Brian K. Holdsworth, David L. Jones; Preliminary radiolarian zonation for Late Devonian through Permian time. Geology ; 8 (6): 281–285. doi: https://doi.org/10.1130/0091-7613(1980)8<281:PRZFLD>2.0.CO;2
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Nine time-successive radiolarian assemblages of wide geographic distribution are recognizable in Upper Devonian through Permian rocks. Identification of assemblages in bedded chert terranes provides a means of dating where other evidence is lacking. Changes in nature of these radiolarian populations with time possibly coincided with inferred fluctuations of world sea level.