Basic structure of amphibians

At the end of the Devonian period, a certain kind of lobe-finned fish ventured out of the water and climbed onto land, becoming the earliest amphibians. Since then, vertebrates have entered a new world completely different from the environment they had lived in for hundreds of millions of years.

The first major problem encountered by early amphibians was breathing. But this was solved by their lobe-finned fish ancestors. The lungs of lobe-finned fishes are well developed and probably used frequently. The main difference between amphibians and lobe-finned fish ancestors in this regard is that most fish with lungs still use gills to breathe as the main method, and the lungs are usually just an auxiliary respiratory organ; however, amphibians basically use lungs They breathe in the air, but in their juvenile stages they use gills to breathe in water.

Labyrinthus—a labyrinthine amphibian

The second major problem encountered by early amphibians was desiccation. Fish don't have a desiccation problem because they're always immersed in water, but when the earliest amphibians stopped being immersed in water, they faced the problem of keeping their body fluids from evaporating. Therefore, on the one hand, early amphibians had to, like most modern amphibians, not leave the water too far and often return to streams or lakes; on the other hand, as they evolved, they developed the ability to resist the drying effect of the air. The body covering or body covering. There is evidence that some of the earliest amphibians retained the scales of their fish ancestors; but as evolution progressed, post-Permian amphibians developed strong skin that often lined the The underside of small bone fragments or bony plates. When the ability of amphibian skin to prevent evaporation of body fluids gradually increases and becomes a tough coat to protect against external aggression, amphibians will be less dependent on water and can stay on land for longer periods of time. Activity. This was an important aspect of amphibian evolution, and even more important for those more advanced vertebrates that developed from amphibians, such as reptiles.

Gravity has little effect on fish because water has buoyancy. For vertebrates living on land, gravity is a powerful factor that has an important impact on the individual's structure and life. After the first amphibians left the water, they struggled with the increased gravity. As a result, they developed strong vertebrae and powerful appendage bones. The relatively simple "disks" or "rings" that make up the vertebrae of lobe-finned fish vertebrae become interlocking structures that together form a strong horizontal spine that supports the body. The spine is supported at two points by the shoulder girdle in front and the belt at the back, and the shoulder girdle and belt are supported by the front and rear limbs respectively.

Basalt frog - a fossil slippery amphibian

Early terrestrial vertebrates also developed an adaptation to a new way of locomotion, in which the limbs played the most important role. They not only overcome the force of gravity and lift the body off the ground, but also propel the body along the ground. In this regard, we have seen the reversal of motor functions between fish and amphibians: fish use the body and tail to complete the motor function, and the even fins play a balancing role; early terrestrial vertebrates gradually taper from front to back, and at a certain point The degree becomes a balancer, and the paired appendages become the main organs of locomotion. This mode of locomotion pioneered by early amphibians continued in various variations during the evolution of terrestrial vertebrates.

The first terrestrial vertebrates also encountered reproductive problems. Fish usually lay unprotected eggs in the water, where they hatch naturally. Terrestrial vertebrates either returned to water to reproduce, or they had to develop ways to protect their eggs on land. Amphibians have not been able to solve the problem of protecting eggs on land. Throughout history, they have always had to return to the water to lay eggs, and individual specialized types have to go to wet places to lay eggs. Therefore, amphibians can only be called the pioneers of vertebrate landings, and they are still far from the real conquerors of land.

Starting from the ancestor of Ichthyostega, amphibians (classes) have differentiated into three subclasses in the process of adapting to the terrestrial environment: Labyrinthia, Conospondyma, and Splenomegaly.

animal tags： amphibians axolotl