Tough to see in the photo, but it's likely a chunk of the timing gear.
Now, try to figure out if it's from the current gear or one of its predecessors.

If you're not sure, change it. There is no given time that a broken gear will last. Fortunately (sort of) the B18 or B20 is not an interference engine and will simply quit when the gear finally gives out without causing severe damage. But that could ruin your day.

It's not terribly difficult to change if you're relatively handy with tools.
You may consider going with a metal gearset as an alternative to the fiber gear. That's not to say there is anything wrong with the fiber gear, it's just that they're getting to be almost the same cost as a metal set.