Revision, distribution, and extinction of the Middle and early Late Devonian archaeogastropod genera Floyda and Turbonopsis

Abstract

Large palaeotrochid gastropods of the genera Floyda Webster (1905a) and Turbonopsis Grabau and Shimer (1909) occur in the late Frasnian Lime Creek Formation of Iowa. Floyda concentrica was designated as the type species of Floyda by earlier workers (Webster, 1905a; Knight, 1941), but is a junior synonym of F. gigantea (Hall and Whitfield, 1873). Three of five species and subspecies of Floyda described from the Lime Creek (Floyda concentrica, F. concentrica multisinuata, and F. gigantea depressa) are considered synonyms of the type species F. gigantea; the fifth species, F. gigantea hackberryensis, is here reassigned to the closely related genus Turbonopsis. Both F. gigantea and T. hackberryensis are redescribed using the original types and additional hypotype material from the collections of Charles Belanski.Floyda, first known from late Givetian rocks of the Rhenish Slate Mountains in Germany, is widespread in the United States Midcontinent and western North America by early Late Devonian time. Turbonopsis was endemic to the Appohimchi Subregion of the Eastern Americas Realm prior to the Taghanic Onlap, and appears to have remained so until late Frasnian time when it migrated to western North America.Eustatic sea-level highstands during the Middle and Late Devonian are thought to have breached barriers to migration, allowing both Floyda and Turbonopsis to disperse by prevailing oceanic currents from the United States Midcontinent into western North America during the late Frasnian. The expected oceanic current patterns of the Middle and Late Devonian paleogeographic reconstructions of Heckel and Witzke (1979, figs. 3, 5) adequately account for the known distribution and dispersal of Devonian palaeotrochid gastropods.The Palaeotrochidae underwent extinction prior to the latest Frasnian. Floyda, Turbonopsis, and Westerna became extinct during the onset of the last eustatic deepening event prior to the close of the Frasnian. The extinction of the palaeotrochid gastropods as well as other invertebrate groups may have been the result of restriction or near elimination of shallow warm-water, well-oxygenated shelf habitats by the onlap of cold anoxic bottom waters prior to latest Frasnian time.