The friend who forwarded this picture said it was a connecting rod from a large shipboard Diesel engine.

Some of these huge engines use articulated connecting rods, so that the upper half of the rod connects the piston pin to a sliding mechanism whose motion parallels the up-and-down movement of the piston. The lower half of the rod (which I think this is) connects the slider to the crankshaft. They do this to control the side-to-side loading between piston and cylinder liner.

I can't conceive of this failure happening at startup; but if the engine has been running at full throttle for a while, a hydraulic lock (say a cracked cylinder head suddenly leaking water into the cylinder) would do it, I think. Just consider how much inertia the rotating mass has, added to the full output of the engine!

Looking at this a little more closely; I can see that the stud that used to retain the missing rod cap is bent, and that side of the eye is mangled. I wonder if the nut didn't loosen and spin itself off, followed by catastrophic failure of the rod.