Several species of conodonts in the Columbus and Delaware limestones of north-central Ohio are segregated stratigraphically and form associations that correspond fairly well to lithofacies in which they occur. Polygnathus linguiformis , P. angusticostatus , P. angustipennatus , P. intermedius , and Panderodus sp. appear to have been benthic creatures sensitive to substrate texture and energy levels. Other species of Polygnathus may have been benthic, but a nektonic habit is possible for some. Prioniodina tortoides preferred deep-water conditions but was likely nektonic. Simple-cone taxa other than Panderodus and species of Icriodus are ubiquitous and considered to have been shallow-pelagic plankton or nekton. Icriodus orri shows some preference for near-shore environments. Associations similar to some in this area occur in New York, central Ohio, and southwestern Ontario. Endemic forms appear to define a faunal realm limited to the Illinois, Appalachian, and southeastern Michigan basins that is distinct from a realm extending westward and northwestward from the northern to westernmost Michigan Basin. Barriers to intermixing of endemics involved tectonic elements but also hypersalinity and other environmental factors.

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.