Fossils, Fruits, and Phylogeny: An Integrative Approach to Understanding the Evolutionary History of Palms (Arecaceae)

Abstract

Reconstructing evolutionary radiations is essential for understanding the processes that have generated the extraordinary biodiversity of modern ecosystems. Fossils are an empirical record of the presence, morphology, and geographic distribution of organisms through time, making them critical for understanding evolutionary history. Integrating data from fossils and living biota can therefore reveal important insights into the history of life and Earth systems. In this dissertation I focused on the palm family (Arecaceae), a diverse and widespread lineage of tropical flowering plants, and used their extensive fossil record to understand their early diversification. In chapter 2, I described new fossil fruits from the Late Cretaceous–early Paleocene Deccan Intertrappean Beds of India and used phylogenetic analyses to understand their relationships with modern palms. Chapter 3 focused on advancing knowledge of basic palm biology and establishing a foundation for future studies of the fossil record. I performed a genus-level survey of palm fruit anatomy using X-ray micro-computed tomography (µCT) and scanned over 200 species representing nearly all extant genera. Using these scans, I created a morphological dataset, which I combined with DNA sequence data to analyze the evolutionary relationships of six fossil fruits. The results of these analyses provided important insights into the origins of major palm lineages, including tribes Borasseae, Trachycarpeae, Cocoseae, and Areceae. Additionally, the production of this new comparative morphological dataset, and recognition of key character suites for major clades, will aid in future identification of fossil fruits. Chapter 4 integrated data from chapters 2 and 3 to investigate the diversification history of palms. I performed molecular dating and diversification-rate analyses using fruit fossils as new age calibrations. The results of these three chapters pushed back age estimates for some groups by over 40 million years, and revealed that palms underwent an extensive Late Cretaceous diversification that coincided with their initial geographic expansion. The radiation of most modern tribes occurred during the warm and wet intervals of the Paleogene, coeval with the expansion of angiosperm-dominated megathermal rainforest environments. Finally, the age of tribe Areceae and a shift towards higher speciation rates associated with the tribe suggest that geologic activity and avian radiations in the Indo-Pacific region may have had an important role in the history of Areceae and other species-rich lineages. This dissertation contributes new data and insights into the timing and environmental context of palm diversification, with broader relevance to fundamental questions in evolutionary biology and Earth sciences.