Article: Heterochronic trends in Namurian ammonoid evolution

Theoretical models of heterochronic processes are based on the comparison of ontogenetic age-shape curves of ancestor and descendant. An existing principal components analysis of an exhaustive body of Namurian ammonoid morphological data is a suitable source of information for assessment of heterochrony in this context. Using size as an indicator of age and principal component score as a shape index, heterochronic analyses of two evolutionary radiations of the Gastriocerataceae demonstrate that one was strongly influenced by neoteny, the other by acceleration. From an ancestor which undertook a change in habitat during ontogeny from benthic to nektonic, the occupation of the benthic habitat was increased in the neotenous trend and decreased in the lineage showing acceleration. Widespread changes in marine benthic conditions are suggested as the cause of these trends.