Not the woodpecker!

No idea what caused these curious marks on this elm tree which is clearly dead, but several such trees had the same holes in the bark.
I've never seen anything like this on a tree before and wonder how they got there... if they could perhaps be insect damage...? scarily big insects if so. (see extra, which is a close up not an insect!!!)
They weren't caused by the woodpeckers, which I did see today, several pairs of the greater spotted variety, beautiful flashes of red in the trees and as they flew overhead, no photo though sadly.