acritarch
noun
ac·ri·tarch ˈa-kri-ˌtärk 
: any of a group of fossil one-celled marine planktonic organisms of uncertain and possibly various taxonomic affinities held to represent the earliest known eukaryotes
Word History
Etymology
Greek ákritos "confused, uncertain" (from a- a- entry 2 + kritós, verbal adjective of krī́nein "to separate, decide") + archḗ "beginning" — more at certain entry 1, arch- entry 1
First Known Use
1963, in the meaning defined above