When we talk about tetrapods, we should know that they constitute one of the most evolutionarily successful and diverse groups of vertebrates on Earth. They are present in all kinds of habitats, since thanks to the fact that their extremities have evolved in different ways, they have adapted to life in the aquatic, terrestrial and even airborne environmentIts most significant characteristic is found in the origin of its limbs, but do we know the definition of the word tetrapod? And where does this group of vertebrates come from?
We will tell you about the origin and evolution of these animals, their most striking and important characteristics and we will show you examples of each of them. If you want to know all these aspects of tetrapods, keep reading this article that we present to you on our site.
What are tetrapods?
The most evident characteristic of this animal group is the presence of four limbs (hence its name, tetra=four and podos=feet). It is a monophyletic group, that is to say that all its representatives share a common ancestor, as well as the presence of said extremities, which constitute a “evolutionary novelty ” (that is, a synapomorphy) present in all members of this group.
This includes the amphibians and amniotes (reptiles, birds and mammals) and, in turn, is characterized by having pentadactyl limbs (with 5 fingers) formed by a series of articulated segments that allow the movement of the limb and the displacement of the body, and that evolved from fleshy fins of fish that preceded them (Sarcopterygians). On this basic limb pattern, various adaptations for flight, swimming, or running occurred.
Origin and evolution of tetrapods
The conquest of the Earth was a very long and important evolutionary process that involved morphological and physiological modifications in almost all organ systems, which evolved in the context of the Devonian ecosystems (about 408-360 million years ago), at which time it inhabited Tiktaalik, already considered a vertebrate land.
The transition from water to land is almost certainly an example of “adaptive radiation” In this process, animals that acquire Certain characteristics (such as primitive limbs for walking or the ability to breathe air) colonize new habitats that are more conducive to survival (with new food sources, less danger from predators, less competition with other species, etc.).). Said modifications are related to the differences between the aquatic environment and the terrestrial environment:
With the passage from water to land, the tetrapods had to face problems such as supporting their bodies on land, which they are much denser than air, and to that of gravity in the terrestrial environment. Therefore, their skeletal system is structured in a different way than that of fish, since in tetrapods it can be seen that the vertebrae are interconnected by means of vertebral extensions (zygapophysis) that allow the spine to bend and at the same time act as a suspension bridge to support the weight of the organs below it.
On the other hand, there is a tendency for the spine to differentiate into four or five regions, from the skull to the caudal region:
- Cervical region: which increases the mobility of the head.
- Trunk or dorsal region: with ribs.
- Sacral region: which is related to the pelvis and transfers the force for locomotion to the skeleton of the legs.
- Caudadal or tail region: with vertebrae simpler than those of the trunk.
In this other article on our site we also explain the origin and evolution of bipedal animals - Examples and characteristics.
Characteristics of tetrapods
The main characteristics of tetrapods are as follows:
- Ribs: they have ribs that help in the protection of the organs and in primitive tetrapods they extended through the entire vertebral column. Modern amphibians, for example, practically lose their ribs and in mammals they are confined only to the anterior part of the trunk.
- Lungs: in turn, the lungs (which already existed before the appearance of tetrapods and which we associate with life in land) evolved into aquatic individuals such as amphibians, where the lungs are simple sacs. However, in reptiles, birds and mammals they are divided in different ways.
- Cells with keratin: on the other hand, one of the most important characteristics of this group is the way they prevent dehydration of their bodies, with scales, hairs and feathers formed by dead and keratinized cells, that is, impregnated with a fibrous protein, keratin.
- Reproduction: another issue that tetrapods faced when moving to land was that of making their reproduction independent of the aquatic environment, this was achieved through the amniote egg in the case of reptiles, birds and mammals. This egg has different embryonic layers: amnion, chorion, allantois and yolk sac.
- Larvae: For their part, amphibians exhibit a variety of reproductive modes with a larval stage (eg, tadpoles in frogs) with external gills, and part of their reproductive cycle takes place in the water, unlike other amphibians such as some salamanders.
- Salivary glands and others: among other characteristics of tetrapods, we can name the development of salivary glands to lubricate food and production of digestive enzymes, a large, muscular tongue that functions to capture food, as in the case of some reptiles, the protection and lubrication of the eyes by the eyelids and lacrimal glands, the reception of sound and its transmission to the ear internal.
Examples of tetrapods
Being a megadiverse group, we will name the most curious and striking examples of each lineage that we can find today:
Amphibian Tetrapods
Includes anurans (frogs), urodelos (salamanders and newts) and gymnophynas or caecilians. Some examples are:
- Golden Poison Frog (Phyllobates terribilis): so peculiar for its striking color.
- Fire salamander (Salamandra salamandra): with its brilliant design.
- Cecilias (amphibians that have lost their legs, that is, they are legless): reminiscent of earthworms, with representatives of large, such as Thompson's caecilian (Caecilia thompsoni), which can reach up to 1.5 meters in length.
To better understand these particular tetrapods, you may also be interested in this other article on Where and how do amphibians breathe?
Sauropsid tetrapods
Includes modern reptiles, turtles, and birds. Some examples are:
- The Brazilian coral (Micrurus brasiliensis): with its powerful poison.
- The matamata turtle (Chelus fimbriatus): curious because of its spectacular mimicry.
- The Birds of Paradise: As rare and intriguingly patterned as Wilson's Bird of Paradise, featuring an incredible color scheme.
Synapsid tetrapods
Today's mammals, such as:
- Platypus (Ornithorhynchus anatinus): a most curious semiaquatic representative.
- Flying fox bat (Acerodon jubatus): one of the most impressive flying mammals.
- Star-nosed mole (Condylura cristata): with such unique underground habits as the star-nosed mole.
