Deck 14: The Origin of Birds

Question 1

How is a bird's skeleton designed to make powered flight possible?

Answer 1

The bird's skeleton is designed in a way that enables it to make powered flight. Following are the skeletal adaptions that are seen in bird which enable them to fly:
• Bones with air cavities : The bones seen in birds are hollow, very light and filled with air cavities. They also contain struts and braces for support. This is a key adaption which aids in the flight. The air-filled bones are attached to the air sacs and lungs and function as both cooling system and supercharger to support the avian metabolism for powered flight. The bones in the avian skeleton are fused to form more firm structures.
• Air sacs : The air sacs lighten the body and assist in flying. Therefore, the air sacs are adaption to the flight.
• Modified forelimbs : In birds the forelimbs are modified into wings which aid in the flight of the bird.

Question 2

What evolutionary novelties were shared by theropod dinosaurs and birds?

Answer 2

There are few evolutionary novelties shared by theropod dinosaurs and the birds which serve as a strong evidence that birds were descendants from dinosaurs.
Following are the revolutionaries features that are common to theropod dinosaurs and birds:
• Both the birds and theropods exhibited furcula (distally fused clavicles) which is also referred to as wishbone.
• The pubis is pointed posteriorly is expanded in both theropods and birds.
• In both theropods and birds, there was reduction of dentition.
• A second fenestra is present in front of the eye in both theropods and bird.
• Other commonality includes modifications of the hip and hind limbs required for a very fast runner.

Question 3

Why do most paleontologists consider Archaeopteryx to be a bird? What similarities does it show to theropod dinosaurs?

Answer 3

Archaeopteryx , the oldest known bird, was initially considered as a bird by majority of paleontologists owing to its bird-like feathers. However, it exhibited skeletal features of small theropod dinosaurs.
Following are features exhibited by Archaeopteryx , which are common to theropod dinosaurs:
• Archaeopteryx had wishbone, or furcula which was also present in some tetanuran theropods.
• The skeletal features that were common to Archaeopteryx and therapods include large eyes and hollow, thick-walled bones without pneumatic openings.
• The forelimb of Archaeopteryx was also theropod-like with three fully developed digits and a flexible and unmodified radius, wrist, and ulna.

Question 4

What other possible ancestors of birds have been identified, and why are arguments for a dinosaur ancestry of birds more convincing?

Question 5

Compare and contrast the arboreal and cursorial hypotheses of avian flight. Why might you favor the cursorial hypothesis?

Question 6

What do Cretaceous birds suggest about the origin and early diversification of the Aves?

Question 7

What impact does their recognition as the ancestors of birds have on our ideas about dinosaurs?