TETRAPODS: EVOLUTION, CHARACTERISTICS, CLASSIFICATION - BIOLOGY - 2025

The tetrapods (Tetrapoda in Greek "four legs") comprise the animals with four limbs, although some members have lost them. Its current representatives are amphibians, sauropsids and mammals.

This group evolved around 400 million years ago, in the Devonian period, from lobe-finned fish. The fossil record has a series of extinct representatives that give birth to the transition from water to land.

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This change of environment led to the development of adaptations for locomotion, respiration, reproduction and temperature regulation, mainly.

Origin and evolution

According to the evidence, the first tetrapods appear at the end of the Devonian, about 400 million years ago. Thus, the colonization of terrestrial environments occurred when the great Pangea continent was divided into two: Laurasia and Gondwana.

The first tetrapods are believed to be aquatic forms that could use their fledgling limbs to move on the ground and navigate into shallow waters.

This event marked the beginning of an extensive radiation, which originated totally terrestrial forms and with limbs that provided sufficient support to allow terrestrial locomotion.

Where do tetrapods come from?

Members of the tetrapods originated from an ancient aquatic form. Although the fins of fish do not appear to be closely related to the jointed limbs of tetrapods, a deeper look makes the homologous relationships clear.

For example, the fossil Eusthenopteron features a forearm made up of a humerus, followed by two bones, the radius and the ulna. These elements are clearly homologous to the limbs of modern tetrapods. In the same way, shared elements are recognized on the wrist.

It is speculated that Eusthenopteron could splash at the bottom of the aquatic environment with its fins. However, it could not "walk" as an amphibian does (this inference is made thanks to the anatomy of fossils).

Another fossil, Tiktaalik, appears to fit between a transitional form between lobe-finned fish and tetrapods. This organism probably inhabited shallow water.

The well-formed limbs are evident in the Acanthostega and Ichthyostega fossils. However, members of the first genus do not appear to be strong enough to support the full weight of the animal. In contrast, Ichthyostega appears to be able to move - albeit clumsily - in totally terrestrial environments.

Adaptations for life on land

Mexican gray wolf

The movement of the first tetrapods from an aquatic environment to a terrestrial one involves a series of radical changes in terms of the conditions that these animals had to exploit. The differences between water and land are more than obvious, such as the concentration of oxygen.

The first tetrapods had to solve a series of problems, including: how to move around in a lower density environment? How to breathe? How to reproduce outside the water? And finally, how to deal with fluctuations in the environment that did not are they present in the water, such as temperature variations?

Below we will describe how tetrapods solved these difficulties, analyzing the adaptations that allowed them to effectively colonize terrestrial ecosystems:

Locomotion on earth

Chameleon

Water is a dense environment that provides sufficient support for locomotion. However, the terrestrial environment is less dense and requires specialized structures for movement.

The first problem was solved with the development of members that allowed the movement of animals on the terrestrial environment, and that gave the name to the group. Tetrapods have a bony endoskeleton that forms four limbs built under the plan of pentadactyly (five fingers).

Evidence suggests that tetrapod limbs evolved from the fins of fish, along with modifications to the surrounding muscles, allowing the animal to rise off the ground and walk efficiently.

Gas exchange

If we imagine the passage from water to land, the most intuitive problem is the issue of breathing. In terrestrial environments, the oxygen concentration is about 20 times higher than in water.

Aquatic animals have gills that work very well in water. However, in terrestrial environments, these structures collapse and are unable to mediate gaseous exchange - no matter how abundant oxygen is on earth.

For this reason, living tetrapods have internal organs responsible for mediating respiratory processes. These organs are known as lungs and are adaptations for terrestrial life.

Some amphibians, on the other hand, can mediate gas exchange using their skin as the only respiratory organ, which is very thin and moist. In contrast to the integuments developed by reptiles, birds and mammals, which are protective and allow them to live in dry environments, preventing potential desiccation.

Birds and reptiles exhibit additional adaptations to prevent desiccation. These consist of the production of semi-solid wastes with uric acid as nitrogen waste. This feature reduces water loss.

Reproduction

Ancestrally, reproduction is a phenomenon linked to aquatic environments. In fact, amphibians are still dependent on water in order to reproduce. Their eggs cost with a membrane that is permeable to water and that would dry out quickly if exposed to a dry environment.

Also, amphibian eggs do not develop into a miniature version of the adult form. Development occurs through metamorphosis, where the egg gives rise to a larva that, in most cases, is adapted to aquatic life and exhibits external gills.

In contrast, the remaining groups of tetrapods - reptiles, birds, and mammals - have evolved a series of membranes that protect the egg. This adaptation eliminates the dependence of reproduction on the aquatic environment. In this way, the mentioned groups have totally terrestrial life cycles (with their specific exceptions).

Environmental variations

Aquatic ecosystems are relatively constant in terms of their environmental characteristics, particularly in temperature. This does not happen on earth, where temperatures fluctuate throughout the day, and throughout the year.

The tetrapods solved this problem in two different ways. Birds and mammals convergently developed endothermy. This process allows the environmental temperature to be kept stable, thanks to certain physiological mechanisms.

This characteristic allows birds and mammals to colonize environments with very low temperatures.

Reptiles and amphibians solved the problem in another way. Temperature regulation is not internal and they depend on behavioral or ethological adaptations to maintain an adequate temperature.

General characteristics

Asian elephant

The Tetrapoda taxon is characterized by the presence of four limbs, although some of its members have them reduced or absent (such as snakes, caecilians and whales).

Formally, tetrapods are defined by the presence of the quiridium, a well-defined muscular limb with fingers in the terminal portion.

The definition of this group has been the subject of wide debate among experts. Certain authors doubt that the characteristics "limbs with fingers" is sufficient to define all tetrapods.

Below we will describe the most outstanding characteristics of the living representatives of the group: amphibians, reptiles, birds and mammals.

Taxonomy

• Superkingdom: Eukaryota.
• Animalia Kingdom.
• Subkingdom: Eumetazoa.
• Superphilous: Deuterostomy.
• Edge: Chordata.
• Subphylum: Vertebrata.
• Infraphylum: Gnathostomata.
• Superclass: Tetrapoda.

Classification

Historically, tetrapods have been classified into four classes: Amphibia, Reptilia, Aves, and Mammalia.

Amphibians

Amphibians are animals with four limbs, although they can be lost in some groups. The skin is soft and permeable to water. Their life cycle includes stages of aquatic larvae, and the adult stages live in terrestrial environments.

They can breathe through the lungs, and some exceptions do so through the skin. Examples of amphibians are frogs, toads, salamanders, and the lesser-known caecilians.

Reptiles

Reptiles, like amphibians, commonly have four limbs, but in some groups they have been reduced or lost. The skin is thick and they have scales. Respiration occurs through the lungs. The eggs have a cover and, thanks to this, reproduction is independent of water.

Reptiles include turtles, lizards and the like, snakes, tuataras, crocodiles, and the now-extinct dinosaurs.

In light of cladism, reptiles are not a natural group, as they are paraphyletic. The latter term refers to groups that do not contain all the descendants of the most recent common ancestor. In the case of reptiles, the group that is left out is the Aves class.

Birds

The most distinctive characteristic of birds is the modification of their upper limbs in specialized structures for flight. The integument is covered by different types of feathers.

They have lungs as structures for gas exchange, and these have been modified so that flight is efficient - remember that flight is an extremely demanding activity, from a metabolic point of view. In addition, they are able to regulate their body temperature (endotherms).

Mammals

Mammals comprise a very heterogeneous class, in terms of the form and ways of life of its members. They have managed to colonize terrestrial, aquatic and even aerial environments.

They are characterized primarily by the presence of mammary glands and hair. Most mammals have four limbs, although in some groups they are strongly reduced, as in the case of aquatic forms (cetaceans).

Like birds, they are endothermic organisms, although this characteristic was developed by both groups independently.

The vast majority are viviparous, which implies that they give birth to an active young, rather than laying eggs.

References

1. Clack, JA (2012). Gaining ground: the origin and evolution of tetrapods. Indiana University Press.
2. Curtis, H., & Barnes, NS (1994). Invitation to biology. Macmillan.
3. Hall, BK (Ed.). (2012). Homology: The hierarchial basis of comparative biology. Academic Press.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
5. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
6. Kent, M. (2000). Advanced biology. Oxford University Press.
7. Losos, JB (2013). The Princeton guide to evolution. Princeton University Press.
8. Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., & Ahlberg, PE (2010). Tetrapod trackways from the early Middle Devonian period of Poland. Nature, 463 (7277), 43.
9. Vitt, LJ, & Caldwell, JP (2013). Herpetology: an introductory biology of amphibians and reptiles. Academic press.