Climate and geological change as drivers of Mauritiinae palm biogeography

Aim: Forest composition and distribution are determined by a myriad of factors, including climate. As models of tropical rain forest, palms are often used as indicator taxa, particularly the Mauritiinae. We question, what characterizes the Mauritiinae pollen in the global fossil record? And when did the Mauritiinae become endemic to South America?. Location: Global tropics.

Taxon: Mauritiinae palms (Arecaceae: Lepidocaryeae).

Methods: Pollen trait data from extinct and extant Mauritiinae pollen were generated from light-, scanning-, and transmission electron microscopy. Statistical morphometric analysis was used to define species and their relationships to other Mauritiinae. We also compiled a comprehensive pollen database for extinct and extant Mauritiinae and mapped their global geographical distribution from Late Cretaceous to present, using GBIF and fossil data. 

Results: Our morphometric analysis identified 18 species (11 extinct and seven extant), all exhibiting exine indentations, a synapomorphy of the subtribe. The fossil taxa and early divergent extant Lepidocaryum are all monosulcate, whereas the extant Mauritia and Mauritiella species are all monoulcerate. Paleobiogeographical maps of fossil Mauritiinae pollen occurrences suggest the taxon originated in equatorial Africa during the Cretaceous, and expanded their range to South America, and to India in the Paleocene. Range retraction started in the early Eocene with extirpation from India, and reduction in diversity in Africa culminating at the Eocene–Oligocene Transition (EOT). In contrast, in South America, the distribution is maintained, and since the Neogene Mauritiinae palms are mostly restricted to swampy, lowland habitats.

Main conclusions: Morphometric analysis shows that since their origin Mauritiinae pollen are relatively species poor, and Mauritiidites resembles Lepidocaryum. We also conclude that the biogeographical history of the Mauriitinae and, by extension, tropical forests was strongly affected by global climatic cooling events. In particular, the climate change at the EOT was a fundamental determinant of current tropical forest distribution.