From Protosphagnum to Sphagnum? Additional support for M. F. Neuburg's hypothesis on the evolution of leaf...

Protosphagnacean mosses, whose relationships and evolutionary history are incompletely understood, were widespread during the Permian across northern regions of the Northern Hemisphere. M. F. Neuburg proposed that these Permian mosses evolved from a state of absent or weak leaf laminal cellular dimorphism (the Intia type) to marked cellular dimorphism (the Protosphagnum type). Here, we test Neuburg's hypothesis by evaluating changes in the diversity of leaf structure of protosphagnacean mosses from three fossil localities of early, middle and late Permian age.

Leaves obtained through bulk maceration of rock samples from three localities in north-eastern European Russia separated from each other by several million years were studied to document the cell differentiation patterns, with special focus on leaf apical portions.

In the family Protosphagnaceae, the sphagnoid areolation pattern (similar to that of extant Sphagnum leaves) developed following two pathways. In the sphagnacean pathway, the sphagnoid areolation pattern derived from one initial cell via two successive unequal divisions; in the protosphagnacean pathway, the sphagnoid areolation pattern resulted from development in neighbouring cells not necessarily descended from the same initial cell. Large laminal hyaline cells became numerous only in the late Permian.

Our results are consistent with Neuburg's hypothesis, showing a progressive increase in the frequency and diversity of leaf cellular dimorphism in protosphagnacean mosses from the early to the late Permian and indicating that a leaf developmental pathway highly similar to that of extant Sphagnum appeared in the late Permian, bolstering the hypothesis that extant Sphagnum originated from Permian protosphagnaceans.