Phytoliths are small silica deposits that some plants form in and between cells. The phytoliths are distinct from the species that created them. For this reason, grass silica short cell 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 to classify grass fossil phytoliths. However, phylogenetic, ecological and intra-individual variability patterns in phytolith shape remain largely unexplored.
Hošková and colleagues analysed the full range of intraindividual phytolith shape variation (3650 2D outlines) from 73 extant grass species, 48 genera, 18 tribes, and eight subfamilies (particularly Pooideae). The team used geometric morphometric analysis based on semilandmarks spanning phytolith outlines.
The scientists found that 2D 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 intra-individual variation in phytolith shape varies among species resulting in a remarkable pattern across grass phylogeny.
The study contributes to a discussion on how phytolith comparisons should be carried out. Hošková and colleagues write: “Should fossil taxa be classified by comparing them with average phytolith shape (representing whole intraindividual shape variation for a single shape) or by comparing them with the whole intra-individual phytolith shape variation within the species of our reference collection? Our study suggests the second option is better since the average phytolith shape of some species does not necessarily reflect the natural variation in phytolith shape (as seen in the extreme case of Eragrostis minor). A reference collection based on the entire range of intra-individual phytolith shape variation of the studied species is required.”
“Geometric morphometrics enables the quantitative assessment of the entire phytolith shape and allows visualization of variation. Moreover, applying geometric morphometrics to 2D phytolith shape is cheap and rapid, making it an excellent tool to process large numbers of phytolith outlines required for palaeoecological reconstruction.”
Hošková, K., Neustupa, J., Pokorný, P., Pokorná, A., 2021. Phylogenetic, ecological and intraindividual variability patterns in grass phytolith shape. Annals of Botany. https://doi.org/10.1093/aob/mcab143