BACKGROUND AND AIMS: Grass silica short cell (GSSC) phytoliths appear to be the most reliable source of fossil evidence for tracking the evolutionary history and paleoecology of grasses. In recent years, modern techniques that quantitatively assess phytolith shape variation have widened opportunities for the classification of grass fossil phytoliths. However, phylogenetic, ecological and intraindividual variability patterns in phytolith shape remain largely unexplored. METHODS: The full range of intraindividual phytolith shape variation [3650 two-dimensional (2-D) outlines] from 73 extant grass species, 48 genera, 18 tribes and eight subfamilies (particularly Pooideae) was analysed using geometric morphometric analysis based on semi-landmarks spanning phytolith outlines. KEY RESULTS: The 2-D phytolith shape is mainly driven by deep-time diversification of grass subfamilies. There is distinct phytolith shape variation in early-diverging lineages of Pooideae (Meliceae, Stipeae). The amount of intraindividual variation in phytolith shape varies among species, resulting in a remarkable pattern across grass phylogeny. CONCLUSIONS: The phylogenetic pattern in phytolith shape was successfully revealed by applying geometric morphometrics to 2-D phytolith shape outlines, strengthening the potential of phytoliths to track the evolutionary history and paleoecology of grasses. Geometric morphometrics of 2-D phytolith shape is an excellent tool for analysis requiring large numbers of phytolith outlines, making it useful for quantitative palaeoecological reconstruction.

Center for Theoretical Study Joint Research Institute of Charles University and Czech Academy of Sciences Husova 4 CZ 110 00 Praha 1 Czech Republic

Department of Botany Faculty of Sciences Charles University Prague Benátská 2 CZ 128 01 Praha 2 Czech Republic

Institute of Archaeology Czech Academy of Sciences Letenská 4 CZ 11801 Praha 1 Czech Republic

Institute of Botany Academy of Science of the Czech Republic CZ 252 43 Průhonice Czech Republic

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