Amphibian (animal)

VII

Evolution

Amphibians are a living reminder of the progression by which animals with backbones first emerged from the water to conquer life on land. Amphibians are believed to have evolved from air-breathing freshwater fish during the mid-Devonian Period, a stage in the Earth’s geologic history that occurred between 416 million to 359 million years ago. Fossils of an ancient fish found on Ellesmere Island in the Canadian territory of Nunavut provide important clues about how this transition happened. Scientists named the creature Tiktaalik in 2006. It had some anatomical features similar to those of land animals, such as wrist and elbow bones, and parts of a primitive hand embedded in the pectoral fins.

The freshwater fish that gave rise to amphibians are thought to have had primitive lungs as well as gills, similar to modern lungfish. Their fins were supported by muscular structures called lobes, which enabled them to move on land. Fossil evidence suggests that one of the earliest amphibians was a creature called Ichthyostega, which lived about 360 million years ago. Ichthyostega had a fishlike tail, but it also possessed strong limbs. It apparently did not walk the way later land animals did but moved more like a seal by hunching its body.

After their successful invasion of land, amphibians diversified into many forms. Unlike modern forms, many of these ancient amphibians had thick skulls and heavy bones. Some even had protective armor and scales on their bodies. The first reptiles evolved from amphibians some time during the Carboniferous Period, which lasted from 359 million to 299 million years before present.

The largest known of the early amphibians was Mastodonsaurus, which was similar in appearance to today’s crocodile and probably measured up to 6 m (20 ft) in length. It lived during the Triassic Period, which lasted from 251 to 200 million years ago. The types of amphibians that exist today—frogs and toads, salamanders, and caecilians—were not among the earliest amphibians. The earliest known ancestors of these groups date to the Triassic Period. The earliest true frogs, salamanders, and caecilians appeared during the Jurassic Period, which lasted from 200 to 145 million years ago.

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VIII

Endangered Amphibians

Amphibians have survived more than 300 million years through drastic environmental changes that led to the demise of dinosaurs and many other species. Yet scientists are alarmed by the recent rapid decline of amphibians in many parts of the world. One study of more than 900 amphibian populations monitored from 1950 to 1997 found that amphibian populations have been declining at an average rate of 4 percent per year for the past 40 years. These declines are evident in places as widely separated as North America, South America, and Australia, and have prompted international concern.

Their two-stage life cycle and permeable skin make amphibians particularly sensitive to environmental disruptions such as drought and pollutants. This sensitivity makes them excellent bioindicators—life forms whose well-being provides clues to the health of an ecosystem. Declines in amphibian populations may be due in part to natural fluctuations, but they more likely suggest that humans are changing the environment more rapidly than amphibians can adapt. One such change is the destruction and modification of amphibian habitats, such as the cutting of forests and the draining of wetlands. In North America, for instance, huge numbers of salamanders are being lost as a result of forest clear-cuts.

Mysteriously, many amphibian groups are dwindling—or even disappearing—in areas where their habitat is not being destroyed. In Australia, two species of gastric brooding frogs have not been detected since the early 1980s, yet their habitat remains relatively pristine. The golden toad of Costa Rica has not been seen since 1989, even though its high-elevation rain forest habitat has been protected as a national reserve since the 1970s. Similarly, the red-legged frog has vanished from large areas of the North American Pacific Coast, where it was once abundant. Scientists now think that the rapid spread of the chytrid fungus (Batrachochytrium dendrobatidis), which affects the skin of amphibians, is responsible for many of the rapid declines among frogs and other types of amphibians recorded in the Americas, Europe, Africa, Australia, New Zealand, and the Pacific Islands. The fungus causes the disease chytridiomycosis, which damages the skin of amphibians. The fungus can be highly contagious and lethal in many species.

Another disturbing development is that large numbers of amphibians, especially frogs, are being found with misshapen, extra, or missing limbs. A study published in 2007 indicated excess nutrients from farm runoff was a major source of the problem. The runoff stimulates growth of algae, which allows populations of snails to increase. The snails then release parasitic nematodes into bodies of water where frogs live. The nematodes infect frogs and cause cysts to form in their limbs, resulting in malformations as the limbs develop.

A variety of other factors may also be responsible for amphibian declines. These may include disease from viral, bacterial, or fungal pathogens; global warming; and increased levels of the ultraviolet-B component of sunlight hitting the Earth as a result of depletion of the protective ozone layer. Global warming and climate change can affect the moisture and temperature conditions where amphibians live. Ultraviolet-B light is particularly suspect in the decline of those amphibians that lay their eggs in shallow water, because such eggs are exposed to sunlight for long periods. At a more local level, chemical pollutants, such as acid rain, pesticides, herbicides, and fertilizers, may be harming amphibians. In some regions, the introduction of nonnative competitors and predators has contributed to amphibian population declines. Habitat loss, particularly loss of wetlands, also has a major impact on amphibians. It is likely that an interaction of some or all of these factors may be exacerbating conditions for amphibians.

The Global Amphibian Assessment (GAA) is a collaboration among more than 500 scientists from 60 countries to monitor the status of amphibians worldwide. After a GAA study published in 2004, the World Conservation Union (also known as IUCN) listed 32 percent of amphibian species worldwide as threatened with extinction. More than 20 amphibian species found in the United States are now listed as threatened or endangered under the Endangered Species Act. The growing global extinction crisis for amphibians led the conservation organization Amphibian Ark to declare 2008 the “Year of the Frog” to raise public awareness and promote conservation efforts to save amphibians.

Scientific classification: Amphibians are the only members of the class Amphibia, which contains three living orders. Salamanders, a group that includes newts and mud puppies, make up the order Caudata; frogs and toads make up the order Anura, and caecilians make up the order Gymnophiona.

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