UNDERSTANDING THE ROLE OF PALEOCEANOGRAPHY IN DIVERSIFICATION AND EXTINCTION IN PERFORATE LARGER BENTHIC FORAMINIFERS USING MORPHOLOGIC EVIDENCE FOR ALGAL SYMBIOSIS

Lineages of perforate large benthic foraminifera (LBF) appeared and diversified during the Paleocene, producing substantial carbonate buildups on shelves and epieric seaways throughout the Tethys region, especially during the Eocene. LBF continued to be important secondary carbonate producers in bryozoan-, red algal- and coral-dominated buildups throughout the Neogene. Modern reef sands in the Indo-Pacific often are dominated by shells of calcarinid or amphisteginid foraminifers, while nummulitids are common in deeper shelf sediments. These taxa host endosymbiotic diatoms and several lines of reasoning suggest that the symbioses originated under intermediate light intensities (i.e., mesophotic conditions). 1. Among extant amphisteginids and nummulitids, the least specialized morphologies are found in most abundantly in mesophotic environments or microenvironments. Taxa living in very shallow, high light environments have morphologic specializations that strengthen the shell while limiting lighting penetration. In contrast, taxa in very low light (oligophotic) environments are extremely thin shelled, typically have specialized structures to focus light, and tend to achieve the largest diameters. 2. Culture studies indicate that optimum light intensities for the least specialized modern taxa are substantially lower than optimum light intensities for their symbiotic algae when isolated in culture. 3. The discovery of bleaching in amphisteginids in 1991 and subsequent field and culture studies indicate the sensitivity of these foraminifers to photo-inhibitory stress. Morphologic adaptation to oligophotic environments probably required relative stability of environmental conditions, particularly light penetration, in the 50-120 m depth range. Conceptual models suggest that episodic changes in ocean circulation and thermocline stratification that accompanied high latitude cooling trends during the Cenozoic (e.g., mid to late Eocene) could account for higher rates of turnover in the LBF, particularly taxa most specialized to oligophotic conditions. Arguments and models supporting this hypothesis have similar implications for coral-dinoflagellate symbioses, as well as implications for carbonate sedimentation under conditions of global change.