Heterochrony in the evolution of Late Devonian Ammonoids
Dieter Korn
In the goniatite family Prionoceratidae, the transition from Mimimitoceras to Balvia provides an example of rapid size decrease resulting from progenesis. In the Prionoceras-Mimimitoceras stock the adult conch continued to be of rather uniform shape and size (about 60 mm) and species diversification was expressed mostly in changing juvenile morphology. In the Balvia branch. which had developed in the Wocklumeria Stufe. the adult size diminished strongly (not more than 16 mm). Progenetic Balvia displays conch morphology of ancestral Mimimitoceras juveniles. with distinct ornamentat~onty pes that were added terminally.
Key words: ammonoids, Late Devonian. Germany, evolution, heterochrony
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