Late Cambrian Plectronocerid Nautiloids and Their Role in Cephalopod Evolution

Abstract

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.

Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules

“…Slightly younger cephalopod fossils are widespread and diverse, suggesting that within a time interval of a couple of millions of years [27], cephalopods began to flourish in the latest Cambrian. The shell morphology of these most primitive forms provides direct evidence for the presence of a Nautilus-type buoyancy regulation already in Plectronoceras [28,29].…”

“…C: shared embryonic features in embryos of Nautilus (Nautiloidea) and Idiosepius (Coleoidea) (simplified from Shigeno et al 2008 [23] on top, as in other molluscs. Also, the funnel was posteriorly positioned, resulting in backwards exhalation and forward direction of locomotion [28,29] (Figs. 3C,D).…”

Ancestry, Origin and Early Evolution of Ammonoids

“…Never theless, it appears like the monoplacophorans are the best candidate as extant sister group (Kröger et al 2011 and references therein). In contrast to earlier views, the oldest generally accepted cephalopod fossil is Plectronoceras cambria Walcott, 1905 from the middle Late Cambrian (Glaessner 1976;Dzik 1981;Kröger 2007;Mutvei et al 2007;Landing and Kröger 2009;Mazurek and Zatoń 2011;. It possessed a small (< 2 cm) simple breviconic (short conical) shell with a subventral ('posterior' sensu Kröger 2007) siphuncle and about ten septa (Webers and Yochelson 1989).…”

The oldest record of Eothinoceratidae (Ellesmerocerida, Nautiloidea): Middle Tremadocian of the Cordillera Oriental, NW Argentina

“…Moreover, from an external view, the adoral expansions of the siphuncular segments are rounded rather than acute as seen in other species of this genus (e.g. Bathmoceras llanvirnense (Roberts) in Evans, 2005; B. linnarsoni Angelin in Mutvei et al, 2007). Considering these aspects of the siphuncle morphology, Bathmoceras australe could be included in the family Eothinoceratidae.…”