Notions about amphibians: Characteristics and origin.

Vertebrate extinction rates have increased over the last century, with many species on the verge of disappearing. Among vertebrates, amphibians are the most threatened group. According to the Red List of the IUCN (International Union for Conservation of Nature), more than 40% of the species in this group could become extinct, losing an exorbitant amount of natural heritage and with it, genetic information that has taken millions of years to build.

What are amphibians and what makes them unique?

The word amphibian (Amphibia) comes from Greek, “μφ”, Amphí (both) and then, Bíos (life), both lives, in other words, of those beings that fulfill part of their life cycle in water and in the terrestrial environment. Today's amphibians are very diverse, being able to live their lives in a totally aquatic environment, terrestrial or both.

Amphibians form an ancient group that was the first able to colonize the terrestrial environment, which until then was dominated by invertebrates. Some characteristics of this group are:

• Ectotherms and permeable skin: Like fish and reptiles, body temperature depends on external environmental sources. Their skin is something that has always attracted attention since we were able to observe and study them. This moist, bare skin is your only protection against aggression. They do not have feathers, hair and in most cases, they do not have hard parts. And thanks to that skin, they control their hydration, since they don't drink water, they absorb it through the skin, which is why they are in humid environments and tend to have nocturnal habits.
• Their diet is very inflexible after metamorphosis. They are absolutely zoophagous. In fact, of all the existing groups on planet Earth, it is the one with the largest zoophagous diet. Without exception, all amphibian species feed on other animals, capturing their prey and engulfing it whole in most cases. There are some species in which fungi and plant matter have been found, but in a very partial way.
• Amphibians grow throughout their lives. From metamorphosis until they reach adulthood, their growth is very fast. Once sexual maturity reaches, growth slows down, therefore, there is a relationship between size and age. When we cross paths with a copy of Bufo spinosus large in size, it is most likely that we are dealing with a very long-lived female specimen (they are larger in size). Some anurans, such as Epidalea calamita they have to increase their weight 300 times since their metamorphosis in just two years, which is how long it takes them to reach their sexual maturity.

From the aquatic environment to terrestrial conquest

We have already commented a little on some curious characteristics of this group, now, we will talk about its origin.

To think that all living beings on the planet have a connection is complicated, unthinkable. But we have common ancestors with butterflies or pinsapos and, of course, also with toads and newts. There is a thin but very long thread, of which, if we pull, little by little we will unravel knots that will lead us to the origin of what we call life. In this thread, we will find species that, due to new attempts to search for new forms of life, disappeared, giving rise to new ones.

The life cycle of amphibians began about 400 million years ago, in the Devonian period. A period of great variations. Immense emerging lands called Laurasia and Gondwana, which would later merge to form what we call Pangea. The first plant species would appear, such as ferns, horsetails or copods, which spread out to form large forests, in which the life of small arthropods that would be non-winged insects emerges. Our beloved amphibians didn't exist yet. Their ancestors were still aquatic, but they were getting ready. “Soon” there would be physiological changes that would be necessary to get out into the Earth's environment. Changes such as minimizing water loss, supporting your own weight or developing new breathing systems, among others. Once all this was over, the amphibians came out and set foot on land.

According to paleontologists' assessment, amphibians evolved from ancient fish such as Sarcopterygians, or lobed finned fishes, which appeared in the early Devonian period 408 million years ago.

True amphibians appeared 365 million years ago, in the Upper Devonian; these amphibians were tetrapods that adapted to the process of land invasion known as Ichthyostegyds (Ichthyostega). Ichthyostega has been considered a link between fish and amphibian. Instead of fleshy fins, it had legs and feet with toes, which it probably used to move, both on land and submerged in water. They had large, broad heads and an aquatic or semiaquatic lifestyle. Similar to current amphibians, they had a lateral line (a sensory organ that allows fish to detect vibrations and movement in the water) and could breathe through the skin. In addition, they laid their eggs in the water, from which tadpoles were born, which later underwent a metamorphosis to become adults like today's amphibians.

Other intermediate amphibians were the Advanced Labyrinthodons, which received their name because their tooth enamel was shaped like labyrinths. These animals had an elongated body, with a large skull compared to their body size, and were dorsally flattened. Amphibians were considered to have developed the primitive form of reproduction through eggs that they deposited in water, such as groups of fish.

But at the end of the Permian, about 252 million years ago, what is known as The Great Mortality (it is said that several reasons came together to make it possible, the largest known natural disaster), 95% of marine species and more than 70% of terrestrial vertebrates disappeared. The surviving amphibians didn't have it easy, as they were overtaken by reptiles, beginning the dominance of dinosaurs. Until, at present, there is only one lineage left: the Lisamphibians. Our current amphibians, currently existing, about 8,000 species within this group.

Bibliography:

Ecology, study and conservation of amphibians. Ricardo Reques. Tundra editions.

DAVILA, Carlos Alberto Cañas. The origin and evolution of snakes. “Venomous Snakes”, painting by Master Juan Carlos Suárez., p. 25.

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