Lecture 2 (origin and evolution of birds)

From which group of animals did birds evolve?

reptiles

What are some primitive features modern birds share with reptilian ancestors?

Single occipital condyle, jaw articulation, single middle ear bone, scales on legs, nucleated red blood cells, egg-laying habits.

What is the significance of Archaeopteryx in avian evolution?

It is the first known bird and serves as a transitional form between reptiles and modern birds

what period are archaeopteryx from

Jurassic (150 mya)

What features of Archaeopteryx resemble modern birds?

Fully developed flight feathers

furculum (wishbone)

pelvis and legs are similar to modern birds.

What reptilian features did Archaeopteryx have

Reptilian skull with teeth

long bony tail

unfused hand bones with claws

unkeeled sternum

ribs not fused by uncinate processes

abdominal ribs

What evidence suggests Archaeopteryx could fly

Asymmetrical feather vanes

heavy furcula for muscle attachment

acute angle of scapula

coracoid like modern flying birds.

Why is there debate about whether Archaeopteryx could take off from the ground

It lacked a triosseal canal, suggesting it may have needed a running start rather than powered

takeoff.

How might Archaeopteryx have compensated for lacking a triosseal canal

It may have had reptilian muscle physiology with high power but rapid fatigue

What is the 'take-home' message of Archaeopteryx

It represents a transitional form between reptiles and modern birds but is not in their direct lineage.

What are the two historical hypotheses for bird ancestry

Pseudosuchian Thecodont Ancestry Hypothesis

Coelurosaurian Theropod Ancestry Hypothesis (Dinosaur ancestry)

Why is the dinosaur ancestry hypothesis widely accepted today

Due to recent fossil discoveries in China showing feathered dinosaurs

What are Pseudosuchians?

Small, bipedal reptiles

from the early Triassic (~200-230 mya)

gave rise to dinosaurs, pterosaurs, and crocodilians

Why is the Pseudosuchian ancestry hypothesis unlikely

over 50 million year gap between Pseudosuchians and Archaeopteryx

Share only a few derived features

what are coelurosaurs

lightly built bipedal dinosaurs

elongated limbs adapted for running

appear in upper jurassic and lasted thru cretaceous (150-65 mya)

What shared features do Coelurosaurs and Archaeopteryx have

vertebral column

limbs

hands

pectoral and pelvic arches

feathers

What is a major issue with the dinosaur ancestry hypothesis

Coelurosaurs appear at the same time or later than Archaeopteryx in the fossil record.

What are the two main hypotheses for the evolution of flight

Arboreal Hypothesis ('trees down') and Cursorial Hypothesis ('ground up').

What is the Arboreal Hypothesis

primitive birds (including Arch) were tree-dwelling and developed the ability to glide and from gliding flight to flapping flight

What evidence supports the Arboreal Hypothesis

Archaeopteryx had forelimb claws and foot structures suited for climbing

a long flat tail for balance

flight evolution is less energetically costly

What is the Cursorial Hypothesis

from the ground up, and birds, flight evolved from running and leaping to catching insects

What evidence supports the Cursorial Hypothesis?

Archaeopteryx had hind limbs adapted for running

wing morphology is different from typical gliding structures.

what are some problems with the cursorial hypothesis

feet were adapted for perching not running

flight apparatus was maybe not developed enough to allow powered takeoff

what are some problems with the arboreal hypothesis

current gliders all use flaps of skin for gliding

hindlimbs appear for running

other than feet adapted for perching, no specialized arboreal adaptations

What is Wing-Assisted Incline Running (WAIR)?

A mechanism where birds use flapping to climb steep surfaces, possibly an intermediate stage in flight evolution

What is a criticism of the WAIR hypothesis

It is unclear whether selection pressure for WAIR was sufficient to drive the evolution of flight.

What significant discovery in China supports the link between birds and dinosaurs

Early Cretaceous (~124-128 mya) feathered dinosaur fossils

Microraptor

strong link between non-avian theropod dinosaurs and birds

What is 'Dino Fuzz' and who had it

the feather precursor, filamentous structures

seen on the sinosauropteryx

who had feathers present on long bony tail

caudipteryx

who had feathers on front and hind limbs

Microraptor

What is Gansus Zheni

An early Cretaceous (120 mya) bird most similar to modern birds