Link to picture of how they were used for vaulting. Scroll down to drawing.
http://artvent-artventures.blogspot.com/2011/02/archeology-in-sicily-part-iii-earliest.html
Hey, nice! That looks like a good standalone topic.
The earliest domes in the Med though are the corbelled domes (such as the so-called “Treasury of Atreus”) built by the Mycenaean Greeks.