Occurrence of retene in upper Silurian-lower Devonian sediments from North Africa: Origin and implications

The biomarkers retene, cadalene, tetrahydroretene and isohexylalkylnaphthalene have been identified in the extracts of Silurian-Devonian clastic sediments from southern Tunisia. The occurrence of retene is unexpected as it is generally considered to be a diagenetic product of the conifer constituent abietic acid; however, conifers evolved only after the Devonian. The retene shows a strong correlation with the abundant terrestrial signal from bryophyte-derived cryptospores in the palynofacies, suggesting that it derives from a terrestrial source. Accordingly, we propose that either abietic acid synthesis evolved prior to the appearance of conifers or retene is a diagenetic product of precursors other than abietic acid, which were synthesized by the earliest land plants. Without excluding the first possibility, it appears likely that retene derives here from the diagenesis of compounds with a kaurane-type skeleton, which were abundantly produced by the early Palaeozoic bryophytes.