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

• 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.

190

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135

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65

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225

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TurtlesLizards &

Snakes Crocodiles Mammals

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So, Reptiles, But Where Birds?Tertiary

Cretaceous

Jurassic

Triassic

Permian

190

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135

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Tertiary

Cretaceous

Jurassic

Triassic

Permian

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Theropods(ground runners)

give rise to Carnosaurs & Coelurosaurs

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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.

Thecodontsgive 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)