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Lecture 7

BIOC50H3 Lecture Notes - Lecture 7: Mosasaur, Tiktaalik, Allantois

Biological Sciences
Course Code
Jason Weir

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Lec 7 – Tetrapod Evolution and diversification
- Shallow waters for ostelepiformes – can raise the upper jaw – adaptation for catching
things in shallow water (crocs can do this as well)
- Lobe fins – set low on the body – may have allowed them to drive after prey on, over, or
through shallow mud banks – making rapid mud trips over surfaces called “land” –
- Evolution is blind to the future – limb-like fins in order to become better fish (adapting)
- Why become a tetrapod – less predation, land plants = more food (more land
vertebrates), predator-free environment, water is drying out (drought)
4 theories
- Adaptation to drought – lobefins to “flop” themselves to the next body of water (flop and
walk) (selection to become better at walking)
- Hunting – using lobefins to hunt for prey then coming back into water – numerous times
enough to become better at it and go onto land more often
- Basking - going to a mud bank to bask = metabolism increase – bask more = more
energy; back first then lobefin evolves to move out of the water = more sun = more
- Reproduction – no predators = reproduction cycle without predators = better chance of
offspring living
- Unknown which one is the correct one – maybe a mix of all 4 theories
Parts are missing from the tree
- Tiktaalik – combines features of fish and ostelepiformes (intermediate)
- Missing link – “fishapod” traits – half-fish, half-tetrapod bones and joints including a
functional wrist joint and radiating, fish-like fins instead of toes; would be able to support
weight on land; half-fish, half-tetrapod ear region
Read this - 405_fish.html
Rib cage – to support weight on land (support the lungs)
Digits – Tiktaalik has ill-formed digits but a formed wrist-like region
Head movement – Tiktaalik and after can move head side by side
Tetrapods: “four footed” - land vertebrates with four limbs, rather than fish fins (amphibians,
reptiles, mammals and birds)
Timing of evolution
- Fossilized tracks of 2 m long tetrapods at 395 mya from Poland described in Nature
2010, suggesting tetrapods predate the oldest fossil by 20 million years
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