THE GATUN FORMATION: A NATURAL LABORATORY FOR ANALYSIS OF NEOTROPICAL PALEOECOLOGIC AND DIVERSITY PATTERNS

The late Miocene Gatun Formation of Panama is regarded as an exemplar among late Neogene marine fossil assemblages due to its high biodiversity and importance as a baseline for studies investigating the effects of closure of the Isthmus of Panama. In addition to its incorporation into ocean-scale paleoecological and biodiversity studies, the unit offers tremendous potential for documenting fine-scale changes in depositional environments and reconstructing spatial and temporal variations of a fossilized neotropical marine ecosystem.

The data used in this study were derived from published studies, museum collections, and ongoing field work. All localities have been georeferenced and placed in a composite stratigraphic framework. This stratigraphy incorporates aspects of previous studies and more recent field work; the precise geographic relocation of collecting localities typically enables their approximate stratigraphic placement. Faunal occurrences and specimen counts (where available) from published historic studies and surveyed museum collections have been tied to this locality register. In addition, high-resolution faunal inventories have been conducted from over 700 stratigraphic horizons in the formation, representing more than 20 sections and boreholes in the Canal Basin.

This compilation of data permits quantitative paleoecological and biodiversity analyses of fossil assemblages through the stratigraphy of the Gatun Formation. Analyses of faunal composition reveal both significant stratigraphic and minor spatial (between sampling sites) variation. The functional ecology and ecospace utilization (feeding habits, mobility, and life habits) of the fossil assemblages also vary within the formation. These differences are thought to represent relative shifts in water depth through the stratigraphy and among localities, reflecting long-term shifts in basin subsidence and accommodation space. Local diversity remains fairly constant throughout the formation, although it is clearly influenced by stratigraphic variations in the packing density of fossils and outcrop weathering. The high richness of the Gatun Formation is therefore partly the result of excellent fossil preservation and extensive outcrops, but also subtle turnover in species composition between assemblages.