Earth Sciences 2266F/G Lecture Notes - Lecture 12: Lissamphibia, Eusthenopteron, Tiktaalik

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Lec 12
Basal tetrapods is the term used for amphibians
In modern cladistics, amphibians are paraphyletic
If you look at living amphibians, including frog, toad, salamander, they lost their limbs in
evolution looking like an earthworm, but they still have vertebrae; they just went through
degenerative evolution
None of the modern amphibians are in the direct lineage to evolving to reptiles (only ancestors)
Modern amphibians and reptiles come from a common ancestor near the Triassic-Permian
boundary – NOT direct lineage
You can trace modern amphibians to temnospondyls
In one group, reptiliomorph, look like half amphibian half reptile, and these are the beginning of
reptiles, will eventually lay eggs (amniota)
But we are going to look at the first part, where fish evolves into amphibians
- Good fossils in early Cretaceous, so ancestors must be earlier
- Salamander with long tail, and long, broad head
Transition was much earlier from fish to amphibian
Good fossils found in Devonian, mostly in Greenland
During the transition, these are the problems we have to solve from going from water to land
Gravity: in water, fish have neutral buoyancy (cartilaginous naturally float; bony fish have swim
bladder), but once they’re on land they have to support their entire body weight so there must be
body modifications for them to move: stronger, elongated limbs
Food: early tetrapods still probably derived their food from water
Breathing: fish solved this problem as lungfish have lungs
Excretion/desiccation: if you look at salamander/frog, they have thin, moist skin so they can’t
stay out of water for too long, so having to urinate is losing too much water
Senses and hearing: sight is okay since they have eyes; fish can sense environment using hair
cells along lateral line, but once they’re out of the water, electromagnetic waves are very
different and don’t transmit as well as water
Have to develop more sensitive organs to hear things
Reduction of hyomandibular bone
In Eusthenopteron (Osteolepiformes), hyomandibular still pretty large
In Tiktaalik, much smaller
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