EARLY EVOLUTIONARY TRENDS IN AMMONOID EMBRYONIC DEVELOPMENT

Kenneth De Baets, Christian Klug, Dieter Korn, Neil H. Landman

Research output: Contribution to journalReview article (Academic Journal)peer-review

54 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)1788-1806
Number of pages19
JournalEvolution
Volume66
Issue number6
DOIs
Publication statusPublished - Jun 2012

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