The paleontological records provide insights to understand how the earth system and biota have responded to past climatic changes. In the Miocene, the climate was warmer and the CO2 concentration was higher than in pre-industrial times, and it has been suggested as an analogue for future climate scenarios. Climatic models have failed to satisfactorily simulate the tropical Miocene climate in terrestrial ecosystems, and the magnitude and rate of climate change in the tropics during the Miocene and how affected the evolution of tropical biodiversity remains unknown. Miocene paleoclimatic estimates from the tropics will validate or refute climate models simulations and would inform if the models can predict with confidence future climate scenarios.The Miocene Climatic Optimum (MCO) is a global warming event that occurred between ~17–14 Ma, followed by a period of cooling (Miocene Climatic Transition, MCT). To study how the MCO and MCT affected tropical ecosystems requires a rich fossil record with precise geologic dates. La Venta in Colombia is the most fossil-rich site of tropical South America. I have been developing a research program in La Venta that provides the basis to study climate change and biotic evolution at this site by establishing a large dataset of fossil records with a precise chronology. This project will take full advantage of the exceptionally rich fossil record of La Venta to (1) reconstruct the climatic changes in a tropical ecosystem during the Miocene and (2) assess how these changes affected the evolution of tropical diversity, using mammals as study system. This project will integrate four interconnected working packages (WPs). In WP1, we will use a geochemical approach to quantify paleoclimate during the MCO and MCT. In WP2, we will establish the changes in taxonomic diversity. In WP3, we will evaluate changes in functional diversity, and in WP4, we will assess the link between changes in community structure and climate change.

Phenotypic disparity and species diversity are the hallmarks of biodiversity. Morphological traits that promote the invasion of, and diversification within, new adaptive zones are expected to be more common across the tree of life. Historically, such traits were defined as ‘key innovations’ since they could explain the evolutionary success of a taxonomic group. Morphologists have embraced the concept of key innovations, and have used it extensively to explain many cases of adaptive radiations. However, the theory behind this concept was strongly criticized due to the lack of evidence demonstrating a causal link between a proposed key innovation and an increase in species diversity. Among mammals, rodents constitute a special case. This 57-million-year-old clade comprises at least 2600 species encompassing an astonishing diversity of forms and showing a markedly uneven distribution of cranial phenotypic variation across families. The ecomorphological diversification of the group has then been considered the result of the acquisition of key cranial innovations, which are considered to be associated with different types of feeding strategies. Several dental characteristics associated either with modifications of the chewing movements or with dietary adaptations, were also proposed as a key explanatory factor to major diversification events. Strong functional constraints affecting mastication have limited the number of possible evolutionary pathways and promoted convergent evolution, which in return has hampered attempts to establish an intra-ordinal classification of rodents. Such a situation, which explains the past difficulty in classifying rodents based on cranial and dental characteristics, also implies that these features have strong adaptive significance, which then strengthened their image as promoters of diversification and their recognition as key innovations. However, the different components of the masticatory apparatus (i.e. bones, muscles, and teeth) have generally been studied in isolation with little integration of their morphology to function. The overarching goal of the DispaRat project is to understand whether osteological, dental, and muscular traits evolve independently or synergistically, and whether their evolution influenced diversification of rodent lineages. We will employ a holistic, integrative approach combining state-of-the-art methods of morphological quantification with the latest macroevolutionary methods to address the following hypotheses: Hypothesis 1 – Osteological, dental, and muscular traits are highly interdependent functionally and characterize dietary habits and lifestyle. By exploring the dynamics of the relationships between morphological and mechanical diversity, we predict that the demand of evolving different ecological strategies will explain cohesive suites of morphological traits. Hypothesis 2 - Specific associations of cranial features act synergistically as a key innovation to promote diversification of rodents. If cranial features act as key innovations, we predict that functional interactions between traits, rather than traits themselves, will explain why diversity is unevenly distributed among rodent lineages. We will take a four-step approach to testing these two hypotheses. First, we will focus on assessing the correlation between osteological, muscular and dental characters, to build predictive models of phenotypic integration (WP1). Second, we will estimate the biomechanical performance of the masticatory apparatus to characterize the morphofunctional link between its components (WP2). Third, we will assemble an unprecedented character/taxon matrix for phylogenetic inferences in order to build a morphofunctional model of the masticatory apparatus in some of the earliest representatives of the group (WP3). Finally, we will explicitly test if some functional interactions between traits, or traits themselves, can trigger diversification in rodent groups (WP4).

The aim of this proposal is to investigate the carbon-climate-ice sheet couplings through the study of major global transitions between icehouse and greenhouse states that Earth already experienced. Although atmospheric CO2 level is generally proposed as the main driver of these climatic changes, processes controlling variations of atmospheric CO2 levels remain enigmatic and require a good deal of more thinking. Additionally, ice-sheet growth over Antarctica during the Eocene Oligocene Transition ~33.9 Ma (EOT) did not happen in one step but appears to have suffered from several melt and growth events till the Middle Miocene Climate Transition (MMCT, ~14.6 – 13.1 Ma) after which the ice volume in Antarctica may have stabilized. Climate ice sheet models applied to the EOT have shown that the threshold in atmospheric CO2 levels required to initiate the Antarctica glaciation lies between 840 and 700 ppmv depending on the model’s climatic sensitivity. More recent studies have focused on the Antarctica ice-sheet melting events after the EOT, in particular those occurring during the Miocene Climatic Optimum (16 Ma) and during the Pliocene (4 Ma) in a particularly low CO2 context (i.e. between 380 and 500 ppmv). How is it possible to simulate a full Antarctica glaciation at 700 ppm (lower limit for the EOT), while remaining consistent with partial deglaciation at 500 ppm during the Miocene? Does this imply that the continental configuration differences between the EOT and the MMCT result in a lower Earth’s sensitivity to ice-sheets initiation? What could cause the drop in CO2 at the EOT but also the subsequent rebound? Recent papers all point to a 100-kyr-scale CO2 variability across EOT and MMCT but quantitative models explaining these oscillations are still lacking. In order to address these issues, we propose to combine two methods of investigation. First, the acquisition of new data is needed to better quantify environmental changes occurring at the EOT and at the MMCT with a substantial spatial cover. Mollusks from coastal areas of Western Europe, Eastern USA and New Zealand have been targeted for their potential significance to decipher ocean atmosphere feedbacks at play during both key events. New Zealand outcrops are closed to Antarctica and will provide us with the local answer to ice growth while Western Europe and Eastern USA outcrops may inform us on the planetary answer. More importantly, data acquisition in USA and in Europe may reveal potential changes in ocean dynamics in the Atlantic Ocean. Second, the use of numerical tools including ocean atmosphere model, ice-sheet model and marine carbon cycle model is an invaluable source of information. Important advances to understand climate – ice sheet interactions have already been made on these periods. Conversely, the role of the interaction of the primary productivity with oceanographic-induced modifications in response to gateway changes but also to the ice sheet itself has been overlooked. Three tasks were identified: 1) a D47-Temperature calibration for living mollusks which should lead to a wider utilization of the mollusks fossils as one of the limit was due to the large salinity fluctuations occurring in coastal areas and impeding the use of d180 measured on mollusks as a paleo-thermometer; 2) characterization of coastal temperatures including the seasonality for both EOT and MMCT using mollusks fossil record from three different localities and 3) several sensitivity studies with the IPSL Earth System model mimicking the suite of events (CO2, ice growth …) at the EOT and MMCT to constrain potential feedbacks on CO2 levels related to the marine carbon cycle. Climate simulations will also provide a physical framework to interpret geochemical data from task 2. Our integrated model and data approach should allow the complex web of interconnected processes associated with EOT and MMCT to be untangled.

The Mesozoic Era (–250 Ma to –66 Ma) records some of the highest atmospheric CO2 levels of Earth’s history. The lack of robust estimates of the oxygen isotope ratios of seawater (d18OSW) for this interval, however, strongly limits Mesozoic climate reconstructions and hence our understanding of past climate sensitivity to changing CO2 concentrations. The aim of this project is to provide the first reliable Mesozoic d18OSW estimates using three complementary approaches, in order to verify or falsify the following hypotheses: 1) the Mesozoic had “low” d18OSW ratios, implying that its climate was not particularly warm and that other factors than CO2 control Earth’s climate on geological timescales; 2) the Mesozoic had “high” d18OSW ratios, implying very warm conditions and higher-than-expected climate sensitivity; 3) d18OSW ratios changed rapidly and globally during this interval in concert with sea-level changes as a result of greenhouse-icehouse cycles. We propose three complementary axes to test these hypotheses: In a first axis, we will compare skeletal d18O values with two different paleotemperature proxies (primarily Mg/Ca ratios; delta47 on selected samples), measured on the same brachiopod specimens, to reconstruct d18OSW values at multiple locations through the Mesozoic. The fossils will be predominantly sampled from academic collections and completed by fieldwork for key, selected time intervals and their preservation will be assessed using well-proven criteria. In a second axis, we will measure d18O values of skeletal remains of marine endotherms, which incorporate 18O and 16O primarily as a function of d18OSW values, to trace temporal and spatial changes in d18OSW values through the Mesozoic. We will first map intra-skeletal d18O variability in both modern (cetaceans) and Mesozoic marine endotherms (i.e., ichthyosaurs and plesiosaurs) to identify the skeletal elements less affected by regional heterothermy (i.e., cooler body extremities used to reduce heat loss to their highly conductive aquatic environment) and therefore able to more reliably track past d18OSW values. A systematic investigation of d18O values of the selected fossil skeletal elements of different age and paleolatitude will then be performed using both museum collections and fieldwork. In a third axis, Mesozoic d18OSW values of surface and deep oceans will be simulated using modern d18OSW databases and General Circulation Models (GCM). These simulations will be used to investigate the impact of paleogeography on salinity and d18OSW values in three selected Mesozoic time slices, and notably to critically examine the hypothesis according to which high freshwater input in Mesozoic shallow seas, which constitute the depositional environment of the bulk of strata of this age now available for skeletal d18O measurements, produced very low d18OSW values, leading to strong paleotemperature overestimates. The results will be integrated to estimate spatial (axes 1+2+3) and temporal changes (axis 1+2) in d18OSW on both long-term and short-term timescales (i.e., during events of major environmental change) during the Mesozoic and hence examine critically the three, above-mentioned key hypotheses. The expected results of this project will shed a long overdue, ancient light on Earth’s climate sensitivity, with obvious but fundamental implications for global environmental and economical policies.