AVES
AVES or birds arose in the Jurassic from orinthischian dinosaurs and have many characters resembling those of reptiles, consequently the two classes together are called sauropsida. It was not until the Cretaceous that birds became modernized, this involved both physiological and morphological changes, such as constant temperature , fusion of skull bones, formation of pneumatic bones, fusion of vertebrate to form a synsacrum and a suppression of the tail. But birds have become highly modified due to an serial life and show many differences from reptiles. Present day living birds number 8590 species , they are serial, terrestrial and aquatic.
AVES are bipedal, endothermal craniates with an exoskeleton of epidermal feathers on the greater part of the body. Body temperature is high and constant in correlation with their energy . Break and claws have hrony sheath, and feet are covered with scales. Bones of the skull undergo early ankylosis, there is a single occipital condyle, the upper jaw is formed largely by premaxillae, and the lower jaw has 5 or 6 bones. Fore- limbs are modified to form wings with a carpo- metacarpus and only the first three digits of the hand. Hind limbs articulate far forwards to support the body weight in standing, head of the femur is at right angles to the shaft, there is a tibiotarsus and a tarso-metatarsus, there are never more than four digits .the are clawed. Long bones are light with air spaces and no marrow. The sternum has a central keel in flying birds.
