The application of fossil grass-phytolith analysis in the reconstruction of cainozoic environments in the South African interior
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Grass-dominated ecosystems occupy a primary position with regard to current debates concerning key events in faunal turnover and human evolution in Africa. Our knowledge of how grass-dominated ecosystems in southern Africa have reacted to periods of global warming and cooling in the past is provided by a broad range of proxy data sources, most notably pollen and stable isotope records. Phytolith analyses provide an alternative fossil record of environmental change and are now progressively becoming a conventional analytical approach in palaeoenvironmental research.The grass family (Poaceae) produces abundant silica bodies, especially within specific and specialized silica cells located in costal zones of the leaf epidermis. In addition to being highly resistant to decomposition, grass phytoliths show markedly varied and distinct morphologies. In this study a central hypothesis, namely that the morphology of grass short-cell phytoliths consistently follows meaningful environmental traits that are rooted in the relationship between phytolith shape and the ecological niche of grasses, was investigated by interpreting morphologically diagnostic grass phytolith assemblages according to their association with the ecological requirements of the grass species that produces them, irrespective of taxonomic affiliation.An effort was therefore made to assign ecological meaning to short-cell phytoliths by comparing a range of ecological preferences in modern grasses with the phytoliths that they produce, rather than just using phytoliths to discriminate between grass subfamilies, tribes or genera. This entailed a systematic investigation of grass leaf epidermis from a collection of 309 species, followed by an assessment of the ecological significance of grass-short-cell phytoliths within a quantitative model. The model allowed for comparison of geographically diverse grass phytolith assemblages by converting them into one homogenous group represented by ecological categories. Several meaningful ecological trends were demonstrated by results in this study, and it is suggested that short cell phytolith association in grasses is primarily driven by a temperature gradient, marked by cool versus warm growing temperatures and reflected by grasses utilizing the C3 and C4 photosynthetic pathway. The study gives emphasis to the importance of investigating phytolith systematics with the aid of adequate comparative reference collections.