A review of the fossil record for Dioscoreaceae

Abstract

Dioscoreaceae (yams) include major tuber crops and numerous wild species of pharmacological interest. Historically the family has received disproportionately little attention from systematists, but molecular phylogenetic studies are now underway on several continents, creating demand for reliably dated and reliably identified fossils. Here a comprehensive review of the fossil record for Dioscoreaceae is presented. Records were compiled from the palaeobotanical literature and, where possible, confirmed by examination of high-resolution digital photographs of the original specimens. Twenty fossil taxa previously identified as Dioscoreaceae were evaluated using morphological characters present in extant members of the family. Three fossil taxa, Dioscoroides lyelli, Dioscorea wilkinii and Dioscorea sp. from Kenya, can be assigned to sections of Dioscorea and are considered appropriate for use in molecular dating studies. Four additional fossils can be assigned to Dioscorea at the generic level only, and another four require additional information to confirm. For Tacca, two fossil records were confirmed, and one, T. buzekii, is deemed appropriate for calibration purposes. The remaining fossils are not members of Dioscoreaceae. Over 40% of published 'Dioscoreaceae' fossil records turned out not to be members of Dioscoreaceae, and only four are considered suitable for calibration of phylogenetic trees, demonstrating the need for strict morphological criteria, caution and ideally, a specialist's eye.