that is the crankshaft sprocket key, if you turn the end of the crankshaft you will notice a little groove in it, you will need to tap the key back into the crank with a hammer, you'll notice the key groove on the sprocket for correct timing alignment, your sprocket looks broken, I'd reccomend a new one from the dealer, they are $12

p.s. dont reinstall the sprocket until after you put the oil pump back on