What evolutionary or physiological components give snakes' heads their characteristic, deltoid shape?

DNA

[filler characters]

In reply to 'harveyh,' don't some lizards have similar shaped heads to snakes? And surely without becoming unhinged?

The ability to unhinge their jaw so they can eat prey larger than their head. 

Having the same head shape does not mean that the function is the same, in the case of lizards that shape is likely the result of the needing  to have a specific shape for the muscles used to feed on food. crushing jaws need more muscles and thus wider and strong attachment points which increase the size of the skull for jaw muscles attachment. For other lizards they live in  tight spaces and benefit when the head is more flattened and thus wider, in which case the wider area of the head when flattened out gives it this type of shape.

In the case of  many snakes they need a wider, diamond-shaped head so the muscles can detach the jaw and then again realigned it after eating.  

Head shape is most often driven by the need for eating and the placement of sensory organs.