Chris - I am not an expert by any means. However I did a brief apprenticeship with a diesel engine manufacturer and I have seen this sort of damage due to bolts on the big-end of the con-rod failing. This causes the bolt to fly out, often taking chunks of the crankcase with it. Now, with the con-rod free to move, it often used to foul with an adjoining conrod causing that one to fail as well.
Can you check if there is obvious bolt failure?
Apologies if I am talking bollocks