Organismal Biology #27
REPTILES and BIRDS

ONLINE CLASSROOM VERSION

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, meaning that males usually have a penis. (Structure of the penis varies; snakes have a two-part structure in which each hemipenis has a groove along its inner side so that they make a tube for the transmission of sperm only when they are put together.)
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:
  • Chorion: Provides protection.
  • Amnion: Encloses the embryo in its own protective watery bag; gives the amniote egg its name.
  • Allantois: Serves as a lung-like respiratory organ for the embryo, also a repository for embryonic wastes that are left behind when the egg hatches.

    The amniote egg allowed reptiles to colonize land habitats far away from the rivers and ponds that amphibians rely upon for reproduction. Evolution of the amniote egg was quickly followed in the fossil reord by a great expansion and diversification (an adaptive radiation) of reptiles into dozens of different groups.
    Illustrations

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:
  • Thecodontia ancestors of all the rest
  • Crocodilia
  • Pterosauria (extinct flying reptiles), with wings and sometimes hair
  • Ornithischia: bird-hipped dinosaurs, often secondarily quadrupedal
  • Saurischia: lizard-hipped dinosaurs, which include the giant swamp-dwellers (Sauropods), carnivorous dinosaurs (Theropods), and the ancestors of birds.

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:
  • Modern flightless birds include the ostriches, rheas, moas, and kiwis.
  • Modern flying birds include owls, gulls, storks, pigeons, eagles, hawks, woodpeckers, and many perching birds (Passeriformes), which make up the largest group.
    The perching birds (order Passeriformes) are the largest and most successful group. The adaptation that characterizes this group is that their clawed toes grasp tightly around small branches, even when their muscles are relaxed (including when they are asleep); they require muscular effort to release their grip, but not to maintain it.
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