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Reptiles all have a type
of shell-covered egg that must be laid on land. Reptiles were the dominant
land animals of the Mesozoic Era; their descendants include the birds
and mammals. Birds are vertebrates evolved from dinosaurs and adapted for flight. Important bird adaptations include feathers, wings, warm-blooded metabolism, hollow bones, and loss of teeth and tail bones. Class Reptilia (Sauropsida): Reptiles differ sharply from amphibians in laying a shell-covered type of egg (amniote or cleidoic egg). These eggs must be laid on land, or else hatched inside the female's body. The shell, secreted by the female, prevents passage of liquids. Since the sperm must swim in a liquid medium, it must enter the egg first; this requires internal fertilization. Reptiles also have a dry, tough (leathery or scaly) skin. Legs are generally stronger than in amphibians; most toes have claws. Body temperature changes to match surroundings. Examples: turtles, snakes, lizards, dinosaurs, and crocodiles. Embryonic membranes: Reptiles, birds, and mammals all have amniote eggs within which several specialized membranes develop from the embryo:
Archosauria ("ruling reptiles"): The most successful group of reptiles were the Archosauria, a group that predominated during Mesozoic times. Archosaur traits include a diapsid skull condition (two openings in the temporal region of the skull), a third opening (the antorbital fenestra) in front of the eye, a bony sclerotic ring around the eye, and an early tendency toward bipedalism, which includes very strong hind limbs and a long, strong tail. Included in this Archosauria are:
Aves (birds): Warm-blooded, feathered vertebrates ("glorified reptiles" or "glorified archosaurs"). Flight adaptations: Birds have adaptations for flying, including wings, feathers (modified scales), and good vision. Modern flying birds also have strong ribs and a rigid sternum (breastbone) with a keel. Metabolism: A high rate of metabolism is needed for flight. Birds are warm-blooded, meaning that their metabolic rates and body temperatures are always rather high, regardless of external temperature. Downy feathers are part of an insulating layer that makes a high internal body temperature possible. The complete separation of oxygen-poor and oxygen-rich blood in the heart also increases metabolic efficiency. Weight reduction: Modern birds have many adaptations that reduce weight, including reduction of the tail bones, loss of the teeth and lightening of the jaws, development of hollow air spaces in the arm bones, and loss of one ovary in female birds. Archaeopteryx and the origin of birds: Archaeopteryx, the oldest bird, was preserved in a fine-grained Jurassic limestone. It had many reptile characteristics, including a long tail, slender ribs, weak sternum, small braincase, and jaws with teeth. However, it also had feathers and was probably capable of flight, and is, therefore, classified as a bird. Scientists now think that birds evolved from carnivorous dinosaurs. Modern birds:
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