Facts about Early Amphibians
By Chandramita Bora [You must be registered and logged in to see this image.]
Amphibians are cold-blooded vertebrates that are adapted to live on both land and water. The ancestors of these animals were the early amphibians, who are believed to have appeared on Earth during the Devonian period. Know more facts about early amphibians and their importance in the evolutionary process...
Amphibian is derived from the Greek words amphi meaning both and bios meaning life. Thus, amphibians refer to those animals who can live on both land and water. Usually they begin 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, namely toad or frog, salamanders and caecilians.
Common amphibians like toads, frogs and salamanders were not the earliest amphibians. The earliest amphibians apparently appeared on Earth in the Devonian period, almost 416 to 359 million years ago, while the true frogs, toads and salamanders are estimated to have appeared around 200 to 145 million years ago, during the Jurassic period. The early amphibians evidently evolved from some fresh water fishes known as crossopterygians. These fishes were characterized by the presence of primitive lungs along with gills, which enabled them to live on land, as they could breathe fresh air.
Besides lungs, paired fins supported by lobes (type of muscular structure) developed in the early amphibians, which could support their weight on land. This equipped them to move on land; however they could not walk like other land animals. Instead, they seem to have crawled on land. Another supposition in this regard, 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 bodies. One of the earliest known amphibians was Ichthyostega, which apparently had short stubby legs and a fish-like tail. Alhough initially these amphibians were believed to possess five toes on their feet, recent studies have shown that they actually had seven toes. On the other hand, another early amphibian Acanthostega, was believed to have eight fingers on its forelimbs. The largest known early amphibian was Mastodonsaurus, which, it was estimated, grew to a length of about 6 meters. It is believed to have inhabited the Earth during the Triassic period, almost 251 to 200 million years ago and looked like the present day crocodile.
Reptiles are considered to have evolved from amphibians almost 359 to 299 million years ago, during the Carboniferous period. Another interesting fact about the early amphibians is that though they lived mainly in water, their eating behavior was quite different from most fishes. According to some studies, even before coming out of the water to invade the land, they used to bite or chew their prey instead of sucking them into their mouths like fishes. Acanthostega, an early amphibian used to eat just like the land animals by biting down on the 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 evolution of legs and feet took place when the animals started to live on land. But this particular finding points to the fact that limbs developed in early amphibians, even before they moved out of the water to inhabit the land.
These various findings about early amphibians reveal some significant clues regarding the evolutionary process. The development of early lungs, limbs and strong skeletal structure in the early amphibians, highlight their evolutionary role in explaining how the vertebrates left the water to colonize the earth.