Evolution of Birds

Question 1

Do we have any complete vertebrate phylogenies?

Answer 1

Yes - the birds

Around 10,000 species with 50% being passerines

Question 2

What was the original hypothesised ancestral form of birds?

Answer 2

The ratites (flightless birds)

Question 3

What is hypothesised to be the sister group to the birds?

Answer 3

Dromaeosaurids

Question 4

What did Thomas Huxley hypothesise?

Answer 4

Suggested birds came from dinosaurs in the 1860s

Question 5

Give some features of the Dromaeosaurs

Answer 5

Included the raptors

Had a small forelimb that was modified for a seizing motion

Some had feathers - either for insulation, sexual selection or both

Had symmetrical feathers so they were flightless

Question 6

What slowly happened for large Therapods to become birds?

Answer 6

Sustained miniaturisation over 50 million years

Skull slowly became larger with body size to eventually form paedomorphic skull found in birds

Digits were lost and some became elongated (ancestral form had 5 toes)

Question 7

Give some features shared by both dinosaurs and birds

Answer 7

Egg laying

Scales - feathers are derived scales

S-shaped neck

Tridactyl foot

Bipedal

Toes bearing weight of the body - known as digitigrade posture

Wishbone and a fused sternum - these features only found in the Therapods and the Aves

Question 8

Give the reptilian and bird like features of Archaeopteryx

Answer 8

Reptilian:

• strong thick bones (bird bones are hollow)
• no fusion of the vertebrae
• toothed jaw

Bird features:

• well developed feathers (they were asymmetrical so could fly)
• three fingers
• S shaped neck

Question 9

Was Archaeopteryx a good flyer?

Answer 9

No - not much evidence for strong flapping behaviour so possibly only used flight for hunting or quickly escaping predators

Not much evidence for perching behaviour either - lived in a fairly treeless environment

Question 10

What feature did Archaeopteryx share with Dromaseosaurs but not the Aves?

Answer 10

Hyperextendible second toe - known as the terrible claw

Probably used for hunting - helping to pin down prey

Question 11

Give some differing features between Archaeopteryx and modern birds?

Answer 11

No paedomorphic skull in Archaeopteryx

Archaeopteryx had 3 digits whereas birds have two on forelimb

Archaeopteryx had a long bony tail whereas birds have a short tail

Archaeopteryx had a small sternum and no fused vertebrae (opposite in the birds)