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The standard Late Devonian conodont zonation between the top of the Upper marginifera Zone and the earliest Carboniferous Siphonodella sulcata Zone is revised to replace the former velifer , styriacus , and costatus zonal groups, which were based primarily on genera other than Palmatolepis , and to eliminate a hiatus above the Upper costatus Zone. The changes were enabled by a new phylogenetic model of Palmatolepis , by range extensions that bridge former gaps between the records of some Palmatolepis taxa, and by further taxonomic revisions of some double-rowed Bispathodus taxa. Basing the standard zonation on conodonts of primarily pelagic biofacies permits construction of parallel, alternative zonations for shallower water biofacies and estimation of phylogenetically more precise zonal timespans.

Abstract Evaluation of time and correlation work within the Devonian suffer greatly from the lack of internationally agreed upon boundaries. The only boundary of the whole system which has been internationally agreed upon is its lower boundaryan excellent example of a precisely defined boundary based on biochronologic evidence. The IUGS Subcommission on the Silurian-Devonian Boundary which defined it suggested this boundary (the stratotype of which is in Czechslovakia) to be within a time span of between 100,000 to 300,000 years, as estimated from biologic evidence. There is no absolute dating in the Devonian that could compete with such a highly precise biochronologic date. The range of error of any published absolute data is many times larger than the time span represented by this lower boundary of the Devonian.