Not an uncommon thing to happen ive seen it a few times and everytime ive seen 1 go like this the studs for the cam caps have started to pull out of the head, it happened on my mates a few months back we had to helicoil just about every stud in the head as when we were trying to tourque them back down with the new cam in the studs were starting to pull out.

We did a compression test and there was no lost compession on that cylinder, and the engine still runs sweet now so you might be lucky as when the cam snapped the valves should have just snapped shut.