They were smaller, denser, different in composition from the rib that surrounded them.

They had been driven into the rib from outside — not broken off from the rib itself, not the result of fracture during excavation, not a natural intrusion of sediment material into a post-depositional crack.

The geometry was wrong for all of those explanations.

The object, or what remained of it, had entered the rib from one direction.

It had penetrated through the outer cortex.

It had stopped inside the cancellous interior.