The tetrapods and invading land

Question 1

Tetrapod forms

Answer 1

Dating back to 365 may were…
Acanthostega and Ichthyostega
Then link form was discovered at 383mya
Tiktaalik

Question 2

Acanthostega and Ichthyostega features

Answer 2

•4 limbs
•appendages quite robust
•reasonable ribs - ichthyostega having even larger ribs and well developed vertebral column (limbs attach to pelvic region - more derived)
•thought to have gills as would’ve been aquatic - ruminants of operculum and lungs found
•robust limbs allowed to pull selves out of water, allowing movement onto at least banks
-icthyostega has paddle-like rear limb, good at swimming

Question 3

Tiktaalik features

Answer 3

•reduced gill covers - suggests reduced water flow through gills and less reliant on
-flexible neck, could have raised its head…
•ribs are larger - could support outside water on land
•longer snout - rather than sucking up prey, snapping it up
•prop-like flexible pectoral fins - pushing itself up, feeding on items close to shore (still fish - wouldn’t have walked on land)

Question 4

Early tetrapods lived in water and their features showed this

Answer 4

•allows them to move and sit in shallow water, taking prey from there
•all features developed in an aquatic E - later became useful terrestrial - event of tetrapods and moving to land are 2 DIFFERENT THINGS
•having fins to move on surface seen in teleosts = advantageous
e.g. epaulette shark - moving on reefs
•eel catfish = flexible neck gaining food from terrestrial E
•mudskipper - fins for moving on surface in terrestrial E

Question 5

Fossil tracks?

Answer 5

Found 395mya…
•10mys before Panderichthys was dated back to… (fish)
•some think rock incorrect type for this kind of fossil to form in
•no fossils of tetrapods found in this area - something else?
•other tracks have been found, slightly younger than these, other tetrapods?

Question 6

No. of digits in early tetrapods

Answer 6

•Acanthostega had 8 toes on front and hind feet
•Ichthyostega had 7 on hind
-polydactyly
•Branching leads to digit formation - little finger branches in different way
-in past just kept branching, selective advantage of polydactyly
•fins and limbs are homologous
•some tetrapods have lost digits - birds and mammals (specialisation for locomotion) e.g. ostriches have 2 toes

Question 7

Methods of losing digits

Answer 7

1. Reduced expression of gene ptch1
2. Digits start forming and then cell death occurs
   - processes evolved convergently
   e. g. horses and jurboas have both lost digits via same method - but not closely related
   e. g. artiodactyls have some cell death occurring and others reduced expression - no real pattern
   - all mammals have come to digit loss independently

Question 8

Theories behind who the amphibians evolved from

Answer 8

1. Temnospondyls (diverse and more aquatic forms - probably had gills and lungs), gave rise to Lissamphibia and then the rest
2. Lesopondyli (some were legless like caecilians) gave rise to Gymnophiona (caecilians)
   - both debatable

Question 9

2 main groups of early tetrapods

Answer 9

1. Batrachomorphs - aquatic forms that gave rise to the amphibians
2. Reptilomorphs (or anthracosaurs) gave rise to amniotes

Question 10

Transition from water to land

Answer 10

1. Protect selves from solar radiation
   - chromatophores in skin
2. Less support to of water
   - strong skeleton and muscles
3. Transformation of eye and hearing appartus
   - eye more complex, lens flattens, eyelids and tear glands evolve
   - tympanic membrane (eardrum)
4. Need for gaseous exchange without water loss (amphibian skin vascularised = cutaneous and buccopharyngeal respiration use skin and mouth, but lungs primary site)
5. Water limited on land - keratin and mucous in skin, in damp places, excrete urea or uric acid
6. Reproduction - have to return to water to breed