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Article Dans Une Revue Review of Palaeobotany and Palynology Année : 2019

Bilobate Phytolith Size Matters for Taxonomical and Ecological Identification of Chad Grasses: A Case Study on 15 Species

Résumé

The Poaceae family produces diagnostic phytoliths commonly called Grass Silica Short Cell Phytoliths (GSSCP), of which shapes and sizes are very distinguishable from those produced by other plant families. Grass Silica Short Cell Phytoliths are usually classified into four main categories: the rondels, bilobates, crosses, and saddles. Among them, the bilobates have in common a general "dumbbell" shape consisting of two lobes inter-connected by a shank. They represent the dominant morphotypes observed in the leaf tissues of many Panicoideae grass species. Besides, they tend to often be related to this subfamily in the fossil record, especially when they occur in large amounts. Several studies based on modern African grasses have however demonstrated that bilobates were also abundantly produced by a few members of the Aristidoideae, Chloridoideae, and Oryzoideae grass subfamilies which, like Panicoideae, are today mainly distributed in tropical low-altitude areas. The relationship between bilobates and Panicoideae is thus not so direct in the tropics. As a result, it can lead to significant misinterpretations when applied to paleoenvironmental and archaeological surveys. This study investigates variations in the size of bilobate phytoliths in 15 African grass species, including eight Panicoideae, three Aristidoideae, two Chloridoideae, and two Oryzoideae, with the intention to explore new alternatives for refining their identification in the past deposits of tropical Africa. Different statistical analyses (Principal Component Analysis, Mann\textendash Whitney tests, decisional trees) applied to the previous dataset allowed testing whether: (i) bilobate length and width significantly vary between grass species; (ii) these variations (if so) have the potential to be used as taxonomical and/or ecological proxies. Our analyses confirm that bilobate length is significant in discriminating dry-adapted Aristidoideae and Chloridoideae species from the other sub-families. In particular, a bilobate base longer than 25 μm is almost exclusively observed for Aristidoideae and Chloridoideae, which is in agreement with previous studies. Although subject to less variations, the length/width of bilobate shanks and the average length of their lobes appear significantly useful to refine grass taxonomical identification and discriminate < 120 cm-high grasses from taller grasses. Yet, it seems that the key towards a robust identification of bilobate phytoliths involves considering the size parameters.

Domaines

Paléontologie

Dates et versions

hal-02871852 , version 1 (17-06-2020)

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Citer

Benjamin Bourel, Alice Novello. Bilobate Phytolith Size Matters for Taxonomical and Ecological Identification of Chad Grasses: A Case Study on 15 Species. Review of Palaeobotany and Palynology, 2019, 275, ⟨10.1016/j.revpalbo.2019.104114⟩. ⟨hal-02871852⟩
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