Amphibians are a class of animal that represents a crucial evolutionary step between water-dwelling fish and land-dwelling mammals and reptiles. They are among the most fascinating (and rapidly dwindling) animals on earth.
Unlike most animals, amphibians such as toads, frogs, newts, and salamanders finish up much of their final development as an organism after they are born, changing from marine-based to land-based lifestyles in the first few days of life. What else makes this group of creatures so fascinating?
There Are Three Major Types of Amphibians
Naturalists divide amphibians into three main families: frogs and toads; salamanders and newts; and the strange, worm-like, limbless vertebrates called caecilians. There are currently about 6,000 species of frogs and toads around the world, but only one-tenth as many newts and salamanders and even fewer caecilians.
All of the living amphibians are technically classified as lissamphibians (smooth-skinned); but there are also two long-extinct amphibian families, lepospondyls, and temnospondyls, some of which attained astonishing sizes during the later Paleozoic Era.
Most Undergo Metamorphosis
True to their evolutionary position halfway between fish and fully terrestrial vertebrates, most amphibians hatch from eggs laid in water and briefly pursue a fully marine lifestyle, complete with external gills. These larvae then undergo a metamorphosis in which they lose their tails, shed their gills, grow sturdy legs, and develop primitive lungs, at which point they can scramble up onto dry land.
The most familiar larval stage is the tadpoles of frogs, but this metamorphic process also occurs (a bit less strikingly) in newts, salamanders, and caecilians.
Amphibians Must Live Near Water
The word “amphibian” is Greek for “both kinds of life,” and that pretty much sums up what makes these vertebrates special: they have to lay their eggs in the water and require a steady supply of moisture in order to survive.
To put it a bit more plainly, amphibians are perched midway on the evolutionary tree between fish, which lead a fully marine lifestyle, and reptiles and mammals, which are fully terrestrial and either lay their eggs on dry land or give birth to live young. Amphibians may be found in a variety of habitats near or in water or damp areas, such as streams, bogs, swamps, forests, meadows, and rainforests.
They Have Permeable Skin
Part of the reason amphibians have to stay in or near bodies of water is that they have thin, water-permeable skin; if these animals ventured too far inland, they would literally dry up and die.
To help keep their skin moist, amphibians are constantly secreting mucous (hence the reputation of frogs and salamanders as “slimy” creatures), and their dermis is also studded with glands that produce noxious chemicals, meant to deter predators. In most species, these toxins are barely noticeable, but some frogs are sufficiently poisonous to kill a full-grown human being.
They Are Descended From Lobe-Finned Fish
At some time during the Devonian period, about 400 million years ago, a brave lobe-finned fish ventured onto dry land—not a one-time event, as is often depicted in cartoons, but numerous individuals on numerous occasions, only one of which went on to produce descendants that are still alive today.
With their four limbs and five-toed feet, these ancestral tetrapods set the template for later vertebrate evolution, and various populations went on over the ensuing few million years to spawn the first primitive amphibians like Eucritta and Crassigyrinus.
Millions of Years Ago, Amphibians Ruled the Earth
For about 100 million years, from the early part of the Carboniferous period about 350 million years ago to the end of the Permian period about 250 million years ago, amphibians were the dominant terrestrial animals on earth. Then they lost pride of place to various families of reptiles that evolved from isolated amphibian populations, including archosaurs (which eventually evolved into dinosaurs) and therapsids (which eventually evolved into mammals).
A classic temnospondyl amphibian was the big-headed Eryops, which measured about six feet (about two meters) from head to tail and weighed in the neighborhood of 200 pounds (90 kilograms).
They Swallow Their Prey Whole
Unlike reptiles and mammals, amphibians don’t have the ability to chew their food; they’re also poorly equipped dentally, with only a few primitive “vomerine teeth” in the front upper part of the jaws that allow them to hold onto wriggling prey.
Somewhat making up for this deficit, though, most amphibians also possess long, sticky tongues, which they flick out at lightning speeds to snag their meals; some species also indulge in “inertial feeding,” clumsily jerking their heads forward in order to slowly stuff prey toward the back of their mouths.
They Have Extremely Primitive Lungs
Much of the progress in vertebrate evolution goes hand-in-hand (or alveolus-in-alveolus) with the efficiency of a given species’ lungs. By this reckoning, amphibians are positioned near the bottom of the oxygen-breathing ladder: Their lungs have a relatively low internal volume, and can’t process nearly as much air as the lungs of reptiles and mammals.
Fortunately, amphibians can also absorb limited amounts of oxygen through their moist, permeable skin, thus enabling them, just barely, to fulfill their metabolic needs.
Like Reptiles, Amphibians Are Cold-Blooded
Warm-blooded metabolisms are usually associated with more “advanced” vertebrates, so it’s no surprise that amphibians are strictly ectothermic—they heat up, and cool down according to the ambient temperature of the surrounding environment.
This is good news in that warm-blooded animals have to eat much more food to maintain their internal body temperature, but it’s bad news in that amphibians are extremely limited in the ecosystems in which they can thrive in—a few degrees too hot, or a few degrees too cold, and they will immediately perish.
Amphibians Are Among the World’s Most Endangered Animals
With their small size, permeable skins and dependence on easily accessible bodies of water, amphibians are more vulnerable than most other animals to endangerment and extinction; it’s believed that half of all the world’s amphibian species are directly threatened by pollution, habitat destruction, invasive species, and even the erosion of the ozone layer.
Perhaps the greatest threat to frogs, salamanders, and caecilians is the chytrid fungus, which some experts maintain is linked to global warming and has been decimating amphibian species worldwide.