Can the morphology of deep-sea benthic foraminifera reveal what caused their extinction during mid-Pleistocene Climate Transition?

ABSTRACT: Over 100 cosmopolitan species of deep-sea benthic foraminifera (Extinction Group, Ext. Gp) became extinct during the late Pliocene-middle Pleistocene (3.6-0.55 Ma). Most had elongate, cylindrical tests and terminal apertures with complex modifications. This study provides new hypotheses on the functions of the morphologies that characterised the Ext. Gp and how these features could have been associated with their demise. From our functional morphological analysis we infer that: i) their elongate cylindrical or flabelliform tests, combined with fine perforations and a complex terminal apertural face are indicative of infaunal k-strategists with a low rate of metabolism; ii) their complex apertural faces may also have been an adaptation for gathering or processing their specific phytodetrital food. We propose three alternative hypotheses for the cause of these extinctions, and where possible test them using our high resolution micropaleontological and geochemical record through the last 1.07 Ma in lower bathyal site MD 97-2114 in the SW Pacific Ocean. Hypothesis 1 is that the Ext. Gp species were unable to adapt to increased variability in the overall quantity or pulsed seasonality of the food supply to the sea floor and were out-competed by opportunistic r-strategist benthic foraminifera. This is supported by the highly variable and increasing abundance of opportunistic foraminifera at our study site during the final phase of the extinction in the mid-Pleistocene Climate Transition, MPT. We doubt however, that there was increased variability in phytoplankton productivity throughout the world’s oceans sufficient to bring about the global demise of the Ext. Gp. Hypothesis 2 is that lowered pCO2 during increasingly severe MPT glacials, which coincided with the final phases of the extinction, may have caused the decline and possible loss of the Ext. Gp’s phytoplankton food source. Declining pCO2 during Neogene cooling was coeval with declining relative abundance of the Ext. Gp and reticulofenestrid nannofossils, but the final demise of this latter phytoplankton group occurred slightly later than the MPT in our study site and cannot be implicated with the extinction. If this hypothesis has any validity maybe the phytoplankton group left no fossil record. Our third alternative hypothesis is that maybe our Ext. Gp had much common DNA which made them the selective target of pathogens that caused their extinction. This does not easily explain their earlier disappearance at abyssal depths than at bathyal depths in our study region, which can be accommodated by hypotheses 1 and 2.