Chances are the timing skipped when that bolt head dropped, possibly into the lower chain gear. If the tensioner plunger wasn't keeping proper pressure on the chain it could have been slapping the bottom of that chain guide which caused the bolt to break. Where is your lower oil pump chain guide? The oil pump chain looks aweful loose and if that was skipping teeth periodically, could explain why the plunger didn't keep proper tension. Just a thought.

-Todd
1987 Montero 2.6l 5-speed (Sold...but I still miss her)
2008 Jeep Patriot Sport 4x4