Love those pictures!
I don't know much about sea level/climate but let's see what I can remember;
Sea level used to be much higher than it is now, once upon a time there used to be a lot of shallow seas with the occasional islands, much as you have drawn. The UK (where I am basing this idea) used to be one of those shallow island/sea places.
(/sigh, those were the days, when the UK floated down at the equator.)
Then again you get ice ages, the UK used to be joined to France for instance, look at the continental shelves if you want a better idea of how America and Africa used to be joined.
Ice sheets also had a big impact on the land above sea, Scotland is rising atm while the south of England is sinking due to the last ice age and the removal of the Scottish ice cap.
I'm not sure of the relation between ice (reflects sunlight) and water (absorbs heat) though, guess other factors must play a part.
Your seymouria looks like it is turning into a giant newt! Love to see where it gets to next and know more about the evolutionary pressures that sent it in that direction.
There was a cool program on the BBC about dinosaur evolution, sorry I can't remember the name, I'll see if I can find it for you.