Taxonomy & Evolution

Image analysis of phytoliths can tell grass species apart

Tiny stones created in plants can help identified which plants were found in past habitats.

Have you looked at the plants in Jurassic Park in awe? Paleobotanists aim to reconstruct past environments based on small pieces of fossils. Phytoliths are microscopic, silica bodies in plant tissues which remain in sediments after a plant decomposes and based on their shape and quantity, different vegetation types can be distinguished (e.g. savannah/forest, grasses/woody plants). 

Research led by PhD student Kristýna Hošková and colleagues from the Charles University in Prague and the Institute of Archaeology of the Czech Academy of Sciences used image analysis of phytoliths from two grass species from the Czech Republic and Poland. The research team found more shape variability between species and populations than within one species. One of the co-authors, Dr Jiří Neustupa has previously used studied the morphology of green algae, shoot apical meristem of herbaceous plants and flowers of spindle trees

The shape of an object can be defined as all its geometric features expect its size, location and orientation. Geometric morphometrics relies on landmarks, well-defined points, of an object and via the Procrustes superimposition, the shape component is mathematically extracted from digital images and is more robust that measuring the width and length of objects with callipers. No one has used geometric morphometrics for studying phytolith shapes and especially not between two plant species.

The scientists collected specimens of the light-demanding, dry-tolerant heath false brome (Brachypodium pinnatum) and the shade-tolerant wood false brome (B.sylvaticum). These two grass species grow in grasslands and forest respectively. In total, 2,400 phytoliths from the multiple populations were used for geometric morphometrics. Hošková and colleagues also compared phytolith shape and size from older and younger leaves. Their shapes were classified according to the latest International Code for Phytolith Nomenclature

The two grass species, Brachypodium pinnatum (left) and B. sylvaticum (right), used by Hošková and colleagues. Source WikimediaCommons (left, right)

Most of the shape difference between the two grass species’ phytoliths was explained by having three (or more) deeply incised lobes against shapes with just two shallow lobes in opposite directions. Phytoliths in older leaves were significantly larger than those in younger leaves but overall, there was low variability in phytolith shape and size within individual populations.

Photograph of in situ charred grass epidermis showing phytolith shape in Brachypodium sylvaticum and how the morphology was captured with landmarks (red dots) and semi-landmarks (49 points along both halves) (blue curves). Source Hošková et al. 2020.

“We demonstrated that it is possible to assign individual phytolith shapes to a specific plant species (our success rate in the two-species model was 83%)”. They add, “[W]e expect this approach will be most useful for the reconstruction of grassland vegetation characterised by the dominance of just a few diagnostic grass species,” the authors said.

“Regarding the general applicability of our approach, we do not expect the landmark-based geometric morphometrics of phytoliths to replace other traditional approaches but rather to complement them and make phytolith analysis more accurate in specific cases, as stated above.”

This research revealed how investigating a long-lived plant structure can tell grass species apart with high efficiency and using this method could inform habitat conservation based on past plant community assemblies. Moreover, it can help speed up hard-working paleobotanist’s work who, clearly, provided the foundation for all the Jurassic Park movies.