The Cenozoic history of palms: Global diversification, biogeography and the decline of megathermal forests

Aim: Megathermal rain forests and mangroves are much smaller in extent today than in the early Cenozoic, primarily owing to global cooling and drying trends since the Eocene–Oligocene transition (c. 34 Ma). The general reduction of these biomes is hypothesized to shape the diversity and biogeographical history of tropical plant clades. However, this has rarely been examined owing to a paucity of good fossil records of tropical taxa and the difficulty in assigning them to modern clades. Here, we evaluate the role that Cenozoic climate change might have played in shaping the diversity and biogeography of tropical plants through time. Location: Global. Time period: Cenozoic, 66 Ma to present. Major taxa studied: Four palm clades (Calaminae, Eugeissoneae, Mauritiinae and Nypoideae) and their fossil pollen record. Methods: We compiled fossil pollen occurrence records for each focal palm lineage to reconstruct their diversity and biogeographical distribution throughout the Cenozoic. We use climatic niche models to project the distribution of climatically suitable areas for each lineage in the past, using palaeoclimatic data for the Cenozoic. Results: For most palm lineages examined, global pollen taxonomic diversity declined throughout the Cenozoic. Geographical ranges for each focal lineage contracted globally and experienced regional-scale extinctions (e.g., Afrotropics), particularly after the Miocene. However, climatic niche models trained on extant species of these focal lineages often predict the presence of climatically suitable habitat in areas where these lineages went extinct. Main conclusions: Globally, the decline in megathermal rain forest and mangrove extent might have led to declines in diversity and range contractions in some megathermal plant taxa throughout the Cenozoic. Although global climatic trends are an important backdrop for the biogeography and diversity of tropical groups at global scales, their continental- or regional-scale biogeographical trajectories might be more dependent on regional abiotic and biotic contexts.