake-living fish eaters and were highly adapted for this way of life. However, they also show a fascinating blend of fish-like and amphibian-like characteristics that give a real insight into this stage of tetrapod evolution (Fig. 11.5).

Both of the Greenland amphibians had streamlined bodies and heads, a long flexible tail, and a pronouned tail-fin. Their teeth were very similar to the teeth of fish. Their limbs were short and the wrists and ankles poorly articulated. Oddly, Icthyostega had seven toes on its hindlimbs, and Acanthostega had eight digits on its forelimbs. This is important because it shows that the five-digit hand and foot, the so-called pentadactyl limb, is a later development and is not one of the shared characteristics of the whole group.

Amphibians radiated into around 40 families during the Carboniferous, thriving in the warm, damp, coal forests. The diversity of form and size make these organisms hard to classify. They include snake-like animals, tiny amphibians with adults less than 10 cm long, and aquatic forms with wide, delta-wing heads. From this highly diverse assemblage two

Single bone Paired bones

Many bones

Fig. 11.4 A comparison of the fin of (a) a rhipidistian, and (b) the limb of a primitive tetrapod.

groups are important for the evolution of more modern tetrapods. These are the temnospondyls, which gave rise to modern amphibians, and the reptilomorphs, which likely included the ancestor of reptiles, mammals, and birds.

There were three things that limited the evolutionary potential of these Carboniferous amphibians: they needed to return to water in order to lay eggs, they were all meat eaters, and they had limited mobility on land. Limbs were generally short and articulated at the knee and elbow, producing a low-slung, ungainly motion.

Modern amphibians are divided into three groups: frogs and toads, newts and salamanders, and caecilians, which lack limbs. They are classified together as lissamphibia, and are thought to have evolved from a temnospondyl ancestor during the Triassic. The earliest frogs are early Jurassic in age and have the characteristic shape of modern frogs. A salamander has been found in late Jurassic rocks from Kazakhstan. The oldest known caecilian is also Jurassic in age and still has small limbs. The fossil record of the lissamphibia is relatively poor.

Carboniferous reptilomorphs included groups such as the anthracosaurs, medium-sized fish eaters around 1m in length. Some anthracosaurs, such as Protogyrinus, had relatively long limbs and would have been fully mobile on land. Others, such as Pholiderpeton, were adapted for living in water. An unknown member of this group evolved the ability to lay eggs on land. These amniotic eggs were critical to the success of descendent groups, such as reptiles and mammals.

Fig. 11.4 A comparison of the fin of (a) a rhipidistian, and (b) the limb of a primitive tetrapod.

Live in terrestrial environments, with secondary colonization of water and air. Efficient methods of locomotion, lay eggs on land, or bear live young. Can be warm blooded

Anapsids: includes turtles and tortoises

Synapsids: includes pelycosaurs, mammal-like reptiles, and mammals

Diapsids: includes dinosaurs, birds, pterosaurs, marine reptiles, lizards, and snakes

Synapsids: includes pelycosaurs, mammal-like reptiles, and mammals

Anapsids: includes turtles and tortoises

Modern amphibians

Hylonomus: arguably the oldest reptile, and likely to have laid eggs

Live in terrestrial environments, with secondary colonization of water. Lay eggs on land. Cold blooded

Modern amphibians

Ichthyostega and other diverse but primitive amphibians

Rhiphidistian fish: an advanced clade of lobefins

Fig. 11.5 The evolutionary relationships of tetrapods. The boxes show the main functional adaptations of each group.

Ichthyostega and other diverse but primitive amphibians

Rhiphidistian fish: an advanced clade of lobefins

Hylonomus: arguably the oldest reptile, and likely to have laid eggs

A mixed aquatic/terrestrial lifestyle. Usually feed from water and have good swimming abilities. Capable of locomotion on land, but have sprawling gait. Usually lay eggs in water. Cold blooded

Lived in water except for very short periods of time. Capable of air breathing, but lacking firm skeletal support for efficient movement on land. Laid eggs in water

Fig. 11.5 The evolutionary relationships of tetrapods. The boxes show the main functional adaptations of each group.

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