Evolutionary history of ferns and lycophytes: aligning molecular divergence time estimates with the global fossil record in amber
Final Report Abstract
Estimates of divergence times based on DNA analysis indicate that, despite the ancient evolutionary history of ferns and lycophytes, several extant lineages are relatively recent, having arisen after the emergence of angiosperm-dominated forests during the Cretaceous. In this project, we have documented the exceptional diversity of the lycophyte genus Selaginella in the Cretaceous, with 20 new fossil species described based on fertile leaf fragments preserved in mid-Cretaceous (ca. 100 Ma old) Kachin amber from Myanmar. The number of fossil species described almost doubles the documented record of free-sporing plants from Kachin amber, and renders Selaginella as the most diverse plant genus ever recorded from any amber deposit worldwide. Ten species were unambiguously assigned to the extant subgenus Stachygynandrum, which accounts for approximately 80% of present-day Selaginella diversity. This suggests that the evolutionary origin of extant species-rich clades of Selaginella predate the early phase of the Angiosperm Terrestrial Revolution. Additionally, we discovered and evaluated a new Selaginella fossil fragment preserved in Miocene (ca. 15-20 Ma) Dominican amber. To place this fossil in a phylogenetic context, we reconstructed the phylogeny of Greater Antillean Selaginella using a herbariomic approach, based on 50 chloroplast and 5 nuclear markers. Among the representatives of the fern order Polypodiales, the fossil fern Cretacifilix fungiformis, preserved in mid-Cretaceous Kachin amber of Myanmar was previously assigned to the eupolypod ferns. However, due to insufficient documentation of character states and the unavailability of the holotype, the proposed placement of this fossil in derived families of eupolypods required further investigation. Using additional leaf fragments of C. fungiformis from the same amber deposit we re-evaluated its systematic position. We revised the morphological characters present in the fossil by reconstructing the ancestral character state using a comprehensive, time-calibrated phylogeny of extant eupolypod families and genera. As a result, we recovered morphological similarity of Cretacifilix to the closely related genera Arachniodes and Dryopteris, supporting the placement of this fossil genus in the species-rich family Dryopteridaceae within the Polypodiineae. DNA-based divergence time estimates have suggested that the establishment of modern tropical rainforests in the early Cenozoic was a major driver of the radiation of epiphytic ferns during the Paleocene and Eocene, with subsequent diversification in the Oligocene and Miocene. We tested this hypothesis by evaluating the phylogenetic position of a newly discovered fossil preserved in Dominican amber, assignable to the Neotropical genus Pecluma (Polypodiaceae). Using herbarium material, we applied a genome-skimming approach to generate a phylogenetic hypothesis of the subfamily Polypodioideae based on 73 plastid markers and evaluated the combination of character states observed in the fossil. We used the newly discovered fossil as a calibration point to calculate divergence times of the Polypodioideae and performed an ancestral area estimation. Our biogeographic analyses suggest an Eocene origin of Pecluma in South America, with several subsequent Oligocene and Miocene colonisation events in Mexico-Central America and the West Indies. The new fossil supports the hypothesis that the epiphytic communities of the Greater Antilles have exhibited a constant generic composition since the Miocene.
Publications
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Selaginella in Cretaceous amber from Myanmar. Willdenowia, 52(2).
Schmidt, Alexander R.; Korall, Petra; Krings, Michael; Weststrand, Stina; Bergschneider, Lena; Sadowski, Eva-Maria; Bechteler, Julia; Rikkinen, Jouko & Regalado, Ledis
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Character evolution of modern eupolypods supports the assignment of the fossil fern Cretacifilix fungiformis to Dryopteridaceae. Frontiers in Ecology and Evolution, 11.
Regalado, Ledis; Schneider, Harald; Müller, Patrick & Schmidt, Alexander R.
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Plastome phylogenomics and new fossil evidence from Dominican amber shed light on the evolutionary history of the Neotropical fern genus Pecluma. American Journal of Botany, 111(10).
Regalado, Ledis; Appelhans, Marc S.; Poehlein, Anja; Himmelbach, Axel & Schmidt, Alexander R.
