Amphibians (Amphibia) are a group of animals that include modern-day frogs and toads, caecilians, and newts and salamanders. The ancestors of modern amphibians were the first animals to venture out of the water and adapt to life on land.
Amphibian larvae are often aquatic and go through a complex metamorphosis process as they grow to adulthood. They have moist skin and do not have scales, feathers or hair. The life cycles of amphibians reflects their evolutionary history of bridging land and water. Most amphibians lay their eggs in freshwater. A few species tolerate brackish water and some species lay their eggs on land. Extraordinarily, some species even carry their eggs inside their body. Although life cycles of amphibians vary from species to species, they all share the following three basic stages of development: egg, larva, adult.
Amphibian eggs do not have a hardened shells which means amphibians are not amniotes like reptiles, birds, and mammals. Instead, amphibian eggs consist of a gelatinous envelope that must remain moist to survive. Eggs hatch to release tiny larvae which later undergo a metamorphosis into the adult form.
Many amphibians can absorb oxygen directly into their bloodstream through their skin and are also able to expel carbon dioxide waste back into the air. The skin of amphibians lacks scales and hair. It is smooth and sometimes moist, making it quite permeable to gases and water. This permeability is thought to make amphibians particularly vulnerable to toxins in air and water such as herbicides, pesticides, and pollutants. Amphibians have been sharp decline throughout many areas worldwide. This is thought to be an early warning sign of a troubled environment.
The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period. Early amphibians included creatures such as Diplocaulus, Ophiderpeton, Adelospondylus, Diplocaulus, and Pelodosotis. The world of those early amphibians was quite different than it is today. No birds, mammals or reptiles lived on land (or in the water for that matter) at that time. Only invertebrates and an assortment of prehistoric plants had yet colonized land. It was a silent place, void of birdsong and lacking the growl of predators. The land lay wide open to amphibians and those with the evolutionary tenacity to set forth from the shallow shores began a new and important stage in the history of life on our planet. Several types of fishes had developed lungs. Among those lung-bearing fishes were the lobe-finned fish and the lungfishes.
The Crossopterygians, a group of primitive lobe-finned fish are believed to be the ancestors of amphibians. They evolved several key features that enabled them to colonize land—a more rigid skeleton that would support the animal's body weight on land, nostrils, and leg bones.
Most lineages of amphibians never fully severed their ties with aquatic habitats. A majority of amphibian species return to the water to breed and some species remain entirely aquatic throughout their entire life cycle. Most amphibians go through a complex metamorphosis process as they grow to adulthood.
Amphibians are divided into the following basic groups:
- Frogs and toads (Anura) - There are about about 4,380 species of frogs and toads alive today, making them the most diverse of all amphibian groups. Members of this group have no tail, a large head, large eyes, and long, powerful hind legs.
- Caecilians (Gymnophiona) - There are about about 170 species of caecilians alive today. Members of this group are sleek, limbless amphibians.
- Newts and salamanders (Caudata) - There are about 470 species of newts and salamanders alive today. Members of this group are slender-bodied amphibians with a long tail and usually two pairs of limbs.