PATTERNS AND PROCESSES OF HETEROCHRONY IN LOWER TERTIARY TURRITELLINE GASTROPODS, UNITED-STATES GULF AND ATLANTIC COASTAL PLAINS

Heterochrony is an important component of evolutionary change in the shell sculpture of turritelline gastropods from Paleocene and Eocene sediments of the Gulf and Atlantic Coastal Plains. A survey of heterochronic modes in these gastropods indicates that peramorphosis is dominant over paedomorphosis, a result counter to the pattern previously reported in gastropods and most other groups. Although lack of ontogenetic age data makes firm conclusions impossible at present, peramorphic patterns may have been produced by more than a single process. Possible explanations for the dominance of peramorphosis in the evolution of shell form in this group include a bigger role for intrinsic constraint in controlling shell form versus soft-part anatomy, selection for larger shell size, and the chance discovery of only peramorphosis in this study.