Evolution of Birds
Evolution of birds
• Kingdom Animalia, Phylum Chordata,
Class Aves
• Where did they come from?
– From the Mesozoic Era, a debate of how they
originated.
• Two Hypotheses exist:
– Thecodont vs.Theropod
Evidence to link birds and
dinosaurs
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Hundreds of similar skeletal features
Hollow bones
Feathers in some dinosaurs
Lungs
Heart
Reproductive and Sleeping Posture
More Evidence
• Similarities:
– Gizzard Stones, Brooding and Care of young,
Pulmonary Tract
– Less DNA repetitions, proposed to aid in flight.
– T. Rex bones found with same amino acid sequence
(made from DNA) as in chickens. This sequence is for
collagen tissue created in common birds.
Archaeopteryx
• First universally accepted bird.
Estimated to be 150 million years old.
• Had teeth, flight feathers, long bony tail,
wishbone (fused collar bone), and many
more theropod features.
At least 10 found by now.
Tertiary
Lizards &
Turtles Snakes Crocodiles
So, Reptiles, But
Where Birds?
Mammals
65
MYA
Jurassic
190
PTEROSAURS
SAUROPODS
135
ORNITHISCHIA
Cretaceous
THEROPODS
CARNOSAURS
COELUROSAURS
MYA
DIAPSIDS
MYA
SAURISCHIA
THERAPSIDS
Permian
225
DINOSAURIA
Triassic
THECODONTS
MYA
Tertiary
Lizards &
Turtles Snakes Crocodiles
Mammals
65
MYA
Jurassic
190
PTEROSAURS
SAUROPODS
135
ORNITHISCHIA
Cretaceous
THEROPODS
CARNOSAURS
COELUROSAURS
MYA
DIAPSIDS
MYA
SAURISCHIA
THERAPSIDS
Permian
225
DINOSAURIA
Triassic
THECODONTS
MYA
Theropods
(ground runners)
give rise to Carnosaurs & Coelurosaurs
THECODONTS
SAURISCHIA
DINOSAURIA
ORNITHISCHIA
THEROPODS
SAUROPODS
CARNOSAURS
COELUROSAURS
Carnosaurs
Coelurosaurs
Velociraptor is
associated with
this group.
Theropod Hypothesis
• Theropod has a 3-toed foot similar to modern
bird.
• Carpal wrist bone in Theropod and
Archeaopteryx similar.
• Shoulder sockets similar to modern birds and
allow “wings” or arms to tuck in close to body.
• Lays eggs in nest that were incubated/protected.
• This Hypothesis aligns with “Ground Up” model
for flight.
Thecodonts
give rise to crocodilians, dinosauria, & pterosaurs
Theropods’ ancestors: Known for being
smaller, long tails, short forelimbs.
Some scientists hypothesize that this group
includes the small, lizard-like, flying reptiles.
These were around before Theropods.
Thecodonts
One example: Microraptor
As seen in DVD “The Four-Winged Dinosaur”
Thecodont Hypothesis
• Gliding seems more reasonable than
“ground-runners” to giving flight.
• Birds bodies are flattened top-bottom,
Theropods were flattened side-to-side.
• Feathers more likely evolved as flight
stabilizers instead of a runners warmth.
• This hypothesis aligns with “Top Down”
model for flight.
Evolution of Flight!
Two Main, Opposing
Hypotheses
• Arboreal Model
– “Trees Down” Model
• (Thecodont)
• Cursorial Model
– “Ground Up” Model
• (Theropod)
“Trees Down” Model
• Ancestors of birds were tree-dwellers (arboreal)
• Powered flight evolved from gliding/parachuting:
– Tree-dwelling animals jumped from branch to branch
– Those with a gliding/parachuting surface could travel
further, so selection favored development of increased
wings
– Eventually, forms modified the gliding surface to give them
additional thrust: powered flight
• Seems like a reasonable scenario for the origins of the
other powered flying vertebrates (pterosaurs, bats)
• Makes sense energetically, since the early gliding phases
can use gravity to help them fly long before the need for
the development of strong arm muscles
• Gliders/parachuters are VERY common, and tetrapod
gliders/parachuters have consistenly evolved
convergently from arboreal animals
“Ground Up” Model
• Ancestors of birds were ground running animals
(cursorial)
• Powered flight evolved from activity useful to runners,
outside of the context of a tree-dwelling phase
– Evolution of the wing stroke evolved in some non-flight
context (possibly food capture; possibly as a speed-aid or an
aid for leaping and jumping)
– Feathers originally evolved in a non-locomotion context, but
were exapted for whatever the possible pre-flight use of the
forelimbs was
– Through enlargement of the proto-wing in the non-flight
context, the forelimbs became large enough and developed
enough to begin to carry the animal through the air
– Birds only got into the trees after having developed the early
phases of flight
“Ground Up” Model
• Birds, unlike bats and pterosaurs, do not make use of a
membrane to fly; and there is no fossil evidence that they
ever did
• Unlike bats and pterosaurs, the hind limb is not part of the
flight surface; in fact, most modern and fossil birds have
perfectly good running legs (just as their out groups had)
• Until recently, all the known member of avialian outgroups
(Deinonychosauria, Oviraptorosauria, Therizinosauria,
Ornithomimosauria) were fairly large bodied animals that
were unlikely to have spent much time in trees
• But there was a lack of good modern analogues for
whatever the ground use scenario would have been
• Also, some questioned whether it would have been
energetically feasible for animals to have achieved
powered flight directly from a running/leaping behavior
Youtube: 1of5--Evolve-Flight by History Channel 1 of 5 (see all 5)