Peculiar mode of occurrence and its preservational history of Polyptychoceras, a Cretaceous heteromorph ammonite

Abstract

Polyptychoceras, an Upper Cretaceous diplomoceratid heteromorph ammonite, commonly preserved in the calcareous nodules with a oblique posture against the bedding plane. In order to interpret this peculiar mode of occurence, detailed observation of fossiliferous nodules, hydrostatic calculations, and sedimentational experiments were carried out. As a conclusion, a presumable taphonomic history of Polyptychoceras spp. is reconstructed as follows: 1) Soon after the death of organism, shells of Polyptychoceras spp. were waterlogged and were laid down on the sea floor just like many shells of other ammonites, 2) when the floor is covered with fine and new sediment, which probably forms a softground and behaves as a viscous fluid, some shells having a highly polarized density like a peculiar growth stage of Polyptychoceras spp. had rotated in the sediments, because the bouyancy acting on a distant point from the center of gravity had considerably increased in such condition, 3) for a while, turning-part of phragmocone in some shells had possibly been jutting out from the sediments, and the part had eroded or sometimes lost away, 4) sooner or later, the shells had entirely buried by the succeeding sediments, and a calcareous nodule had been formed before the sediments around the shells were highly compressed throughout the diagenetic process. The post-burial movement of some ammonite shell, above mentioned, is named as "Zombie model", and this mechanism is possibly applicable to several other heteromorphic ammonite shells buried in the muddy sediments.