The modern newts and salamanders, frogs and toads are the survivors of the amphibians that first ventured out on land some 370 million years ago. Their pioneering land effort was not a total success, however, since amphibians must still return to water to breed. It was their descendants, the reptiles, that truly conquered the land.
The very word amphibia defines the essential quality of these animals, for it means "both lives." It refers to their ability to live in 2 worlds — the world of water that their fish ancestors still inhabit, and the world of land that their descendants, the reptiles, inherited.
The young amphibian larva that emerges from the egg is adapted to life in water — it has gills and a swimming tail. Later, there is a fairly rapid change in structure (a metamorphosis), when the larva loses these features, and replaces them with lungs and stronger limbs to adapt it to life on land.
There are several reasons for believing that the fossil amphibians of the Paleozoic Era passed through a similar aquatic larval stage of development. In some cases, small specimens have been found in which traces of the gills have been preserved, and a series of progressively larger forms link them to an adult with no trace of gills.
In other cases, such as Seymouria (see p.-S3), the head of young specimens still shows traces of canals in which were located the sensory, lateral-line organs (inherited from their fish ancestors) that could only have been used in the aquatic environment of a larval stage.
Finally, some living amphibians, such as the mudpuppy of North America, have returned to a wholly aquatic life, retaining into adulthood the gills that previously only the larva had possessed. This is true also of some of the Paleozoic amphibians, such as Qerrothorax with its 3 pairs of feathery gills (see p. 53).
Amphibians colonized the land during Late Devonian times. Several groups of large amphibians (labyrinthodonts) dominated the land during the Late Carboniferous and Early Permian. At the same time, smaller snake- and salamanderlike amphibians (lepospondyls) evolved. Only 2 groups of amphibians survive today — frogs and toads, and newts and salamanders.
Paleontologists can, as yet, establish few linkages between the various groups of fossil amphibians. (For key to silhouettes, see p. 312.)
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