Evolutionary origin of Aves

Question 1

Features of birds

Answer 1

• considered to be diapsids, but bone has lost teeth and chewing mechanism to lighten skull
• lost fenestra and fused with orbit to make bigger hole

Question 2

Bird origin

Answer 2

•birds have complete phylogeny, in dinosaur clade
•evolved with archosaurs, but phylogeny position is debated
H1 = birds descended from crocodiles
H2 = descended from unknown archosaurs
H3 = descended from theropods, small carnivorous dinosaurs
H3 is accepted via fossil discoveries
Diverged 150mya

Question 3

Where features of birds evolved

Answer 3

1. Skull of archaeopteryx is paedomorphic (compared juvenile to adult) - birds are too
2. Digit changes occurred, ancestral dino had 5, then lost 5th and 4th
   - archaeopteryx longer 2nd digit - less symmetrical
   - living birds, 2 and 3 fused
3. Bones become lightweight and hollow very early
4. Many had bristle-like feathers - showed colours and patterns from melanocytes
5. warm-blooded dinos early (primary feathers for insulation, then display)

Question 4

Dinosaur features of birds

Answer 4

• parental care
• egg-laying (became asymmetrical and elongate)
• scales - bird legs, feathers derived from scales
• S-shapred neck
• tridactyl foot
• bipedal
• toes bearing weight of body - digitigrade posture
• wishbone and fused sternum (theropods)

Question 5

Archaeopteryx

Answer 5

Transitional form
•reptilian and avian features
-toothed jaw
•flight musculature attached to furcula (wishbone)
•fossil feathers like modern
•much smaller keep than modern
•has thick bones, no fusion of vertebrae (reptilian features)
•3 fingers and S-shaped neck (bird features)
-is it a member of Aves?