Shell structures in Carboniferous bactritid-like coleoids (Cephalopoda) from South Central USA

Morphological features of the shell including internal structures of the phragmocones of three different bactritoid cephalopod taxa assigned to the Order Bactritida: Bactrites quadrilineatus, Ctenobactrites lesliensis (L. Carboniferous) and the Order Parabactritida: Rugobactrites jacksboroensis (U. Carboniferous) were studied. The material comes from South Central USA. The features examined include the conch shape, siphuncle shape and ultrastructure of the shell wall, septa, septal neck and the connecting ring. These traits were then compared with Shimanskya postremus from Upper Carboniferous of Texas, which has been originally referred to bactritids and later placed within the coleoids in the Order Spirulida. Based on similarities and differences observed, B. quadrilineatus and C. lesliensis are now also considered being coleoids similar to S. postremus; R. jacksboroensis remains placed in the Order Parabactritida.