Silent past: Biogeographic gaps in the Cenozoic fossil archive

Exposed sedimentary rocks are unevenly distributed across space, interfering in our ability to reconstruct the spatial and temporal dynamics of past climates and ecosystems. In this study, we quantified the extent of “lost” informative areas—regions where sedimentary layers from the past 66 million years (Ma) are not presently exposed—and assessed gaps across climate zones and geological time bins. Using Chorlton's global geological map, we reconstructed the distribution and depositional ages of exposed sedimentary rocks, and linked these with palaeoclimate simulations from the HadCM3 model, classified under the Köppen–Geiger system. This framework allowed us to compare the distribution of palaeoclimates predicted by HadCM3 for each time bin with the distribution of climates represented only by currently exposed rocks of that interval. We further integrated fossil occurrence data from the Paleobiology Database (PBDB) to evaluate mismatches between climatic coverage in the geological and fossil records. Our results indicate that more than 72 % of past continental areas across the Cenozoic lack accessible sedimentary rocks, implying a substantial loss of information about past biodiversity. Of the remaining 28 %, exposed sediments are disproportionately derived from regions that experienced tropical, temperate, and arid climates, while deposits from cold and polar climates are underrepresented. The fossil record, in contrast, shows distinct biases, with a particularly large proportion of fossils originating from past temperate environments, likely reflecting excavation effort. These findings underscore the need to account for geographic and climatic biases when interpreting macroevolutionary trends, biodiversity patterns, and species' responses to climate change.