Vertebrates 2 Flashcards
What are two evolutionary theories?
- Phyletic Gradualism (like uniformtarianism and james hutton), small changes over a long period of time, rapid changes accounted for by erosion of intermeidate forms from the fossil record.
Darwin was thought to be a gradualist, but thought lingula (brachiopods with long range) had gone through short periods of change whcih looks like.. - Punctuated equilibrium: long periods wtih no change intersperced with periods of abrupt change (niles edridge and stephen jay gould). Mostly genetically stable, mutations and variations snuffed out by population unless there was a change in environment which would shove population numbers towards a selected gene.
i.e. peppered moths: selected for color depending on colour of the trees. (not total example cause no new species) OR modern day HIV: drugs come out but some of the HIV are reisistant, so leave the person alone for a while then hit the non-resistant population with a lot of it.
Mostly leaning towards punctuated equilibrium but some microfossils show phyletic gradualism too.
What are transitional forms?
In between species that aren’t quite like either one.
- Archaopteryx: bird/dinosaur with teeth and feathers and claws
- Horses, from Eohippus (in forest) to Equus (on plains)
- Ardpithecus Ramidus: evolved in the forest, when bipedalism was previously thought to evolve in plains (selective advantage for those who could see over the grass) so we aren’t really sure of the story here.
What do tetrapods have?
- have or have had 4 limbs.
- pelvis attached to backbone
- spine with interlocking spurs
- curved rib cage to protect internal organs
- nostrils for breathing
- limbs with 1 -2 5 pattern.
But how did they evolve? We need to find the fishapod!!
Who are the likely ancestors of tetrapods?
Lobe fins! Fins joined to body with one bone like tetrapods, specifically Rhipidistia (eusthenopteron: walking fish) which had similar bone pattern. Humerus, ulna, radius in pectoral fins, femur tibia and fibula in pelvic fin. Limbs like our own.
What was the devonian thought to look like to support this Eusthenopteron idea?
Desert, devoid of vegitation, few open water sources so that the DRIVING FORCE of legs would be fish who had to drag themselves to the next pool over. This would promote limbs.
Who were the 1st tetrapods?
Erik Jarvik found ICHTHYOSTEGA in Greenland. a fish with legs that had 1-2-5 pattern. Could walk on land but was also happy in the water. however it didn’t really look transitional, it looked like a full blown tetrapod! so we had to keep looking, Jenny Clack went to greenland and found some other things: found that Ichthy had 7 digits and that it wasn’t really great at walking on land. and found ACANTHOSTEGA which had 8 toes and limbs protruding from its sides. thought to be paddles rather than limbs for walking. (no ankles/wrists not load bearing hips or pelvis, no for walking)
Why did limbs evolve? hypothesis
walking was an exaptation, limbs initially evolved for navigating NEW devonian interpretation. Navigating swamps and not walking on deserts. devonian sees first forests and swamps, rivers lakes etc.
Tetrapods would have gone to these areas and limbs would have helped them navigate root tangled swamps. These areas were beneficial cause they hid from predators like placoderms (Hyneria 4m long) and had rich nutrient organisms and invertebrates to feed on.
Where do we find the fishapod and what the heck is it?
Catskill clastic wedge: sediments from rising mountains caused by acadian orogeny in devonian and closure of the iapetus ocean. Close to equator. Found by Neil Shubin.
Looked at rocks in upper devonian river deltas and found lots of primitive tetrapods but needed to look further back. Found Canadian Arctic ELLESMERE ISLAND.
TIKTAALIKS: 375ma 1-2m long fish thing with scales, fins and a primitive jaw, but had a flat crocidile like head and reduced webbing on its limbs (only to wrist and hand) - more flexible movement and a mobile neck, sturdy shoulders with lungs. Lived closer to fresh water margins eating small fish and invertebrates. Could “push up” out of the water and get onto land, snapping at passing insects as an ambush predator. good transitional form.
Why are tiktaaliks not the perfect fishapod?
Because trace fossils were found in Zachelmie quarry in poland from MARINE environment of basically a muddy footprint indicating WALKING. which pre-dates tiktaaliks (mid devonian 395ma) and in the wrong environment. Found in intertidal mud flat, which has a rich food source and tons of organic debris.
walked like a salamander probably. BUT its only a trace fossil so we’re still saying tiktaaliks are the fishapods.
