PHYTOLITH VARIABILITY IN ZINGIBERALES: A TOOL FOR THE RECONSTRUCTION OF PAST TROPICAL VEGETATION

Phytoliths (plant silica bodies) are useful microfossils for vegetation reconstruction due to their preservation even in the absence of other plant fossils, and for their morphotypes that are diagnostic of specific taxonomic or functional groups. Zingiberales (gingers, bananas, and relatives) are a pan-tropical order of flowering plants with a fossil record based primarily on fruits, seeds, and leaves that extends back to the Cretaceous; pollen is not preserved. Zingiberales are phytolith producers, and previous work suggests that distinctive phytolith morphotypes exist for different families. Thus phytoliths could be used to expand the fossil record of Zingiberales, and in situ phytoliths to confirm taxonomic placement. Our study examined the variation in phytolith type and size of the eight extant Zingiberales families to identify characteristic morphotypes and inter-family differences for use in identification of fossils. Over 150 samples have been processed, representing more than 65 species (>30 genera, all families) and including leaf, reproductive, and rhizome material. Druse-like morphotypes are found in families Cannaceae, Costaceae, Marantaceae , and Strelitziaceae, and vary in degree of roundness and depth of facets. Lobe-edged hat-shaped morphotypes are found in Marantaceae and Lowiaceae, but are smaller with a round base in the former and larger with a rectangular base in the latter. Musaceae and Heliconiaceae both produce “trough” phytoliths with those of Heliconiaceae being more echinate. Rugose or granulate spherical to subspherical morphotypes are found in Zingiberaceae and Marantaceae, and rarely in Cannaceae and fruits of Heliconiaceae. Zingiberaceae have generally small bodies, typically granulate sphericals or silica sand, compared to larger and more spherical bodies in Marantaceae. Reproductive material often has larger and different morphotypes than vegetative organs, e.g., hat-shaped with columellate base in Musaceae seeds, polygonal plates with reticulate patterning in Costaceae seeds, and very large knobby bodies in Heliconiaceae and Marantaceae fruits. Variation in morphotypes between families can be utilized for identification of tropical plants for paleoenvironmental reconstruction and as a tool for systematic classification of extinct Zingiberales.