How did smooth-bodied amphibians evolve?

Produced by: Science Popularization China

Author: Life Pulse Team

Producer: China Science Expo

Frogs, toads and giant salamanders are familiar amphibians that are still active in ponds, rice fields, swamps, streams, rivers and wet places on land. All of these living amphibians belong to a large family, namely "smooth-bodied amphibians", formerly known as "armorless amphibians". As the name suggests, these are animals with smooth bodies and no armor.

There are about 4,000 species of these living amphibians, which live in warm, wet environments on all landmasses except Antarctica. Scientists divide them into three major groups: tailless amphibians, tailed amphibians, and legless amphibians.

(Photo source: veer photo gallery)

Legless amphibians

Legless amphibians are a group of highly specialized amphibians that resemble earthworms, have no limbs, and have short tails or no tails at all.

Most anapsids live in tropical areas and live in underground caves. Ichthyostega is one of the representatives of this type of animals. Their skin is naked, with many annular wrinkles and rich mucus glands; their eyes are degenerate, but their sense of smell is well developed.

(Photo source: Guangxi News Network-Contemporary Life News)

In addition to the above specialized characteristics, legless amphibians also exhibit some primitive characteristics. For example, most legless amphibians have degenerate bony scales, but these scales are not covered on the body surface like fish, but are sunken into the annular wrinkles of the skin. These degenerate small scales are regarded by some scholars as the remains of the ancient labyrinthine body armor, reflecting the primitive characteristics inherited by this type of animal.

Caudate amphibians

Salamanders are "amphibians with tails." There are more than 350 species living today, mainly distributed in the Northern Hemisphere. Only one type of tailed amphibians called "pulmonary salamanders" has entered South America in the Southern Hemisphere.

(Photo source: veer photo gallery)

Many people confuse salamanders with lizards, but they are easy to distinguish. Salamanders are amphibians, while lizards (such as geckos) are reptiles; lizards have scales on their bodies, while salamanders have bare skin; and salamanders generally have four "fingers" on their front feet, while lizards have five.

The history of tailed amphibians can be traced back to the Middle Jurassic period (about 170 million years ago). The earliest known representatives were found in Central Asia and Western Europe, but these fossils are very scattered and fragmentary.

Tailless amphibians

Tailless amphibians are usually collectively referred to as "frogs". They include frogs in a narrow sense (i.e., what we usually call frogs, etc.) and toads. The main difference between the two is that frogs have a smooth body surface, a light body, like a humid environment, are good at jumping, and have a solid chest-shaped shoulder girdle; while toads have a rough and uneven body surface, a bulky body, poor jumping ability, but strong drought resistance, and an arc-shaped chest-shaped shoulder girdle. However, the difference between the two is not very strict in biological taxonomy. Toads can also have strong jumping abilities, and frogs have also been found to have arc-shaped chest-shaped shoulder girdle (such as the wrinkled frog).

The more primitive species of tailless amphibians are all toads, such as the tailed toads of North America, the slippery toads of New Zealand, and the disc-tongue toads of Europe and North Africa. At the same time, fossil evidence shows that the arc-chest shoulder girdle appeared earlier than the solid-chest shoulder girdle. From this perspective, toads are the predecessors of frogs. In other words, the elegant frogs evolved from some weird-looking toads.

(Photo source: veer photo gallery)

In animal taxonomy, amphibians without legs, amphibians without tails and amphibians with tails constitute three orders in the subclass Lymantria. In addition, the subclass Lymantria has a fourth order, the Proto-anura, represented by the trident frog found on the island of Madagascar in Africa.

While studying these animals, scientists are also troubled by another question that must be answered: from which ancient amphibians did the smooth-bodied amphibians evolve? The currently dominant view is that the common ancestor type of all smooth-bodied amphibians evolved from a certain type of lamellipodan among the labyrinthine amphibians, and all modern amphibians have a common ancestor. This view is called the "single-source origin theory." In contrast, the "multi-source origin theory" believes that tailless amphibians evolved from lamellipodan, while tailed amphibians and legless amphibians may have evolved from shell vertebrae.

However, it is still difficult to determine which of the two hypotheses is correct, and this is also an unsolved mystery in the evolution of modern amphibians.