Meaning of acritarch

ac•ri•tarch

Pronunciation: (ak'ri-tärk"), [key]
— n. Paleontol.
  1. any of various microfossils, of unknown biological affinities, having a central cavity enclosed by a wall of chiefly organic composition.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.