Phylogeochemistry – the what, how and why

Biodiversity dynamics in deep time are influenced by climatic changes. However, linking these two aspects is challenging. At a basic level, a phylogenetic tree can be directly compared to global averages of a palaeoclimate proxy of interest. This approach is only qualitative and neglects regional climate differences. In a recent article, we introduced the concept of phylogeochemistry, which links the – typically distant – topics of geochemical proxy data and comparative phylogenetics. As a case study, we applied this technique to main and trace element concentrations in belemnite rostra, showing how and at what rates these proxies change through time with respect to macroevolutionary links. Here, we introduce phylogeochemistry more broadly, explaining the motivation and background, and provide a brief overview of the methodology and potential research questions. Specifically, in a phylogenetic tree, each tip (representing, e.g., a species or population) can be associated with proxy data (e.g., oxygen isotope ratios used for the reconstruction of palaeo-seawater temperatures). Phylogenetic comparative methods can then be used to reconstruct ancestral states at internal nodes or the rate of changes. Potential applications include, e.g., testing whether temperature affects extinction rates, whether distinct warm- or cold-water clades can be identified. Due to the flexible probabilistic framework, the models can be specifically tailored to the research objectives at hand. We further discuss challenges and caveats that need to be considered in study design. Last but not least, phylogeochemistry calls for closer collaborations between geochemists and palaeobiologists for the best results.