Sphaeroidal unicellular fossils 10 to 62 μm in diameter are abundant in black shales referred to the Stoer Formation of lower Torridonian age at Stoer Bay, in northwest Scotland. The Rb-Sr whole-rock age is estimated as about 935 m.y. The fossils are assigned to the leiosphaeridacean acritarch genera Favososphaeridium and Protosphaeridium, common in younger pre-Paleozoic rocks of the U.S.S.R. Their biological affinities are obscure, but it is consistent with what is known so far of their associations and distribution to consider them as some form of marine phytoplankton.

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