Whorl Width in the Body Chamber of Ammonites as a Sign of Dimorphism

  • Carlo Sarti

Abstract

The clearest indication of dimorphism in ammonites is considered to be the shell diameter between forms which have identical inner whorls but which reached maturity at different sizes. Callomon coined the apt terms macroconch (M) and microconch (m) for the large and small forms, respectively. The character of the more or less inflated and broadened whorls of the body chamber, clearly represented in the width:diameter ratio W:D, is another very important manifestation of dimorphism, always present, in my opinion. This character has been used to separate “male” (LARGIVENTER conch = L) from “female” (LEVIVENTER conch = 1), and in some Mesozoic ammonites presents a clear bimodal distribution. The present paper gives some examples of dimorphism from the literature and personal studies on faunas from the Trento Plateau (Northern Italy), demonstrating the great importance of the W:D ratio with which dimorphic pairs can be recognized.

Keywords

Shale Cretaceous Jurassic Stratigraphy Monites 

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Copyright information

© Springer Science+Business Media New York 1999

Authors and Affiliations

  • Carlo Sarti
    • 1
  1. 1.Dipartimento di Scienze GeologicheUniversità di BolognaBolognaItaly

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