Siphuncular structures in Calciosiphonate nautiloid orders Actinocerida, Orthocerida and Barrandeocerida (Cephalopoda)

摘要

The calciosiphonate type of the connecting ring was probably present in the oldest known nautiloid cephalopods, plectronocerids, from the Late Cambrian. This type also occurs in the nautiloid orders Actinocerida, Orthocerida and Barrandeocerida that appeared in the Ordovician. The calciosiphonate connecting ring is composed of two calcareous layers: the outer, spherulitic-prismatic layer, and the inner, calcified-perforate layer. The calcified-perforate layer contains cavities that are separated by calcified walls. The cavities open into the shell chamber by pore canals. In Adamsoceras holmi (actinocerid) and Orthoceras scabridum (orthocerid), the cavities have large diameters and thin calcareous walls, whereas in Archigeisonoceras folkeslundense (orthocerid), and probably also in Danococeras gotlandense (orthocerid) and Boionautilus sp. (barrandeocerid), the cavities have small diameters and thick calcareous walls. The structure of the connecting rings is discussed and reconstructed.