First Successful Terrestrial Vertebrates
Reptiles were the first vertebrates to successfully and definitively conquer the terrestrial environment.
This is because they have developed some adaptive features such as: presence of calcareous shell surrounding the egg and waterproof, dry, gland-free skin, lined with epidermal scales (in snakes and lizards), by corneal plaques (in crocodiles and alligators) or by bone plates (in turtles), forming a carapace that protects the animal against dehydration.
The waterproofing of the skin occurred thanks to the intense production of a protein molecule, the keratin, the great biochemical novelty produced in large quantities by the reptiles epidermis, a fact that will also be repeated in birds and mammals. In fact, in the skin of amphibians, this molecule already exists, but in very small quantities, being unable to make the skin impervious to water and breathing gases.
This adaptation allowed the reptiles to save water, enabling life in the most diverse habitat, including desert. On the other hand, the lack of moisture in the skin and the richness in keratin prevent gas exchange, which is then performed exclusively by the lungs.
The lungs have a larger relative surface and are more efficient than amphibians, relieving the skin of respiratory function. Air inlet and outlet is also more efficient due to the aid of rib muscles.
Even reptile excretion is adapted to the least possible loss of water. The nitrogenous excretion product is the uric acid, eliminated by the cloaca together with the feces in the form of a semi-solid paste.
Fig. 1, epidermal scales (present in snakes and lizards)
Fig. 2, corneal plaques (present in crocodiles and alligators)
Fig.3 bone plates (present in turtles)
Another important adaptation to life in the terrestrial environment is internal fertilization, regardless of water, in which the gametes (eggs and sperm) are protected from external influences. Females are usually oviparousthat is, when fertilized they lay eggs and embryos develop within them, hence outside the maternal body.
Embryonic development occurs entirely within an egg with a porous protective calcareous shell, which allows gas exchange to occur.
A bag full of liquid, the amniotic gallbladder, ensures the development of the embryo in aqueous medium. An vitelline gall replete with food reserves, the calf, ensures the survival of the embryo with food from the egg. And to complete the efficiency of this new reproductive method, an excretory bag, the allantoid, It collects uric acid and immobilizes it in the form of crystals that do not interfere with the life of the embryo.
Clinging to the shell membrane is another embryonic annex, the chorium, in the form of a richly vascularized membrane, which ensures respiratory gas exchange with blood that routes oxygen to embryonic cells.
There is no larval phase. Finishing development, the young individual, with more adult characteristics, breaks the shell and leaves the egg.
Some venomous lizards and snakes may be ovoviviparous (the egg is laid by the female after remaining for a certain period of embryo development within the mother's body) or viviparous (Embryo development occurs entirely within the female's organism).
The name reptiles derives from the mode of locomotion: the four legs (absent in snakes) lie on the same plane of the body, determining the crawl of the belly in the ground (from Latin). reptare = crawl). To perform these movements, they have well-developed muscles. The reptile skeleton is completely bony. The Earth has already known gigantic forms of these animals, such as dinosaurs, which have populated and dominated our planet for years as an undisputed superiority.