Carboniferous orthocerid-like coleoid Mitorthoceras with unique shell structure (new order Tuborthocerida, Coleoidea, Cephalopoda); comparison with bactritid-like coleoid Shimanskya

Abstract

ABSTRACTThe Carboniferous genus Mitorthoceras from USA, previously assigned to orthocerids, is here restudied and assigned to the new coleoid order Tuborthocerida. Mitorthoceras has several orthocerid characteristics: longiconic shell with long body chamber; central or subcentral siphuncle containing endosiphuncular deposits; and a tubular, uni-layered connecting ring probably consisting of mixed calcareous and chitinous elements. Coleoid characteristics are: shell wall rich in a chitinous material, and lack of a nacreous layer. Characteristic for the new order is the unique structure of the outer layer of the shell wall; this layer is composed of numerous, narrow, transverse furrows, separated by distally expanding ridges and closed from the exterior by thin calcareous layers, transforming the furrows into tube-like structures. The apical end of the shell forms an acute tip. A similar tube-like furrow-layer probably occurs within the shell wall of the bactritoid-like coleoid Shimanskya that has a smooth shell surface.