Reptiles and Their Evolutionary Trends

. Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs. The origin of the reptiles lies about 320310 million years ago, in the swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph labyrinthodonts. The oldest known animal that may have been an amniote a reptile rather than an amphibian, is Casineria The upper part of the skull of reptiles is modified giving the reptiles a far more efficient and powerful jaw action and making the skull light. The reptiles are classified mainly on the structure of their skulls, in which there are temporal vacuities or fossae or empty spaces in the temporal region. Three different groups of reptiles developed fossae in different places, parapsid, synapsid and diapsid types, and these remained unchanged throughout the evolutionary history of these groups. Key features of Class REPTILIA Body varied in shape, covered with horny epidermal scales, sometimes with dermal plates, integument with few glands. Paired limbs, usually with five toes with claws, adapted for climbing, running or paddling, limbs absent in snakes and some lizards. Skeleton well ossified, ribs with sternum except in snakes, forming a complete thoracic basket, skull with single occipital condyle. Respiration by lungs. Three-chambered heart, except in crocodiles which have four-chambered heart. Metanephric kidney, uric acid is the main nitrogenous waste. Ectothermic animals. Nervous system with primitive brain, spinal cord dominant. There are 12 pairs of cranial nerves. Sexes separate, fertilization internal, hemipenis as copulatory organ. Eggs covered with calcareous or leathery shells. Extra embryonic membranes, amnion, chorion, yolk sac and allantois are present during embryonic life. Reptiles arose about 310320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds and mammals that are descended from early reptiles. A definition in accordance with phylogenetic nomenclature which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.