A geological perspective on calcareous nannoplankton evolution : biotic response to global change and environmental perturbations?

Recent environmental changes and climate instabilities pose urgent questions regarding biota ability to keep pace with concurrent excess CO2, global warming and ocean acidification. Major concerns are addressed to the possibility of near future extinctions causing a biodiversity loss and revealing a biota failure in sustaining rapid and progressive environmental changes accelerated by anthropogenic impacts. However, the findings of new organisms in various ecosystems raise the possibility that global change might stimulate biota speciation and/or innovation: novel life forms might represent temporary adaptation to environmental stress or might be real new species evolved in response to global change. The ocean, the oldest and largest ecosystem on Earth, best recorded global changes in climate and oceanic physical, chemical and trophic parameters. Within the oceanic biosphere, calcareous phytoplankton plays a special role as: (1) is common and widespread and consists of cosmopolitan and endemic taxa; (2) has a 220 My-long evolutionary history; (3) is one the most effective calcite producers of the planet since the Jurassic; (4) is relevant for both the inorganic and organic carbon cycle; (5) is extremely sensitive to environmental variations; (6) may directs climate change by altering albedo and absorption/ emission of atm-CO2 at large scale. The Phanerozic geological record of global change unambiguously indicates that the Earth system already experienced conditions of (super)greenhouse and (super)icehouse. Diversity pulses of calcareous nannoplankton are grossly coeval with major events such as climate and sea-level changes, large magmatic episodes and variations in ocean structure and composition, suggesting that evolutionary patterns are intimately linked to environmental modifications. Our study aims at reconstruction of both tempo and mode of nannoplankton evolution and the causal/casual role of environmental pressure. We explored time-intervals of (1) evolutionary acceleration in absence of coeval environmental change, during a period of presumed climatic stability (Tithonian, latest Jurassic), and (2) global change marked by calcareous phytoplankton adaptation/evolution (Early Aptian Oceanic Anoxic Event 1a; PETM). For each case history calcareous nannofossils have been investigated in sections from different oceans in order to discriminate among local, regional or global causes, and to verify possible diachroneity in calcareous phytoplankton evolution and/or in response to global changes. Calcareous nannofossil species richness, first and last occurrences and abundance (relative and absolute) have been achieved. Morphometric analyses of selected taxa have been performed to separate malformation, under-calcification, over-calcification, and real ultrastructure changes. Three speciation models were proposed: phyletic gradualism, punctuated equilibrium and punctuated gradualism. Phyletic gradualism holds that new species arise from slow and steady transformation of populations providing gradational fossil series linking separate phylogenetic species. Punctuated equilibrium explains the appearance of new species by rapid speciation occurring in small peripheral isolated populations, followed by migration to other areas where the fossil sequence usually shows a series of sharp morphological breaks. Punctuated gradualism implies long-lasting evolutionary stasis interrupted by rapid, gradual phyletic transformation without lineage splitting. The Jurassic-Paleogene case-histories investigated provide examples of all evolutionary patterns in calcareous nannoplankton. Preliminary results suggest: (1) potentially different speciation styles of coccoliths versus nannoliths; (2) that punctuated gradualism prevails in intra-generic speciation, whereas punctuated equilibrium dominates inter-generic speciation; (3) that most new morphotypes associated with extreme, global change represent ephemeral adaptations rather than true species. Although climate and environmental changes have been instrumental for directing nannoplankton evolution, episodes of major innovation occurred during times of ecosystem stability suggesting very successful diversification and adaptations to steady conditions. Contrary to general models, extreme events such as OAE1a and PETM did not cause extinctions among calcareous nannoplankton.