Seed phytoliths in West African Commelinaceae and their potential for palaeoecological studies

During the investigation of phytoliths in Holocene sediments from the site complex of Ounjougou/Mali, we recognized several conspicuous types which were initially attributed to Marantaceae and Cyperaceae. Systematic screening of West African herbarium specimens revealed that they originate from the seeds of Commelinaceae, a family of mostly tropical and subtropical monocotyledonous herbs. In West Africa, this family is represented by a limited number of taxa, most of them with clear ecological preferences in anthropogenic (segetal and ruderal) plant communities and wetlands. From 22 species occurring in the Sahelian, Sudanian and Guineo–Congolian zones of West Africa, phytolith extracts were prepared and studied with transmittent light. Microscopic seed sections were prepared and examined with SEM to document the original anatomical position. Three morphotype groups were distinguished: 1. polygonal platelets; 2. flat polygonal prisms with conical top; and 3. subcylindric, distinctly anisopolar, the upper part polygonal prismatic with conical top. Depending on the presence or absence of central protuberances and peculiarities of surface vesturing distinct subtypes could be described. The investigations have shown that identification of Commelinaceae seed phytoliths is often possible at low taxonomic levels. We report here the results of this comparative study and discuss the potential inherent for palaeoecological interpretations in West Africa.