Lecture 2: Avian Origins and Evolution

Question 1

Archaeopteryx

Answer 1

• Original link between birds and reptiles
• Oldest undisputed fossil bird, but evolutionary offshoot, not primitive model
• capable of flight, not long or maneuverable

Question 2

Late Jurassic of Europe-oldest undisputed bird…or is it?

Answer 2

• 12 fossils and a feather

- Other contenders for the oldest: Fossils from Texas, Africa, N. A., S. A., China, North Korea

Question 3

Is Archaeopteryx a bird?

Answer 3

2011- fossil find suggests dinosaur

2014- new fossil find suggests bird

Question 4

“Reptillian” Characteristics (Archaeopteryx)

Answer 4

• teeth
• 3 free digits w/claws, manual phalanges
• long boney tail
• Sternum not keeled
• No uncinate processes on ribs

Question 5

Avian features (Archaeopteryx)

Answer 5

• feathers; vanes asymmetrical
• furcula
• brain; vision and hearing enlarged, forebrain enlarged
• 1st toe pointed backwards (hallux)

Question 6

Shared derived characters with Theropod Dinosaurs

Answer 6

• Adaptation for bipedal; hinged ankle joint, fibula reduced, metatarsal elongated
• Fifth toe lost/hallux
• pubis boot, later extends backwards
• Furculum
• Hand; 5 to 3 digits, semi-lunate wrist, (later carpometacarpus)
• Shoulder blade; long and thin
• Pneumatic bones
• Feathers

Question 7

Mesozoic evolution of Aves

Answer 7

-Large gaps in fossil record; Archaeopteryx considered representitive

Question 8

Phylogenetic relationships of Mesozoic birds

Answer 8

-Greater diversity than previously thought

Question 9

Main clades (Mesozoic)

Answer 9

• Avialae
• Pygostylia
• Enantiornnithes
• Ornithurae
• Aves

Question 10

Avialae

Answer 10

• Feathers
• Reversed hallux
• Unserrated teeth
• < 25 tail vertebrae
• *Archaeopteryx

Question 11

Pygostyle

Answer 11

• Pygostyle
• Tail < 8 free vertebrae
• Pubic boot lost
• Carpometacarpus
• Confuciusornis (Jurassic- Cretaceous; wing +feet = Archaeopteryx)
• No teeth (convergence with modern birds)

Question 12

Enantiornithes

Answer 12

• Opposite birds (articulation of scapula and coracoid, fusion of metatarsals)
• Keeled sternum
• Strut like coracoid
• Alula (bastard wing-tiny group of asymmetric feathers on tip of first digit)
• Lines of arrested growth
• Scapula has fossa and coracoid has process for articulation, fusion of tarsometatarsus-top down
• Opposite in birds

Question 13

Ornithurae

Answer 13

• Ribs uncinate processes
• Air sacs
• Bones without arrested growth
• keel developed

Question 14

Aves

Answer 14

• Modern birds

- loss of teeth

Question 15

The evolution of feathers

Answer 15

• Traditionally: tied to questions of avian ancestry and origin of flight
• Once thought to have resulted from splitting scales
• Now considered novel epidermal derivatives

Question 16

EvoDevo

Answer 16

-Development (5 stages) of feathers gives clues to evolution of feathers

Question 17

Evidence

Answer 17

a. Feather stages found on modern birds
b. Feather development- morphology and molecular signaling
c. Fossil evidence; when and who had diff stages of feathers
- Initial advantage of feathers (still unknown)

Question 18

The evolution of flight

Answer 18

Evolution of a flapping wing from a forelimb with no locomotor function

Question 19

Initial selective advantages of flight (multiple hypothesis)

Answer 19

1. To escape from predators
2. To catch flying prey
3. To help move from place to place
4. To free hind legs for uses as weapon (balancing raptor)
5. To gain access to new food sources or unoccupied niche
   Parental care?

Question 20

How to generate lift: Hypotheses

Answer 20

• Tree down: glider
• Ground up
• Wing assisted incline running
• Flight evolved gradually-(improve traction, gain speed)
• *Fossil record of changes in anatomical components of flight and biophysics support arboreal hypothesis

Question 21

Some general evolutionary trends associated with flight

Answer 21

• Smaller size (early trend in certain dinosaur lineages) flight adaptation?
• Changes in body proportions (wings longer than legs, tail shorter)

Question 22

Tree down hypotheses

Answer 22

• glider; ancestors of Archaeopteryx lived in trees and glided into flapping flight
• Anatomy evolved gradually, consistent with anatomical changes

Question 23

Ground up hypothesis

Answer 23

• cursorial ancestors used their long, powerful legs to run fast with their arms outstretched (to catch insects tc.)
• At some point lifted up by air currents and carried into flapping flights
• Flight evolved against gravity
• Two biophysical problems; complex flight stroke, biology of archaeopteryx would not permit asymmetric upstroke

Question 24

Wing assisted incline running hypothesis

Answer 24

the feathered forelimbs of small two legged dinosaurs may have helped them run up hills or other inclines to escape predators

• traction, gain speed
• Relevance modern birds have modern shoulder girdle