The Taphrognathus transatlanticus, Clydagnathus windsorensis, and Gnathodus zones, initially defined in the Codroy Group of Newfoundland, were identified in the Havre-aux-Maisons Formation of the Windsor Group of the Magdalen Islands, Quebec, Canada. Recognition of these zones permits correlation within the islands and with Newfoundland and Nova Scotia. The Taphrognathus transatlanticus and Clydagnathus windsorensis zones correlate with the lower and upper macrofaunal B Subzone, respectively, whereas the Gnathodus Zone corresponds to the C, D, and E macrofaunal subzones.Tectonism and plastic movement of sulphates and chlorides have resulted in fragmentation and transport of parts of the Havre-aux-Maisons Formation. The conodont zonation complements the macrofaunal zonation permitting correlation of fragmented blocks. When macrofaunas are scarce or absent the conodont zonation provides an alternative to one based on macrofauna only. It has been used effectively in assessing previous zonal determinations based on macrofauna.Conodonts of the Magdalen Islands are dominated by Clydagnathus, Cavusgnathus, Mestognathus, and Taphrognathus, all asymmetric cavusgnathiform genera. This dominance, the total lack of Gnathodus, abundant sulphates and chlorides, as well as foraminiferal and algal evidence, suggest that Lower Carboniferous conodonts of the Magdalen Islands lived in shallow-water shelf environments under conditions of fluctuating salinity.

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