REIN, S. (1999): On the swimming abilities of Ceratites DE HAAN and Germanonautilus MOJSISOVICS.-
Freiberger Forschungsheft, C 481, Heft 7, 39-47, 7 Fig., Freiberg.

Abstract

The phragmocone function of ectocochleate cephalopods is discussed from a biomechanical viewpoint using true epibiotism of Placunopsis ostracina Berg on steinkerns of Ceratites De Haan and Germanonautilus Mojsisovics. The phragmocone of the nectonic modern Nautilus is a highly sensitive weight-regulating organ. For maintaining the critical condition of neutral buoyancy it reacts to a weight increase during growth by formation of new camerae. Sometimes an additional weight increase because of a pathological behaviour due to captivity gives rise to the formation of further chambers and a simultaneous shortening of the body chamber as a reaction. Living Ceratites as well as Germanonautilus tolerated a considerable weight increase caused by the settlement of Placunopsis without visible changes of the growing phragmocone. This behaviour is interpreted as an indication that the phragmocone was not necessarily a weight-regulating device of swimmers. Obviously, Ceratites and Germanonautilus even without epizoans attached to their shell were too heavy for a floating lifestyle, and therefore both genera were bottom-dwellers. However, because of their remarkably different shell morphology and the acting hydrostatical laws in detail they differ in their lifestyle. Ceratites had to rely on a steady ground contact for horizontal mobility and had to carry its shell like a hat, whereas Germanonautilus, normally resting on his broad venter, due to its wide hyponomic sinus was capable of actively swimming vertically and horizontally utilizing his powerful funnel thrust.