Palaeozoic Protosphagnales constitute the largest group of extinct mosses of a still uncertain affinity. Their leaf areolation resembles that of modern Sphagna, which is strikingly different from any other mosses.
Ivanov et al. use original methods to analyse digitised images of fossils. They argue that the similar type of areolation may have developed through different pathways: unequal oblique cell divisions in Sphagnum and equal cell divisions with subsequent uneven cell elongation in the Palaeozoic plants. Such different mechanisms make the relatedness of Sphagnales and Protosphagnales doubtful, and explain the remarkable intra-leaf variability in Protosphagnales, thus calling for the reassessment of the key characters used in their taxonomy.
Ivanov, O. V., Maslova, E. V., & Ignatov, M. S. (2018). Development of the sphagnoid areolation pattern in leaves of Palaeozoic protosphagnalean mosses. Annals of Botany. https://doi.org/10.1093/aob/mcy046