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Functional morphology of Cretaceous helically-coiled ammonite shells

Published online by Cambridge University Press:  08 February 2016

Peter Ward
Peter Ward*
Affiliation:
Geology Department, University of California, Davis, California 95616
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Abstract

Torticonic (helically-coiled) ammonoids have been most commonly interpreted as vagile, benthonic forms. Their mode of coiling, however, places the siphuncle in a functionally dorsolateral, rather than ventral position on the shell whorl due to: 1) their probable apex-upward shell orientation during life, and 2) size asymmetry of the lateral sutural saddles due to helical coiling, associated with upward siphuncle displacement. The resultant positioning of the siphuncle produces cameral liquid de-coupling soon after initiation of cameral liquid removal (emptying of liquid from a newly formed chamber within the phragmocone). Since in Recent chambered cephalopods cameral liquid de-coupling increases efficiency of vertical migration, a similar mode of life was indicated for the torticonic ammonites.

Type
Research Article
Information
Paleobiology , Volume 5 , Issue 4 , Fall 1979 , pp. 415 - 422
Copyright
Copyright © The Paleontological Society 

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