Vertebrates are animals that have a backbone and belong to the phyllum "Chordata"
Within the phylum Chordata in the subphylum Vertebrata, there is a further subdivision known as the infraphylum Tetrapoda. Tetrapoda, or four-legged animals, is the largest group of terrestrial vertebrates.
This diverse group of animals includes amphibians, reptiles, birds, and mammals.
They have limbs or ancestors that had, and they live in land or have ancestors that did. Their bodies have adapted to terrestrial living evolving the following characteristics:
- With no buoyancy to support their weight, they have developed body support structures, like strong limbs attached to the main skeleton through girdles and a stronger flexible spine.
- Developed a neck that allows for head movement to gather food and control its environment without affecting their lower bodies’ movement.
- Paired fins have developed as limbs
- They show adaptations to live in air instead of water, mainly to protect the body structures from drying excessively. Lungs instead of gills, a cornified skin, protected eyes and modified ears and nose and oral glands to moisten foods that are drier than in water. A major adaptation to terrestrial living was the development of eggs and foetal membranes to protect their embryos from drying.
Some Tetrapods developed these changes only partially. These are the Amphibians.
Ectotherms are organisms that rely on external sources of heat to raise their body temperature. Fish, amphibians and reptiles are mostly ectothermic.
In contrast to this, endotherms can produce their own heat internally via the process of metabolism. Endothermy is an expensive process in terms of energy required to maintain a constant raised temperature. Even though it may be advantageous because it increases efficiency in certain processes in the body, such as power in muscle contraction, faster digestion and faster nervous system processes.
Some fish have developed partial endothermy, such as Tuna and other species of fast swimmers from the SubOrder Scombroidei (albacores, billfishes, bonitos, mackerels, ribbonfishes, sailfishes, swordfishes, and tunas) and sharks from the Lamnidae family. Internal temperature has been observed to be up to 20° C above water temperature.