Rob, I am speculating here, but I cant imagine that a cog like that would have a metal failure like that, I wonder if the bolts that attached it to the hub had become loose, and the back and forth loose movement did this (although one would think you would feel a strange "delay" if they were really loose--who knows, but wow, what a weird failure.
how many bolts held this cog onto the hub?
and what a curious design of the shaped parts swirly shaped that lead to the missing inner part with the attachment bolt or bolts. (doesnt look structurally strong to this non engineers eyes)