Dominici, Stefano¹, Thorsten Kowalke²
(1) University of Florence, Firenze, Italy
(2) LMU Munich, 80333 Munich, Germany

ABSTRACT: Relative Sea Level Change, the Intermediate Disturbance Model, and a Turnover Event in the Evolution of Cerithioidean Gastropods

Empirical evidences throughout the geological record explain why the idea of a connection between cycles of sea level variation and episodes of benthic turnover has always gathered sustainers. Current ecological and paleoecological thinking offers explanation by indicating that abiotic disturbance is a major controller of the structure of shallow marine communities. In this vision, turbidity increase, overcrowding, and reduction of shelf area experienced by shallow marine ecosystems during relative sea level lowstands would lead to habitat destruction and fragmentation, thereby increasing extinctions of species. On the other hand, authors have suggested that disturbance is intermediate during transgressive episodes, causing an increase in formation and persistence of isolate populations, thereby resulting in increased allopatric speciations. During intervals of maximum transgression, when populations of newly originated species are larger, fossil assemblages from the same habitat should record stability of composition.
This idea is tested against a high-resolution framework of composite depositional sequences, fifth- to third-order, from the Eocene of the Spanish Pyrenees. Available detailed facies analysis of lower Eocene strata give independent clues as to the extent and nature of environmental disturbances through high-frequency episodes of delta-forestepping, at the same time allowing fine levels of chronostratigraphic correlation across the basin. Multivariate analysis of sampled fossil assemblages allow to recognize mudflat, estuarine, and shallow subtidal benthic communities. The distribution of several clades of Cerithioidean gastropods, a group experiencing during the lower Eocene high levels of overall diversity, is then analysed. A major turnover episode is recognized and discussed in the framework of the intermediate disturbance model of allopatric speciation, suggesting that this model can be fruitfully extended to other time intervals and other clades.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.