Gfyuki

Myr Superion is a 2 mana 5/6 that comes with the following casting restriction:

Spend only mana produced by creatures to cast Myr Superion.

Under most circumstances this wouldn't be ambiguous, but I'm not sure how it interacts with type-changing effects. Specifically, I want to know whether Myr Superion could be cast with mana produced in the following ways:

• 
  Blinkmoth Nexus is tapped for mana, then animated using some other mana.


• 
  Palladium Myr is tapped for mana, then stops being a creature e.g. due to Song of the Dryads.


• 
  Deathrite Shaman is tapped to activate its first ability, which goes on the stack because it is not a mana ability, and then stops being a creature before that ability resolves.

I wasn't able to find any lands which could produce mana using an ability that is not a mana ability, but I suspect I will be able to deduce that hypothetical outcome based on the others listed.

In other words, when casting Myr Superion does a mana source need to be a creature when the ability was activated, when the ability resolved, or when the Superion is cast?

The ability checks what produced the mana. This means that, to be used to cast Myr Superion, the source of the ability that produced the mana must have been a creature at the time the mana was produced (i.e. when the ability resolved).

As @Natedogg2 of #magicjudges-rules said, "Producing the mana, then making the source into a creature does not count."

Yes, the mana source would need to be a creature on the battlefield when the mana was created. Mana is not tied to the object that created it, but has some characteristics based on that object when the mana was created, just most things in magic, the source stops mattering once the effect goes on stack and/or resolves.

Myr Superion does not check the source of the mana in the way you're thinking, but looks for the characteristics of that mana, and the mana "remembers" it's characteristics until it is spent or it empties from the pool, we know this because mana with conditions that is prevented from emptying from the pool by effects like Omnath, Locus of Mana or Kruphix, God of Horizons still keeps any restrictions or other characteristics from turn to turn, even after modified by Kruphix's ability, as seen in the ruling on Kruphix:

If unused mana in your mana pool has any restrictions or riders associated with it (for example, if it was produced by Cavern of Souls), those restrictions or riders will remain associated with that mana when it becomes colorless.