The amphibians were the first vertebrates who conquered earthly life. The oldest known amphibian by fossil record is the Acanthostega , which lived in the Devonian period.
The area of biology responsible for studying the fossils of such animals is paleontology. The fossils of the first amphibians indicate that they were more aquatic than terrestrial animals, but many of them already had characteristics indicative of lung breathing .
Amphibians are individuals capable of exploring the aquatic and terrestrial environment.
What are amphibians?
Amphibians are animals that received this name because they are able to live a phase of life in water and another on land. Therefore, this is a characteristic of the group.
Frogs are amphibians classified in the order Anura (Photo: depositphotos)
The word amphibian means ” double life” ( amphi = two; bio = life). Amphibians, in fact, were the first in the Cordados phylum to make the transition from marine to terrestrial life. However, this transition is not complete, as they still needed the aquatic environment for their reproduction.
At the time of the conquest by a new environment, the rest of the terrestrial fauna was composed only of invertebrates. Large predators were restricted to the aquatic environment and were mainly represented by fish.
This set of factors enabled a great expansion of amphibians on the mainland, with gigantic representatives, measuring up to 4 meters in length.
Amphibians can be classified into three types of orders: Gymnophiona (apodes), Urodela and Anura .
Amphibians in the gymnophiona group have an elongated, vermiform body and are apodes, that is, without legs. They live buried or in aquatic environments in tropical regions. They are called boiacicas, caecilias or blind snakes , because they have vestigious eyes, sometimes covered by a membrane.
Those in the urodela group ( uros = tail) are represented by salamanders, amphibians with elongated bodies, with four limbs used in locomotion and tail.
And finally, the anura group are those amphibians that do not have a tail, such as frogs, frogs and tree frogs. Such animals have a body adapted to jump, with more elongated hind limbs with the function of propelling the animal.
Amphibians and breathing
In the early stages of the life of amphibians they have aquatic life (larval phase) called tadpoles, which breathe through external gills. After undergoing a metamorphosis, the amphibian appears in the form of an adult, terrestrial, with legs, which breathes through the lungs and through the skin.
During this process, the gills disappear. The lungs are simple, with little contact surface for gas exchange, and are not as efficient, which is why the existence of cutaneous breathing.
This type of breathing takes place through the skin and it is necessary that it is moist for gas diffusion. Amphibian skin, in turn, has good vascularity.
Amphibian habitat is not exclusively terrestrial. Despite the structures that allowed this group to adapt to the terrestrial environment, amphibians remain restricted to humid or aquatic environments .
The main factor responsible for this restriction is the skin, which is thin and has no structures to prevent water loss. The skin is rich in mucous glands, which keep it moist and permeable.
Nervous system of amphibians
The nervous system of amphibians is formed by the brain organized in three parts: telencephalon, midbrain and rombencephalon. Each part of the animal’s brain is responsible for an area of its sensory system.
The telencephalon is linked to smell, the midbrain to vision and the rombencephalon to hearing and balance. Animals have a backbone that transmits information from the brain to other parts of the body via nerves .
Amphibians were the first vertebrate animals to have movable eyelids and tear glands that lubricate, clean and protect the eyes. These are important adaptations for life in a dry environment with particles in suspension, as is the case of the terrestrial environment.
The eyes of amphibians have adaptations for day and night vision , and can distinguish colors.
How do amphibians reproduce?
Amphibians in general need water to reproduce , but the place where reproduction takes place goes from a river, even a simple puddle. The mating ritual begins when the male begins to croak, thus attracting the female.
She, in turn, is full of eggs, lets herself be held by the male in a hug until she launches her gametes into the water together with the male, who will release his sperm. So, fertilization occurs in water, that is, in an external environment – except salamander and blind snake that have their fertilization internally.
Fertilized eggs give rise to larvae, which are tadpoles , they will live in the water until they perform the metamorphosis. This process of metamorphosis is slow and it undergoes several transformations until the transition to the adult stage, where they will be in a totally different shape than they used to have.
What do amphibians feed on?
In most species of this class, tadpoles are vegetarian and feed on small pieces of vegetables that are suspended in water.
However, as they move into adulthood, they become carnivores and feed mainly on insects and some invertebrate animals . There are also cases where adult amphibians eat eggs and tadpoles of other species.
The amphibian tongue has a lot of elasticity and is very sticky at its tip. All this to favor the capture of the prey, because, when the tongue is thrown at the insect that has prey, it sticks to it and it is immediately pulled into the mouth.
All of this happens with tremendous speed and before swallowing the insect, the amphibian presses it against its good sky.
Some of the amphibians even have a type of tooth in the roof of the mouth that is responsible for mobilizing the captured insect. Amphibians have a developed stomach, their intestines end in the cloaca and there are glands.
They also have substances that help digest the insect shells. Its circulation is incomplete, as there is a mixture of arterial and venous blood. Its heart has three cavities, two of which are atria and one ventricle.
What amphibious animals?
Current amphibians are commonly small animals . Today’s largest amphibians are restricted to giant salamanders that occur in rivers in Japan and China, about 1.5 meters long and those that live in North America, in fresh water, and reach 60 cm in length.
The most well-known amphibians are: frogs, tree frogs , frogs , blind snakes, caecilians, newts and salamanders.
Amphibians and reptiles: What are the differences?
Amphibians dominated terrestrial fauna for about 100 million years, until reptiles appeared , with more efficient adaptations to life on earth and that started to compete with them.
As a result, there was a drastic reduction in the number of amphibians and a large proliferation of reptiles.
The main difference between amphibians and reptiles is the fact that reptiles have achieved their independence from water for breeding .
With the appearance of the amniotic egg, the embryo is isolated from the external environment through a porous shell, allowing gas exchange.
A young man hatches from the egg and does not need to go through the larval stage, that is, development is direct.
Amphibians have defense structures that are the poison glands in the skin . In certain frogs and salamanders, the poison glands are concentrated mainly in a pair of structures called parotoid glands.
The unpleasant taste and toxic effect of the poison make certain predators avoid these animals.
Many amphibians have strong colored skin, which acts as a warning to predators, it is called warning or apomatic coloring.