EARLY EVOLUTIONARY TRENDS IN AMMONOID EMBRYONIC DEVELOPMENT
Version of Record online: 14 FEB 2012
© 2012 The Author(s). Evolution © 2012 The Society for the Study of Evolution.
Volume 66, Issue 6, pages 1788–1806, June 2012
How to Cite
De Baets, K., Klug, C., Korn, D. and Landman, N. H. (2012), EARLY EVOLUTIONARY TRENDS IN AMMONOID EMBRYONIC DEVELOPMENT. Evolution, 66: 1788–1806. doi: 10.1111/j.1558-5646.2011.01567.x
- Issue online: 1 JUN 2012
- Version of Record online: 14 FEB 2012
- Accepted manuscript online: 5 JAN 2012 02:10AM EST
- Received May 6, 2011, Accepted November 29, 2011, Data Archived: Dryad doi: 10.5061/dryad.k84c3k8j
- embryonic shell;
- predatory pressure;
- reproductive strategy
During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.