Biogeography, evolutionary relationships, and biostratigraphic significance of Ordovician platform conodonts

Fewer than 15 of the nearly 100 currently recognized Ordovician conodont genera have platform elements in the apparatus, but these are of special interest because they represent the oldest such types known and they include some types of platform developments not known in younger strata. Eight of these genera have their origin and main occurrence in the North Atlantic Province, three in the Midcontinent Province, and three are present in both provinces. The data at hand suggest that especially during Middle Ordovician time, some North Atlantic Province platform taxa invaded parts of the Midcontinent Province, but there is much less evidence of such migrations in the opposite direction. Five evolutionary lineages are recognized and discussed (those of Amorphognathus, Cahabagnathus nom. nov., Eoplacognathus, Icriodella, and Pygodus) and other genera (Complexodus, Nericodus, Polonodus, Prattognathus n. gen., Rhodesognathus, Sagittodontina, Scyphiodus, and Serratognathus) are dealt with in less detail. Species of Amorphognathus, Cahabagnathus, Eoplacognathus, and Pygodus are of major biostratigraphic significance but most other taxa are either too long-ranging or too restricted in their distribution to be useful biostratigraphically. Two new generic designations (Cahabagnathus, Prattognathus) and three new species (Cahabagnathus chazyensis, C. eamesi, Pygodus anitae) are proposed.