How does this happen??
 

Went to change my transmission oil and found this toothless wonder staring back at me....

It looks like another example of living long enough and seeing it all...And apparently you're living right if you escaped damage free...:thumbsup:...Ben

In my opinion you are risking more (possibly catastrophic) damage if you try to glue that piece back on. The load on that area of the tooth is pretty high and I seriously doubt if any glue, epoxy etc has much chance of holding things together for very long. As Ben said, you got of lucky this time. Next time [whistle][guitar] Think that you have two choices here, drive it as and hope that nothing else breaks, or install a new gear.
Are you positive that there are no other pieces at the bottom of the case? When I look at the picture I see what looks like a gouge on the RH end of the damage. That makes me suspect that something else (FOD) got in there and broke the tooth. Also take a close look at the the end of the tooth above the chipped one. Is that a crack I see?

Plan is to run it for now and then take it in the new year to be rebuilt with a new gear.