EVOLUTION OF THE TETRAPODS Flashcards
What are tetrapodmorpha fishes?
- Those that gave rise to the tetropods
- Vertebrate fish with early tetrapod features
- Such as 4 limbs
What made the tetrapodomorpha fishes different from other fish?
- Changes in spiracular regions (different from other fish)
- Beginnings of tetrapod middle ear
- Had gills and lungs
- 4 limbs
Groups of tetropodomorph fishes
- Osteolepiforms - Eusthenopteron
- Well developed fins
- Short ribs extended dorsally
what are the features of the more derived forms of tetropodomorph fish?
- Elpistostegalians - Panderichthys
- Eyes on top of head
- Reduced tail
- Dorsoventrally flattened
- No dorsal or anal fins
- Large lateral ventral ribs
- Still had paired fins
- Possibly used fins to move around in shallow water
- Heavy bodied
What are the features of the Tiktaalik?
- Reduced gill covers - reduced water flow through gills
- Longer neck & could raise its head & long snout - snapping up prey
- Ribs large & could lift chest & move head
- Flexible pectoral fins - prop? Wrist bones - homologous with wrist bones seen in tetrapods, but still had fin rays (fins).
- Recently discovered
- Derived species
Give some examples of early Tetrapods
- Ichthyostega
- Acanthostega
- Date from 365 MYA (Greenland)
- Primarily aquatic (gills)
- Suggestion there was an opurculum
- Fore and hind limbs
- Hind limbs had contact with vertebral column
- Ichthyostega more specialised (more robust skeleton)
How was Ichthyostega specialised?
Early Tetrapods
- Paddle-like back legs (good for swimming?)
- Adapted hearing? Hearing under water?
- Robust ribs, hauling out of water ?
What are the features of Parmastega alidae
Basal tetrapod
- Large eyes positioned on top of head - looking out across water
- Nostrils (nares) positioned under the water (suggests it used gills)
- Had a spiracle - possibly used for air breathing
- Large teeth - may have fed on larger vertebrates
How were early tetrapods adapted for aquatic and terrestrial environments?
- Early tetrapods were aquatic
- Some fish use their fins to move around (walk?)
- Eel catfish - flexible neck - able to come out of water
- Mudskippers - Flexible neck and use fins for locomotion on land
- Believed that Ichthyostega moved like a mudskipper
- Different modes of respiration in early tetrapods
Early tetrapods were adapted for aquatic life how did this change?
- Features became advantageous for land
- Evolution of tetrapods and evolution of terrestrial life were 2 seperate events.
How did fossilised tracks change the opinion on when tetrapods evolved?
Tracks found from 395 MYA - predate early tetrapod
- Some debate about them
- More complicated picture
- Maybe an earlier diversification of tetrapods
What is different about early tetrapod digit formation?
- Polydactyl
- More than 5 digits
Examples of Polydactyl early tetrapods
- Acanthostega - 8 toes on front & hind feet
- Ichthyostega - 7 toes on hind feet. Forefeet unknown
How are digits formed?
Postaxial Formation:
- starts on digit 4 (ring finture)
- then 3, 2, 1(thumb),
- 5 (little finger) forms last
- Can get further digit addition
Preaxial branching forms radius etc
How are limbs and fins homologous?
- Coded by the same set of genes
- Hoxd13 expressed in postaxial side of fin
- Alx4 expressed in preaxial side of fin
Same sets of genes but differential expression in the tetrapods
