UCMP MYSTERY FOSSIL #6 shows . . .
early single-celled eukaryotes

UCMP Mystery Fossil Number 6
  Or are they really eukaryotes? Eukaryotes are defined by the presence of a nucleus, but what were originally interpreted as nuclei may simply be the shrunken contents of the cells. If that’s the case, then these might be fossils of cyanobacteria. You can find out more about early eukaryotes here.  

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