Coiled nautiloids from the lower Carboniferous Fayetteville Formation in Arkansas, Midcontinent North America

As the first installment of our study to clarify the cephalopod fauna of the Fayetteville Formation (lower Carboniferous; middle Chesterian) in northwest to north-central Arkansas, Midcontinent North America, eight species of coiled nautiloids are described from the lower and upper shale units. They include a tainoceratid, Tylonautilus gratiosus (Girty), the koninckioceratids, Endolobus clorensis Collinson, E. sp., Valhallites tuberculatus sp. nov. and V westforkensis sp. nov., a trigonoceratid, Epistroboceras nzangeri sp. nov., and the liroceratids, Liroceras? sp. and Bistrialites bicostatus (Gordon). Only T gratiosus and B. bicostatus survived into the late Chesterian. Embryonic to early juvenile shell morphologies of T gratiosus are revealed and suggest close phylogenetic relationships between this genus and the trigonoceratid genera. Morphologies of new material of B. bicostatus warrant the present new combination for this species, which was questionably placed in Liroceras by Gordon in 1964.