Development of the sphagnoid areolation pattern in leaves of Palaeozoic protosphagnalean mosses.

Background and Aims: Protosphagnalean mosses constitute the largest group of extinct mosses of still uncertain affinity. Having the general morphology of the Bryopsida, some have leaves with an areolation pattern characteristic of modern Sphagna. This study describes the structure and variation of these patterns in protosphagnalean mosses and provides a comparison with those of modern Sphagna. Methods: Preparations of fossil mosses showing preserved leaf cell structure were obtained by dissolving rock, photographed, and the resulting images were transformed to graphical format and analysed with Areoana computer software. Key Results: The sphagnoid areolation pattern is identical in its basic structure for both modern Sphagnum and Palaeozoic protosphagnalean mosses. However, in the former group the pattern develops through unequal oblique cell divisions, while in the latter the same pattern is a result of equal cell divisions taking place in a specific order with subsequent uneven cell growth. The protosphagnalean pathway leads to considerable variability in leaf structure. Conclusions: Protosphagnalean mosses had a unique ability to switch the development of leaf areolation between a pathway unique to Sphagnum and another one common to all other mosses. This developmental polyvariancy hinders attempts to classify these mosses, as characters previously considered to be of generic significance can be shown to co-occur in one individual leaf. New understanding of the ontogeny has allowed us to re-evaluate the systematic significance of such diagnostic characters in these Palaeozoic plants, showing that their similarity to Sphagnum is less substantial.