assemblage of microscopic fossil structures uncovered in the Bitter Springs Formation, a rock layer about 800,000,000 years old exposed in central Australia. Collections first made in 1965 revealed at least four general groups of organisms that possibly inhabited shallow seas of central Australia in Late Precambrian times (ending about 540,000,000 years ago); these groups resemble bacteria, filamentous blue-green algae, green algae, and fungi. The demonstration of cell division in a fossil green alga named Glenobotrydion is evidence that an evolutionary stage that would later lead to sexual reproduction and genetic variation already had been attained.