Résumé
An unusual molecular fossil (onocerane I) has been detected for the first time in Quaternary lacustrine sediments (Lagoa do Caçó, NE Brazil). This molecule was initially thought to attest to the former presence of ferns or club mosses. According to possible plant precursors of onocerane-related molecules recorded in the literature and by comparison with palynological results and palaeoclimatic data, we here provide evidence that club mosses and ferns cannot account as sources of onocerane I in this setting. Onocerane I is abundant in the lipid extracts of sediments deposited during the two driest periods recorded in Northern Brazil (Last Glacial Maximum and Younger Dryas). This molecule is therefore suspected to be diagnostic of the development of plants adapted to dry or semi-arid conditions in this region.