Lecture 2: Avian Origins and Evolution Flashcards
Archaeopteryx
- Original link between birds and reptiles
- Oldest undisputed fossil bird, but evolutionary offshoot, not primitive model
- capable of flight, not long or maneuverable
Late Jurassic of Europe-oldest undisputed bird…or is it?
- 12 fossils and a feather
- Other contenders for the oldest: Fossils from Texas, Africa, N. A., S. A., China, North Korea
Is Archaeopteryx a bird?
2011- fossil find suggests dinosaur
2014- new fossil find suggests bird
“Reptillian” Characteristics (Archaeopteryx)
- teeth
- 3 free digits w/claws, manual phalanges
- long boney tail
- Sternum not keeled
- No uncinate processes on ribs
Avian features (Archaeopteryx)
- feathers; vanes asymmetrical
- furcula
- brain; vision and hearing enlarged, forebrain enlarged
- 1st toe pointed backwards (hallux)
Shared derived characters with Theropod Dinosaurs
- 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
Mesozoic evolution of Aves
-Large gaps in fossil record; Archaeopteryx considered representitive
Phylogenetic relationships of Mesozoic birds
-Greater diversity than previously thought
Main clades (Mesozoic)
- Avialae
- Pygostylia
- Enantiornnithes
- Ornithurae
- Aves
Avialae
- Feathers
- Reversed hallux
- Unserrated teeth
- < 25 tail vertebrae
- *Archaeopteryx
Pygostyle
- Pygostyle
- Tail < 8 free vertebrae
- Pubic boot lost
- Carpometacarpus
- Confuciusornis (Jurassic- Cretaceous; wing +feet = Archaeopteryx)
- No teeth (convergence with modern birds)
Enantiornithes
- 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
Ornithurae
- Ribs uncinate processes
- Air sacs
- Bones without arrested growth
- keel developed
Aves
- Modern birds
- loss of teeth
The evolution of feathers
- Traditionally: tied to questions of avian ancestry and origin of flight
- Once thought to have resulted from splitting scales
- Now considered novel epidermal derivatives
EvoDevo
-Development (5 stages) of feathers gives clues to evolution of feathers
Evidence
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)
The evolution of flight
Evolution of a flapping wing from a forelimb with no locomotor function
Initial selective advantages of flight (multiple hypothesis)
- To escape from predators
- To catch flying prey
- To help move from place to place
- To free hind legs for uses as weapon (balancing raptor)
- To gain access to new food sources or unoccupied niche
Parental care?
How to generate lift: Hypotheses
- 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
Some general evolutionary trends associated with flight
- Smaller size (early trend in certain dinosaur lineages) flight adaptation?
- Changes in body proportions (wings longer than legs, tail shorter)
Tree down hypotheses
- glider; ancestors of Archaeopteryx lived in trees and glided into flapping flight
- Anatomy evolved gradually, consistent with anatomical changes
Ground up hypothesis
- 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
Wing assisted incline running hypothesis
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
