The Cretaceous–Paleogene (K-Pg) extinction of the non-avian dinosaurs (66 Ma) led to a 25 million year gap of megaherbivores (>1000 kg) before the evolution of megaherbivorous mammals in the Late Eocene (40 Ma). The botanical consequences of this 'Paleocene megaherbivore gap' (PMHG) remain poorly explored. We hypothesize that the absence of megaherbivores should result in changes in the diversification and trait evolution of associated plant lineages. We used phylogenetic time- and trait-dependent diversification models with palms (Arecaceae) and show that the PMHG was characterized by speciation slowdowns, decreased evolution of armature, and increased evolution of megafaunal (≥4cm) fruits. This suggests that the absence of browsing by megaherbivores during the PMHG may have led to a loss of defence traits, but absence of megaherbivorous seed dispersers did not lead to a loss of megafaunal fruits. Instead, increases in PMHG fruit sizes may be explained by Late Paleocene rising temperatures, rainforest expansion, and the subsequent radiation of seed dispersing birds and mammals. We show that the profound impact of the PMHG on plant diversification can be detected even with the overwriting of adaptations by the subsequent Late Eocene opening-up of megaherbivore-associated ecological opportunities. Our study provides a quantitative, comparative framework to assess diversification and adaptation during one of the most enigmatic periods in angiosperm history.

See manuscript text: 10.1098/rspb.2021.2633. Fossil data: We searched the literature for palm fossil fruits and seeds following a review of the palm fossil record. This resulted in 90 observations for which we assembled the taxonomy of the fossil and its nearest living relatives (tribe, genus, species), fossil site location, fossil age, and length and/or width of the fossil fruit or seed. Trait data: on fruit lengths and armature type in palms (Arecaceae) Analytical pipeline: All R script and markdown files, and underlying trait and phylogenetic data, to conduct the analyses as presented in Onstein et al. 2022, including ancestral state reconstructions and diversification rate analyses.

Funding provided by: Deutsche ForschungsgemeinschaftCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001659Award Number: DFG–FZT 118, 202548816Funding provided by: SYNTHESYS*Crossref Funder Registry ID: Award Number: GB-TAF-6695Funding provided by: Netherlands Organization for Scientific Research*Crossref Funder Registry ID: Award Number: 824.15.007Funding provided by: University of AmsterdamCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001827Award Number: Faculty Research Cluster 'Global Ecology'