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17 Cards in this Set
- Front
- Back
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present in ancestors
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Primitive
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not present in ancestors
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Derived
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What do most biologist believe about the descendants of birds?
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Most biologist believe that birds represent an evolutionary transition from reptiles because of shared primitive characteristics
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Primitive traits linking birds and reptiles
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Single occipital condyle (connection between skull and spine)
Jaw articulation (different bones in mammals) Single middle ear bone (mammals have 3) Scales on legs Nucleated red blood cells (mammals anucleate) Egg-laying habits |
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The two competing theories of bird evolution
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1. Theropod descendants
~TREX ~VELOCIRATORS 2. Thecodont ~Primitive reptiles ~Gave rise to dinos plus other reptiles |
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1. Theropod descendants
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Pros
Many recent fossil discoveries Avian features like feathers, furcula, pygostyle, bill, and flow through lungs thought to be discovered Cons Alan Feduccia Disagrees with: Foot anatomy Dominant theories of feather evolution BUT: no transitional forms found for 50 million year period between Thecodonts and Archaeopteryx |
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Archaeopteryx Similarities with birds
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Fully developed flight feathers like modern birds
A furcula (wishbone) which an avian adaptation for modern flight Pelvis and Legs similar to modern birds |
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Archaeopteryx Differences with modern birds
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Bony jaw with teeth
Lack of a keeled sternum Long bony tail Separate digits on hand Skeleton “reptilian” Abdominal ribs present Ribs not fused with uncinate processes |
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Sinornis
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Toothed bird similar to Archaeopteryx but
Hand Forearm Pectoral girdle Foldable wing Pygostyle |
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Confuciusornis
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1st with horny beak
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Microraptor
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Flight feathers on the legs
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Primitive birds were arboreal and developed the ability to glide
Flapping flight came with adaptations for greater power |
Arboreal Hypothesis - Top Down
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Flight came about from running and leaping to catch insects or avoid predators
Flight feathers provided stability Flapping flight came with adaptations for greater power |
Cursorial Hypothesis- Bottom up
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Arboreal Hypothesis - Top Down Evidence
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Anatomical structures presumably advantageous for climbing
Long flat tail better for gliding Less energetically expensive |
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Arboreal Hypothesis - Top Down Problems
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No evidence that gliding has evolved into flying in other taxa
Hind limbs of Archaeopteryx appear adapted for running |
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Cursorial Hypothesis- Bottom up Evidence
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Hind limbs appear adapted for running (similar to all theropods)
Wing morphology different from other gliding structures |
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Cursorial Hypothesis- Bottom up Problems
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Flight apparatus of Archaeopteryx may not be well enough developed to permit takeoff from ground
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