Resolving polyphyly within the Endocerida: The Bisonocerida nov., a new order of early palaeozoic nautiloids

摘要

Although the Early Palaeozoic cephalopod order Endocerida is generally regarded as monophyletic, alternative views have been expressed. These invoke origins from different lineages within the order Ellesmerocerida. To test these alternatives, various aspects of endocerid morphology (including conch shape, structure of the ectosiphuncle, form of the endosiphuncular deposits, nature of the apical portion of the conch, and distribution of muscle attachment scars) are reviewed and compared with those of potentially related ellesmerocerids, taking regard of their stratigraphical distribution. The taxonomic distribution of endocones and endocone-like structures, combined with the potential for endosiphuncular diaphragms to evolve into endocones, suggests that the presence of endocones cannot be used to diagnose the Endocerida. Two distinct groups of Early Ordovician cephalopods that bear endocones may be recognised. One includes the longiconic Proterocameroceratidae possessing septal necks of variable length, conical and simple endocones, unflated apical portion of the siphuncle, and probable dorsomyarian muscle attachment scars. The second group includes the Piloceratidae, Manchuroceratidae and younger Allotrioceratidae and Najaceratidae. All are characterised by complex endosiphuncular deposits of endocones with conchiolin and calcareous crests, holochoanitic to macrochoanitic septal necks, and probable oncomyarian muscle attachment scars. The earliest pilocerids possess compressed cyrtochoanitic conchs. The coeval appearance of these groups and the similarity of each group to distinct ellesmerocerid taxa suggest that the Endocerida are polyphyletic, and a new order, the Bisonocerida is proposed for this second group.