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Morphological pathways in the evolution of Early and Middle Devonian ammonoids

Published online by Cambridge University Press:  08 April 2016

Dieter Korn
Affiliation:
Institut für Geowissenschaften der Eberhard Karls-Universität Tübingen, Sigwartstraße 10, D-72076 Tübingen, Germany
Christian Klug
Affiliation:
Institut für Geowissenschaften der Eberhard Karls-Universität Tübingen, Sigwartstraße 10, D-72076 Tübingen, Germany

Abstract

The principal conch parameters—whorl expansion rate, whorl overlap rate, umbilical width, and whorl thickness—of Early and Middle Devonian ammonoids have been extensively investigated. Stratophenetic analyses show long-term trends in the transformations of these characters over long periods of time, but sudden and rapid reversals can also be observed. On the basis of these four quantifiable conch parameters and supplementary qualitative characters, ten ammonoid morphs were distinguished. Reconstruction of the evolutionary history of these morphs reflects the existence of two major phylogenetic lineages, both already visible in Early Devonian faunas. The agoniatitid lineage is characterized by slow character development and leads to the Frasnian gephuroceratids; the anarcestid lineage displays rapid morphological evolution that leads to the late Givetian pharciceratids as well as the Middle and Late Devonian tornoceratids. Morphological evolution is interpreted as partly limited by geometrical and physical constraints.

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Copyright
Copyright © The Paleontological Society 

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