Learn English – the proper term for placeholder messages

Normally, you have something to follow:

• And, what's more, he also did that!

With the following text, it is clear that an exclamation mark after the phrase is inappropriate. The dash is not appropriate either - it is used to mark a break of some sort, but the text that follows "what is more" or its contraction "what's more" is a continuation of the previous thinking, not a contrasting thought.

A comma is fine; what's more, it is the punctuation I'd use automatically.

Whether the comma is necessary, or allowable, after the "and" is perhaps a little debatable. As you can see, I'd use it; and, what's more, it feels right because there is a small parenthetical break around the "what's more" part of the phrase.

Ambiguity:

• the characteristic of having more than one possible interpretation or meaning (AHD)

Amphibology:

• a sentence or phrase (as “nothing is good enough for you”) that can be interpreted in more than one way. (M-W)

• the use of ambiguous phrases or such as can be construed in two senses. A good example is Shakespeare's 'The duke yet lives that Henry shall depose' (Henry VI).( Chambers's Twentieth Century Dictionary)

Etymology:

• Gr. amfibolos ambiguous + logos speech.