Ammonite Shells as Habitats — Floats or Benthic Islands? (Abstract)

  • A. Seilacher
Conference paper

Abstract

In Erle KAUFFMAN’s view, which has brought so much new air into the stagnant Posidonia Shale discussion, “shell islands” acted as outposts for benthic life on an otherwise uninhabitable mud bottom. Of the potential islands, vertebrate bones and belemnite rostra became available only after soft part disintegration. They never show epibionts. The remaining substrates — driftwood and ammonite shells — could become colonized also while they were still afloat. The following observations regard this distinction:
  1. 1.

    Uneven colonization: In monospecific ammonite clusters, overgrowth contrasts in degree and kind from one shell to the other.

     
  2. 2.

    Substrate selectivity: None of the epibionts occurs with similar frequency on all substrates. Inoceramus, Seirocrinus and gooseneck barnacles occur almost exclusively on drift wood, Orbiculoidea, oysters (2 kinds) and byssus-attached bivalves other than Inoceramus (Pectinids, Limids, Bakevelliids, Monotids), only on ammonite shells. Even more specific, Serpula has been found only on Harpoceras and Nautilus, which also carries bryozoa. This selectivity holds true even if wood and ammonite shells are now closely associated on the same bedding plane.

     
  3. 3.

    Equal encrustation on both flanks: Right and left side of the examined ammonites never differ significantly with respect to kinds, sizes, distribution and numbers of encrusters (Ostrea, Exogyra, Serpula), suggesting simultaneous rather than subsequent colonization.

     
  4. 4.

    Unequal distribution around ambitus: In contrast to the encrusters, byssusattached pelecypods (Gervillia, Plagiostoma, Pectinids) on Lythoceras are significantly accumulated on the side that faced down in the swimming or floating cephalopod shell. This is also true for one Harpoceras specimen with Inoceramus.

     
  5. 5.

    Slope orientation: While oysters and serpulids fail to show preferred orientations, 177 Orbiculoidea on the flank of a large Lythoceras are statistically aligned conforming with the swimming position of the host shell.

     

Keywords

Prefer Orientation Bedding Plane Unequal Distribution Substrate Selectivity Detailed Version 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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Copyright information

© Springer 1982

Authors and Affiliations

  • A. Seilacher

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