Article: Phragmocone implosion in Ordovician nautiloids and the function of siphonal diaphragms and endocones
David H. Evans
Two species of nautiloid - a new genus and species of Cyclostomiceratidae and a new species of Dideroceras from the Llanvirn Series of Wales and western Shropshire - show preservational features which suggest that the adapical portions of the phragmocones imploded after the death of the organism. The limit of phragmocone damage is close to the most adoral development of siphonal diaphragms or endocones. It is thought that the apical part of the phragmocone was partly or completely filled with gas, failing to flood or flooding very slowly after the death of the organism. Either increased hydrostatic pressure or a decrease in shell strength, or a combination of both factors, brought about phragmocone failure. The function of siphonal diaphragms, and in part that of endocones, is interpreted as reducing the metabolic cost of pumping liquid away from camerae which would otherwise become flooded, and would consequently disturb the organism's hydrostatic equilibrium.