I am always amused by Creationists’ denials that transitional forms exist among either fossil organisms or living species. After all, the definition of “transitional” depends on the existence and forms of two other organisms that the transitional form is being compared to. Thus Archeopteryx is called transitional because it has characteristics of both birds and theropod dinosaurs, which are reptiles. But if neither birds nor reptiles existed, then Archaeopteryx would not be transitional to anything.
It is the same with amphibians. Their very existence as a class is evidence of evolution, for they are clearly transitional between fishes (which are almost entirely confined to water, breath through gills, have fins instead of legs, and lay jelly-like eggs), and reptiles (which often have legs, can thrive even in dry deserts because they breath through lungs, and lay hard or leathery shelled eggs on land). Most amphibians as adults can live on land by walking on legs and breath air through lungs, but they lay eggs like those of fish and have a larval stage that lives in water and breaths through gills. So one could argue that a salamander is a modified fish or that a lizard is a modified salamander, thus regarding the salamander as a transitional form having characteristics of both.
One might also think a Creator of all living things from scratch would not bother to make animals with such lifestyles as amphibians. What would be the point, except to trick us into accepting evolution?
The forms of living amphibians also give us some insights into how evolution works:
- The Urodeles include the news and salamanders which have short and weak limbs, long bodies and tails, and a larval form that resembles the adult.
- Anuarans (frogs and toads) have very short bodies, powerful limbs, and no tails as adults, but have a very different larval form with a strong tail for swimming and no limbs. They are the most common amphibians.
- Gymnophiores, also known as caecilians, have no limbs at all and have extremely long bodies but little or no tail and live almost entirely underground. Indeed, they resemble giant earthworms.
It is easy to see how both frogs and caecilians could have evolved from salamanders by going in completely different directions. One might also suspect that salamanders are a transitional form between frogs and caecilians, but in fact the salamanders are the primitive forms most closely resembling the fish and lizards mentioned earlier, while the other amphibian forms resemble neither fish nor lizards, but are more specialized in their lifestyles.
Most fossil amphibians from the Devonian period to the Permian period have the form of salamanders, thus providing support for the hypothesis that salamanders are indeed the primitive form. In the mass extinction at the end of the Permian period, the giant amphibians that had been so common on the land disappeared. Later, one line of smaller amphibians developed into frogs in the Triassic period. Today, with so much competition from reptiles, birds and mammals, the smaller amphibians, numbering only 4,000 species, are a remnant of what the class once was.