Siphuncular structure in the Recent Nautilus, compared with that in Mesozoic nautilids and ammonoids from Madagascar

Abstract

The connecting ring in the Recent Nautilus is composed of two layers: a fibrous, organic inner layer, and a porous, spherulitic-prismatic outer layer. The organic layer is an uncalcified continuation from the nacreous layer of the septal neck. In the Mesozoic nautilids Cymatoceras and Paracenoceras from Madagascar, that have the Nautilus type of siphuncles, the inner fibrous-organic layer of the connecting ring has been destroyed by diagenesis. The outer spherulitic-prismatic layer is twice as thick as in Nautilus, which indicates that, in addition to jet-powered swimming, they also used short-term buoyancy changes for vertical migrations, and that the mode of life of these nautilids was somewhat different from that of the Recent Nautilus. Moreover, the Madagascar nautilids lack the auxiliary ridge by which the connecting ring in Nautilus is firmly attached to the inner surface of the preceding septal neck. The siphuncle in Nautilus can therefore withstand a much higher hydrostatic pressure than that in the fossil nautilids that lacked this deposit. In ammonoids the fibrous-organic layer of the connecting ring has a solid, semi elastic structure. In all 11 ammonoid taxa from Madagascar here studied, this layer is preserved whereas the nautilids from the same stratigraphic horizons have lost this layer. This clearly indicates that the fibrous-organic layer of the connecting ring in nautilids is chemically different from that in ammonoids.