AMPHIBIANS IN GENERAL
The best-known amphibians are frogs, toads, and salamanders. Compared to the classes of other higher animals -- animals with backbones -- amphibians are very primitive creatures.
According to the fossil record, here is approximately when the major groups of land animals appeared:
Birds...... about 170 million years ago
Mammals.... about 220 million years ago
Reptiles... about 320 million years ago
Amphibians: about 400 million years ago
Fish....... about 500 million years ago
Life originated in the seas. The first animals were simple ones without backbones -- "invertebrate" animals such as segmented worms, sponges, and corals. The first fish were so simple that they didn't even have jawbones for opening and closing their mouths. The first land animals with backbones were the amphibians. The oldest amphibian fossils look like fish with thick, muscular fins. That's because amphibians arose from early fish. The first amphibians lived more in water than on dry land.
Keeping the above points in mind, we can say that, from an evolutionary viewpoint, amphibians as a class developed before many of nature's most useful animal "inventions" had appeared. For instance, look at what amphibians don't have (and this includes today's amphibians):
FOUND IN NORTH AMERICA
To see a more complete, and confusing, breakdown, visit the NCBI taxonomy database page for amphibians
ON THE WEB
EVOLUTION: Major events during the gradual evolution from fish to amphibians are described in detail, fascinating illustrations based on critical fossils that have been discovered are presented, and links to more information are provided at the MyHerp.com
IDENTIFICATION: If, for instance, you identify a Spring Peeper and want to confirm your identification and know more about the species, Amphibiaweb provides a searchable database where you can type in the Spring Peeper's name (Pseudacris crucifer in Latin), and be presented with a picture of the species, as well as links to much more information. You also might benefit from browsing through the California Reptiles & Amphibians website.
CLASSIFICATION: At Amphibian Species of the World there's a searchable database where you can generate lists of amphibian species present in different countries.
GETTING INVOLVED: At the North American Amphibian Monitoring Program site you can learn about an amphibian- monitoring program, with projects including frog-call surveys and terrestrial-salamander surveys. You can participate in monitoring frog populations in your area if you visit the site called Frog Web and sign up to be an official frog-watcher!
In most backyards, toads are the most likely of all amphibians to appear. That's because toads have a special adaptation that helps them range farther from water than other amphibians: Their thick, warty skin keeps water inside the toads better than other amphibian skins.
Because water and air so easily flows into and out of amphibian skin, amphibians are much more vulnerable to pollution in water and air than other higher animals. In fact, today amphibian numbers are collapsing all over the world. Acid rain is thought to be responsible for amphibians disappearing from streams, lakes and ponds where they were common only a few years ago. Nowadays we often hear of two-headed or five-legged frogs. Apparently this high frequency of deformity results from pollutants interfering with natural development at the genetic level. Of course, as pollution levels increase, eventually other kinds of animals, even humans, are sure to suffer similar fates...
You can review some amphibian-oriented books available at Amazon.com in both the US and the UK by clicking here.