LATE CAMBRIAN PLECTRONOCERID NAUTILOIDS AND THEIR ROLE IN CEPHALOPOD EVOLUTION
Numerous plectronocerid nautiloids appear in the Upper Cambrian of China. We have restudied their siphuncular structure, first described some 20 years ago. The siphuncle is characterized by: (1) long and holochoanitic septal necks dorsally but short and recurved necks laterally and ventrally; (2) strongly expanded connecting rings laterally; (3) two calcified layers in each connecting ring, outer spherulitic-prismatic and inner compact, the latter perforated by numerous pore canals; and (4) highly oblique siphuncular segments. The strongly expanded lateral sides of the connecting rings, together with the highly oblique course of the siphuncular segments, considerably enlarged the surface area of the connecting rings in each chamber, thereby increasing the transport capacity of cameral liquid. Thus, from their first appearance, plectronocerid nautiloids had developed a siphuncle for the replacement of cameral liquid with gases, and this system had a better and a more sophisticated design than that seen in stratigraphically younger nautiloids. However, their small orthoconic or slightly cyrtoconic shells were not well adapted for jet-powered swimming.
Document Type: Research Article
Affiliations: 1: Swedish Museum of Natural History, Department of Palaeozoology, Box 50007, 10405 Stockholm, Sweden; s: ; [email protected], Email: [email protected] 2: Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;, Email: [email protected]
Publication date: 01 November 2007