Chapter 4: PARADE OF THE VERTEBRATES IN TIME AND TAXA

AVES

AVES - known as Birds - are endothermic vertebrates with feathers - descendants of some bipedal dinosaurs, most likely,ornithischians.- exhibit endothermy

Common Morphological Features of Avians

1. Have retained reptilian scales on their beak, legs and feet.

2. Have a single occipital condyle.3. Have a diapsid skull.4. Birds have feathers.

Common Morphological Features of Avians

Common Morphological Features of Avians

Feathers – are made up of keratin derived from the epidermis.

- replaced reptilian scales on all surfaces except the beak, legs and feet.

- with pigments facilitating recognition by other members of species.

- function as: to make bird flight possible,

and for insulation against seasonal heat and high-altitude cold.

Birds’ Adaptation for Flight

1. Long bones become slender2. Most bones, including vertebrae lost central

marrow leaving cavities containing air-filled extensions of air sacs from the lungs.

3. Skull is lightened by thinning of compact layer.

4. All teeth have been lost, replaced by a lighter-weight beaker.

Birds’ Adaptation for Flight5. The bones of wrist, palm and digits reduced in

size.6. Volume of muscles in forelimbs also reduced.7. Urinary bladder have been lost.8. Large intestine have become shortened.

Birds’ Adaptation for Flight

Common Skeletal Structures That Are Present in Birds

Carina – a large sternal keel for attachment of the massive flight muscles. (Absent in birds that that do not fly.)

Synsacrum – formed when entire skeleton of the trunk except the ribs has become a rigid unit by ankylosis of trunk vertebrae and their fusion in the pelvic girdle.

Modification of Soft Parts

1. Esophagus with crop for storage of seeds and grain.

2. Stomach has become a grinding gizzard like that of crocodiles.

3. Sizes and shape of eyes – result in increased activity of vision from high in the air.

4. Enlaged orbits.5. Presence of hyperstriatum, a thick new stratum of

nerve cells in the cerebral hemisphere, responsible for the struggled avian behavior.

Modification of Soft Parts6. Enlarged cerebellum.

ClassificationSubclass Archaeornithes - the primitive birds.

Archaeopteryx – the oldest known bird, discovered in 1986.

Archaeopteryx

Morphological Features of Archaeopteryx

1. Had a long reptilian tail, thecodont teeth on both jaws, and feathers on the tail and wings.

2. Skull was more reptilian than avian3. Nostrils were far forward4. No beak5. Braincase had not expanded to accommodate an

enlarged brain.6. Cervical vertebrae was not saddle shaped at the ends as

in today’s birds.7. Trunk vertebrae was not rigidly fused.8. The sternum was not well developed.

Morphological Features of Archaeopteryx

7. The synsacrum was not well developed.8. The sternum was small, not accommodated

with strong pectoral muscles for sustained flight suggesting that these birds may have soared more than they flew.

Protoavis – crow-like birds of Triassic Period, 75 million years older than Archaeopteryx.

- two fossils was discovered in 1986 in a mudstone quarry in Texas, U.S.A.

- more dinosaur-like than Archaeopteryx.

- may have been closer to the evolutionary line leading to modern birds than was Archaeopteryx.

Protoavis