京都大学21世紀COEプログラム 活地球圏の変動解明 アジア・オセアニアから世界への発信

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Seminar Series of Active Geosphere #32

T. Satomura

Did Ammonoid Carcasses Surface or Sink?

by
Haruyoshi MAEDA
Graduate School of Science, Kyoto University

To be preserved as fossils, how and why did ammonoid carcasses sink to the seafloor after death? This amateurish and primitive question should not be neglected because it establishes the most essential bridgehead for ammonoid taphonomy. The present talk focuses this point.
In life, ammonoid animals probably managed to maintain neutral buoyancy by way of the osmotic cameral emptying as does the living Nautilus. After death, sea water could penetrate into the under-pressurized camerae by way of the natural permeability of the siphuncular sheath, i.e., waterlogging. Volume of cameral gas might have decreased until gas pressure within the camerae equaled the ambient hydrostatic pressure. As cameral gas produced buoyancy, a certain critical depth (= ambient pressure) might divide their postmortem histories into two taphonomic pathways, the never-surfacing and the surfacing ones as marine vertebrates (cameral gas vs. decay gas).
Most ammonoids with no signs of damage or puncture and/or mouth apparatus in situ probably represent the never-surfacing pathway under high ambient pressure below the depth limit. Their distributional patterns are more or less facies-controlled. Like most ammonoids, Gaudryceras from the offshore mudstone facies in the Cretaceous Yezo Group well represents the never-surfacing pathway.
In contrast, phylloceratids such as Cretaceous Hypophylloceras usually lack a body chamber and show signs of phragmocone puncture. Unlike Gaudryceras, their distributional patterns are widespread and facies-independent. These features may suggest 1) their shallow-water habitat above the depth limit or slow waterlogging owing to their peculiar long amphichoanitic septal necks, and 2) surfacing and floating first, followed by resinkig by cameral puncture.

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