Tetrapods are a sub-group of animals known as Sarcopterygii. The term “tetrapod” refers to land-living animals such as hawks, frogs, and turtles. They also include marine animals such as sea lions, whales, sea snakes, seals, dolphins, and sea turtles. Besides the existing animals, tetrapods also refer to extinct animals including mosasaurs, plesiosaurs, and ichthyosaurs.
Origin of the Tetrapods
Evolution played a significant role in the origin of the tetrapods. These vertebrates evolved from animals known as Tetrapodomorpha. On the other hand, Tetrapodomorpha also evolved from ancient lobe-finned fishes known as Sarcopterygii. These fish existed during the Devonian period which was about 390 years ago. The first tetrapods are believed to have been aquatic, however, most of the modern tetrapods are amniotes which either lay eggs on land or fertilize eggs within the females. The amniotes further evolved into two groups: reptiles and mammals. Examples of reptiles are dinosaurs (including birds), turtles, lepidosaurs, and crocodilians. Amniotes include flying creatures such as the bats and birds.
Morphology of the Tetrapods
The word “tetrapod” means “four feet” or "four limbs.” The four-limbed animals use their limbs for flying, walking, crawling, swimming, and running. However, there are specific tetrapods that have lost some or all of their limbs through further evolution. They include whales, dolphins, seals, aquatic birds, coral snakes, and sea cows among others.
The skull in tetrapods is entirely different from that of fish. The tetrapods’ skull is longer than that of the fish resulting in the orbits being farther back.
Most tetrapods initially had a tooth structure called plicidentine that was characterized by an infolding enamel. Today, the tooth structure has evolved into fangs and large teeth growing on small jaws. These teeth enable animals to feed on their prey.
Regarding vision, the eyes of the tetrapods evolved as they changed their habitat from land to water or vice versa. They developed eyelids, tear ducts, and rod and cone opsins.
Circulation and Respiration of Tetrapods
Just like the early tetrapods, modern reptiles and amphibians possess three-chambered hearts. The spiral valve found within the heart prevents mixing of the circulating oxygenated and de-oxygenated blood. Consequently, the tetrapods experience higher metabolic rates and are more active.
The early tetrapods used four methods of respiration namely digestive tract lining, gills, skin breathing, and lungs. These respiratory methods have also been adopted by present-day fish and amphibians and the tetrapods at large.
Tetrapods inhabit a wide variety of habitats such as scrublands, Polar Regions, forests, mountains, rivers, oceans, lakes, grasslands, and deserts. The tetrapods that live on land are those that are adapted for survival on earth. Other tetrapods are aquatic such as the fish while others are semi-aquatic including the amphibians.