Homology of the dark cells of Paleozoic liverworts with the specialized oil body cells of modern liverworts (Marchantiophyta)

O_LIOil bodies are a synapomorphy of liverworts (Marchantiophyta), a major group of land plants with a sparse fossil record. Paleozoic liverworts sometimes possess dark cells that appear similar in distribution to liverwort oil body cells. The Middle Devonian Metzgeriothallus sharonae provides an opportunity for comparison with modern liverworts. C_LIO_LIShale samples were collected and the carbonaceous fossils isolated by acid maceration. Museum shale specimens of Pallaviciniites devonicus were also obtained and processed. The relative location, composition, frequency and spatial distribution of the fossil dark cells was compared to that of oil cells in extant taxa. C_LIO_LIChemical characterization of the fossil dark cells is consistent with cells that contained lipids, the main component of oil bodies. Quantitative analyses revealed that the frequency and spatial distribution of dark cells are comparable to oil body cells. The dark cells of M. sharonae show clumping near the thallus edge, suggesting an anti-herbivore function. C_LIO_LIThese results support the hypothesis that the dark cells of Paleozoic liverworts and the oil body cells of extant lineages are homologous structures with a shared developmental origin, providing a new character that can be used to assign fossils and that sheds light on the evolution and function of liverwort oil bodies. C_LI