Abstract

New material from the Late Mississippian Fayetteville and Caney Formations of the south-central USA supports the taxonomic revision of the Mississippian actinoceroid cephalopods. New representatives of the previously poorly known Rayonnoceras solidiforme, Campyloceras striatulum, and Campyloceras imoense are described. Carbactinoceras procerum new species and Elmoceras graffhami new genus and species are described, and the genus Campyloceras is emended. Character evaluation suggests the shape of the endosiphuncular deposits in the Actinoceratida and Pseudactinocerida are homoplastic because similar patterns of endosiphuncular deposits occur in distantly related nautiloid clades. Apex shape, however, is shown to have previously unrecognized potential for taxon discrimination. Cladistic analysis of 13 actinoceroid and pseudorthocerid nautiloids supports the splitting of the Orthocerida and the creation of a sister group to the Actinoceratida which is identical with the largely neglected Pseudorthocerida.

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