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Iso communities and correlation of the north american didymograptus bifidus zone ordovician

Iso communities and correlation of the north american didymograptus bifidus zone ordovician

Newsletters on Stratigraphy 6(2): 85-96

Some of the current problems related to inter- and intra-continental chrono-correlations of Lower and Middle Ordovician North American strata are due to the practice of basing such correlations on generic identifications of trilobites and/or brachiopods. Two or more associations of fossils where generic compositions are similar but species differ and when evolutionary relationships between such assemblages are not established are referred to as isocommunities (Bergstrom 1973). The potential fallacy of using isocommunities for chrono-correlations is discussed and illustrated by showing how Zone M of the Ross (1951)-Hintze (1952) trilobite-zonation for Utah [USA], which is supposedly largely correlative with the North American Didymograptus bifidus Zone of Berry (1960 and later), spans the interval middle Arenig to lower Llanvirn when identifed on the basis of generic similarity of faunas. Independent conodont evidence also supports a correlation suggesting that the North American D. bifidus, Isograptus caduceus and D. protobifidus Zone are partly or entirely (D. protobifidus Zone) within the range of the British D. nitidus Zone.

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