Protosalvinia revisited, new evidence for a land plant affinity

The Upper Devonian Protosalvinia phytoclasts from Ohio Shale and its host rock were analyzed using GC/MS, GC-ir/MS, in situ FTIR and pyrolysis-GC/MS, in order to determine its chemical composition and to discuss the taxonomic position of this enigmatic fossil. Based on molecular ratios, the maturity of the organic matter present in the phytoclasts and rocks is comparable to a bituminous rank coal. Diagenetic products of Chlorobiaceae carotenoids were observed in the hosting rocks of Protosalvinia, indicating that anoxia prevailed during the sedimentary deposition. The soluble organic matter of phytoclasts shows land-derived biomarkers including long chain n-alkanes with a slight odd/even predominance and bicyclic sesquiterpenes with eudesmane skeletons. The carbon isotopic composition of n-alkanes is consistent with a land plant origin. The macromolecular fraction of Protosalvinia consists of a condensed aromatic network. The pyrolysis products of the phytoclasts show polycyclic aromatic hydrocarbons commonly observed in coals, including naphthalenes, phenanthrenes and dibenzofurans. Based on what is known about the thermal evolution of organic matter, these components could derive from demethylation and condensation of phenylpropanoids or polysaccharides present in the original structure of Protosalvinia. The former components are exclusively metabolized by land plants.