The Great Ordovician Biodiversification Event (GOBE), occurring between approximately 488 and 444 million years ago, represents the most significant increase in marine biodiversity during the Phanerozoic Eon. Following the Cambrian Explosion, which saw the emergence of most animal phyla, the GOBE witnessed a rapid diversification at lower taxonomic levels. While the GOBE's impact is undeniable, the underlying mechanisms remain a subject of intense debate. Several hypotheses have been proposed, including: a decrease in tropical ocean temperatures from levels detrimental to marine organisms in the Cambrian to more moderate levels in the Early and Middle Ordovician; the fragmentation of the supercontinent Rodinia and associated eustatic sea-level rise creating new ecological niches; a long-term oxygenation of the early Paleozoic atmosphere; frequent asteroid impacts creating new ecological niches; and an increase in phytoplankton biodiversity modifying trophic structures and promoting the diversification of plankton-feeding groups. Recent research has largely discounted the asteroid impact and atmospheric oxygenation hypotheses. This study aims to investigate the relative contributions of long-term Ordovician cooling and paleogeographic evolution to the GOBE, using a coupled paleoclimatic and macroecological modeling approach to move beyond simple temporal correlations and establish causal relationships.

Previous research on the GOBE has explored various potential drivers, focusing on correlations between biodiversity changes and environmental factors. Studies using conodont thermometry and oxygen isotope data have suggested a link between ocean temperature decrease and increased biodiversity. Other studies have examined the influence of continental configuration and sea-level changes, while still others explored the role of atmospheric oxygen levels. The extraterrestrial hypothesis, linking asteroid impacts to the GOBE, has been largely refuted by updated dating of geological strata. Similarly, evidence suggests that atmospheric oxygen concentrations were already relatively elevated during the Cambrian and Early Ordovician, and ocean redox conditions remained stable during the main phase of the GOBE, arguing against oxygenation as a primary driver. These findings highlight the need for a more process-oriented approach to disentangling the complex interplay of factors involved in the GOBE.

This study utilizes a coupled paleoclimatic and macroecological modeling approach to quantitatively assess the individual contributions of extrinsic mechanisms to the GOBE. The Fast Ocean Atmosphere Model (FOAM), a mixed-resolution ocean-atmosphere general circulation model, was employed to simulate ocean temperatures over a period of 60 million years (from 490 to 430 Ma). The model incorporated changes in atmospheric pCO2 to reproduce the long-term Ordovician cooling trend observed in oxygen isotope data. The resulting temperature fields were then used to force a macroecological model based on the interaction between numerous modeled pseudo-species and their environment. This model, drawing on the MacroEcological Theory on the Arrangement of Life (METAL), captures biodiversity patterns well in the modern ocean. Thousands of pseudo-species, each occupying a unique thermal niche, were generated, and biodiversity was defined as the number of pseudo-species whose thermal niche overlapped the mean annual ocean temperature. The model focused on continental shelves, excluding deep ocean and polar regions due to limitations in paleontological databases. Sensitivity analyses were conducted to assess the robustness of the results to uncertainties in temperature reconstructions and organism physiology, including variations in the simulated cooling scenario and the thermal upper limit of the model pseudo-species. Model results were compared with three different fossil global biodiversity curves based on brachiopods, trilobites, and conodonts, and regional biodiversity patterns were compared with a brachiopod database. Additional simulations with constant tropical sea-surface temperatures served as sensitivity tests to assess the influence of continental drift alone. A Taylor diagram was used to compare the biodiversity spatial patterns under contrasting warm and cool climatic states and quantify the impact of global climate cooling on biodiversity.

The coupled climate-macroecological model simulations revealed a striking shift in biodiversity patterns in response to global cooling. In warmer early Ordovician conditions, biodiversity peaked at higher latitudes, contrasting with the modern latitudinal biodiversity gradient (LBG). As the climate cooled, the peak biodiversity shifted towards lower latitudes, coincident with an overall increase in global biodiversity. This increase in biodiversity in the model is directly linked to the expanding habitable area as temperatures decreased to levels more suitable for marine life. The model accurately captured the monotonic increase in global marine biodiversity during the Ordovician, which is in agreement with the general trend observed in the fossil record. The sensitivity analysis demonstrated that the simulated increase in biodiversity in response to global cooling was robust to changes in both the temperature reconstruction and the thermal upper limits of the model pseudo-species, unless highly unrealistic physiological adaptations were assumed. The study further showed that continental drift alone did not trigger a substantial increase in marine biodiversity, confirming that global climate cooling was the primary driver of the simulated biodiversification. The model also reproduced the establishment of a modern-like LBG during the GOBE as a consequence of global cooling. Regional comparisons between simulated and observed brachiopod biodiversity revealed varying degrees of model-data agreement, suggesting potential influences of other regional factors. Despite some discrepancies in specific regional patterns and timing of biodiversity change, the model broadly captured the overall increase in biodiversity in Laurentia, Siberia, and South China.

The findings strongly suggest that global climate cooling was a primary driver of the GOBE, as it resulted in an overall increase in global marine biodiversity and the establishment of a modern-like LBG. While biological evolution and ecological changes undoubtedly contributed to the diversification process, the model results demonstrate that a significant biodiversity increase could occur primarily due to global cooling, without requiring additional biological drivers. The robustness of the findings across various sensitivity analyses reinforces the importance of climate as a leading factor in the GOBE. The varying degrees of model-data agreement in regional biodiversity patterns suggest that other factors, such as regional environmental changes, local ecological processes and sampling biases, played important roles in shaping regional biodiversity. Uncertainties in paleoclimate reconstructions and the spatial resolution of both the model and the fossil record, also influence the extent to which the model findings precisely reflect the historical biodiversity dynamics. However, even considering these limitations, the model successfully reproduced the major trends of the GOBE, illustrating the significant impact of global climate cooling on early Paleozoic marine ecosystems. This research demonstrates the effectiveness of combining paleoclimate and macroecological models to understand the complex factors driving biodiversity changes in deep time. The results highlight the importance of considering global environmental factors in addition to regional-scale processes when evaluating the causes of major biogeographical events.

This study provides strong evidence supporting the hypothesis that global climate cooling was the primary driver of the Great Ordovician Biodiversification Event. By integrating a global climate model with a macroecological model, the research demonstrates that a significant increase in marine biodiversity and the establishment of a modern-like latitudinal biodiversity gradient could be largely attributed to the decrease in ocean temperatures. While regional factors and biological evolution undoubtedly played important roles, the findings highlight the dominant influence of large-scale climate change in shaping the biodiversity patterns of the Ordovician. Future research should focus on improving the accuracy of paleoclimate reconstructions and incorporating more detailed biological interactions into macroecological models to further refine our understanding of this crucial period in Earth's history.

The study acknowledges limitations inherent in the modeling approach. The spatial resolution of the climate model and the availability of paleontological data might influence the accuracy of model-data comparisons, particularly at regional scales. Uncertainties in the reconstruction of early Paleozoic ocean temperatures introduce some level of uncertainty into the analysis. The model simplifies biological interactions and does not fully capture the complexity of ecological processes, which may impact the precision of biodiversity simulations. Additionally, potential biases in the fossil record, such as preservation issues or uneven sampling, might also affect the accuracy of model-data comparisons. Nonetheless, the findings provide significant support for the leading role of global climate cooling in the GOBE, even considering these limitations.