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Characteristics features of aves is
(a) Presence of beak and feathers
(b) Ability to lay eggs
(c) Air spaces in the lungs
(d) All of the above

Last updated date: 13th Jun 2024
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Hint: Hint: Aves or birds are a group of highly evolved vertebrates, extremely specialized in structure, adaptations, ways of life, habits and behaviour. Birds are pre- eminently creatures of the air, marvelously designed and masterly constructed for an aerial life.

Complete answer:
Aves are warm-blooded, oviparous, bipedal, feathered, winged and toothless vertebrates, with tetra- locular heart, elaborate respiratory mechanism, exceedingly high body temperature, intense metabolism and great powers for flight. Some of their distinguishing features are:
1) An exoskeletal covering of epidermal feathers and scales, scales are present only on shank and foot.
2) Jaws are modified into toothless beaks or bills., covered by an epidermal sheath called rhamphotheca.
3) Fore-limbs are modified into wings for flight.
4) digits are only three in the fore- limb and never more than four in the hind limb.
5) An oil gland is the only cutaneous gland present.
6) Hollow or Pneumatic bones, with air cavities inside.
7) Monocondylian and diapsid skull
8) Neck vertebrae are heterocoelous.

Additional information:
- Birds have large and keeled sternums, for attachment of flight muscles. They have double- headed ribs with an uncinate process.
- In the fore-limb, distal carpals and metacarpals fuse to form the carpo- metacarpous. In the hind limb, proximal tarsals fuse with the tibia forming tibiotarsus.
- Similarly, distal tarsals fuse with metatarsals and form the tarso-metatarsus.

- Several avian features are shared by some reptilian groups. Birds share with dinosaurs bipedality, Pneumatic bones, pre-lacrimal fossa, reduced number of fingers and simplified hind limb with mesotarsal joint, etc.
- Birds share with mammals warm-bloodedness, and with mammals and the crocodilians' tetra-locular heart.
- It is believed that some mammal-like reptiles were also warm-blooded and some dinosaurs had a four- chambered heart.