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

Oil 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. Shale samples were collected, and the carbonaceous fossils were isolated by acid maceration. Museum shale specimens of Pallaviciniites devonicus were also obtained and processed. The relative location, frequency, and spatial distribution of the fossil dark cells were compared to those of oil body cells in extant taxa. Quantitative analyses revealed that the frequency and spatial distribution of dark cells are comparable to those of oil body cells. Microscopy results show evidence of oil body membranes within dark cells. The dark cells of M. sharonae show clumping near the thallus margin, suggesting an anti-herbivore function. These 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 aid in the classification of fossils and that sheds light on the evolution and function of liverwort oil bodies.