co-discoverer vs spotter

co-discoverer and spotter both are nouns.

Word	Noun	Adjective	Verb	Adverb
co-discoverer	Yes	No	No	No
spotter	Yes	No	No	No

As nouns, spotter is a hypernym of co-discoverer; that is, spotter is a word with a broader meaning than co-discoverer:

co-discoverer: someone who is the first of two or more people to discover something
spotter: someone who is the first to observe something

Other hypernyms of co-discoverer include discoverer, finder.

co-discoverer (noun)	spotter (noun)
someone who is the first of two or more people to discover something	someone who is the first to observe something
	a worker employed to apply spots (as markers or identifiers)
	a worker employed at a dry-cleaning establishment to remove spots
	a person employed to keep watch for some anticipated event