The term, 'amphibian' is derived from two Greek words, amphi meaning both, and bios meaning life. Thus, amphibians are those animals that can live both on the land and in water. Usually they begin their life in water as aquatic larvae, and then undergo metamorphosis to reach adulthood. So far, almost 5,000 species of amphibians have been discovered, which can be classified into three main groups - toads or frogs, salamanders, and caecilians.
Evolutionary History of Amphibians
The common amphibians like toads, frogs, and salamanders were not the earliest amphibians. The ancestors of these amphibians apparently appeared on the Earth in the Devonian period, i.e., almost 416 to 359 million years ago. On the other hand, the true frogs, toads, and salamanders were estimated to have appeared around 200 to 145 million years ago during the Jurassic period.
The early amphibians were believed to evolve from some freshwater fish, known as crossopterygians. These fish were characterized by the presence of primitive lungs along with gills, which enabled them to breathe fresh air and thus survive on the land.
Besides lungs, paired fins supported by lobes (a type of muscular structure) were developed in the ancestors of present-day amphibians, in order to support their weight on land. Though this equipped them to move to the land, they could not walk like other land animals. Rather, they seemed to crawl on the land.
Another supposition is that during the Devonian period, which was characterized by alternating dry and wet periods, some early amphibians, whose habitat dried up, crossed the land in search of alternative water bodies. According to many experts, this led to the evolution of the first four-footed animals.
The early amphibians were believed to be much larger than the present-day ones, with thick skulls and heavy bones. Protective armors or scales were also present on their body. One of the earliest known amphibians was Ichthyostega, which apparently had short, stubby legs and a fish-like tail. Initially, these amphibians were believed to possess five toes on each foot, but recent studies have shown that they actually had seven toes.
Another early amphibian Acanthostega, was believed to have eight fingers on its forelimbs. The largest known early amphibian was Mastodonsaurus, which was estimated to grow to a length of about 6 meters. This huge animal was believed to inhabit the Earth during the Triassic period (almost 251 to 200 million years ago), and looked like the present-day crocodile.
Reptiles were considered to evolve from amphibians almost 359 to 299 million years ago during the Carboniferous period. Though they lived mainly in water, their eating behavior was quite different from most fish. According to some studies, even before coming out of water to invade the land, they used to bite or chew their prey instead of sucking them into the mouth.
Acanthostega, an early amphibian, used to eat just like the land animals by biting down its prey. So, it can be said that they evolved for a life on land much before emerging out of the water. Though Acanthostega was a tetrapod (four-footed vertebrate), its limbs were not strong enough to support its weight on land, and hence it was mainly an aquatic animal. This points to another important fact about the development of legs and feet. Earlier, it was thought that the evolution of legs and feet took place when animals started to live on the land. But this particular finding points to the fact that limbs developed in the ancestors of present-day amphibians, much before they moved out of water to inhabit the land.
These findings reveal some significant clues regarding the evolutionary process of amphibians. The development of early lungs, limbs, and strong skeletal structures in the early amphibians highlights their evolutionary role in explaining how the vertebrates left the water bodies to colonize the earth.