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Recognition of differences in the habitats, apparatuses, and ranges of Late Devonian Icriodus and Pelekysgnathus permits refinement of their biofacies interpretations and construction of an alternate icriodontid zonation. Icriodus is a euphotic genus that predominated in most environments during the early Late Devonian (Frasnian) but died out during the early Famennian. Its apparatus consists of platform (I) elements; four larger, acodiniform cones; and two smaller, oneotodiform, scolopodiform, or drepanodiform cones. Pelekysgnathus is a shallow-water genus, which shortly after Icriodus died out, produced somewhat deeper water taxa with triple-rowed I elements that are homeomorphs of Icriodus I elements. Apparatuses for both single-rowed taxa (Pelekysgnathus) and triple-rowed taxa (“Icriodus”) contain oistodiform cones, apparently in place of one or more of the acodiniform cones. Biofacies models for southern Belgium and Utah show that Icriodus lived not only in nearshore environments but ranged into the pelagic palmatolepid-bispathodid (I) and palmatolepid-polygnathid (II) biofacies. Younger “Icriodus”, however, inhabited mainly the deeper subtidal polygnathid-“icriodid” (III) and polygnathid-pelekysgnathid (IV) biofacies. Pelekysgnathus inhabited mainly the polygnathid-pelekysgnathid biofacies and ranged shoreward into the shallow-subtidal clydagnathid (V), scaphignathid (VI), patrognathid (VII), and pandorinellinid (VIII) biofacies, but has not yet been found in the hypersaline antognathid (IX) biofacies (new).

The Late Devonian, subdivided by 28 mainly Palmatolepis-based zones from the Lower (Polygnathus) asymmetricus to Upper (Siphonodella) praesulcata Zones in the standard conodont zonation for pelagic biofacies, can be subdivided into nine icriodontid-based zones in nearshore biofacies. In ascending order, these are the Icriodus symmetricus; Lower and Upper Pelekysgnathus planus; Lower, Middle, and Upper “I.” cornutus; and Lower, Middle, and Upper “I.” costatus Zones. Taxonomic revisions involve mainly relegating several previously described species to subspecies and morphotypes and raising some subspecies to species. Two new biostratigraphically significant subspecies, I. iowaensis ancylus n. subsp. and I. alternatus helmsi n. subsp., are recognized to occur both in the western United States and in Europe. Pelekysgnathus brevis n. sp. is described as new on the basis of a Middle Devonian occurrence in Utah.

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