(Ive been told summoning is always the highest chainlink and therefore always happens first, but i figure its best to confirm this for sure)
That isn't how Chains work.
A Chain is created when a card/effect is activated in response to another card/effect.
To use your scenario: If Dust Tornado is activated, then the Paleozoic card is activated in response, then the Chain is as follows:
Chain Link 1: Dust Tornado; target something (and nothing else at this moment in time)
Chain Link 2: Paleozoic
You continue building the Chain until neither player has anything they wish to add to the Chain.
After that, you resolve the Chain backwards.
Chain Link 2: Paleozoic (carry out the effect that was activated here)
Chain Link 1: Dust Tornado, destroy the target (if possible)
MadRest already described how the Chain you provided resolves, so I'll get to my main point, which was: An effect being able to Summon does not magically make it the highest Chain Link. It depends on how the Chain is built.