Squirrel Treefrogs, Hyla squirella, image by Karen Wise of Kingston, Mississippi
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 thought of as primitive creatures.

AMPHIBIAN EVOLUTION

According to the fossil record, here is approximately when the major groups of land animals appeared:

Birds...... about 160 million years ago
Mammals.... about 210 million years ago
Reptiles... about 315 million years ago
Amphibians: about 380 million years ago
Fish....... about 480 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 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.

Tiktaalik fossil, technically a fish, but showing features of an evolutionary transition from fish to amphibians; display in the Field Museum, Chicago; image courtesy of Eduard Solà
Tiktaalik fossil, technically a fish, but with flat, froglike head and sturdy fins enabling it to prop itself up in shallow water, indicating an evolutionary transition from fish to amphibians; display in the Field Museum, Chicago; image courtesy of Eduard Solà

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):

WHAT AMPHIBIANS CAN'T DO

Bullfrog, Rana catesbeiana

THE WATER PROBLEM

Red-spotted Toad, BUFO PUNCTATUS, side view

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 -- seen on the Red-spotted Toad, Bufo punctatus, at the right -- keeps water inside the toads better than other amphibian skins.

Because water and air so easily flow 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.

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