EVOLUTION OF PHYTOLITH DEPOSITION IN MODERN BRYOPHYTES—IMPLICATIONS FOR EARLY LAND PLANTS

Silica accumulations in the inter- and intra-cellular spaces of plants, known as phytoliths, are well known within all the major lineages of vascular plants. These phytoliths can remain in the fossil record even when the rest of the plant is not preserved. Because they can have shapes that are diagnostic of certain clades of plants, they can be useful indicators of past vegetation and ecosystems. Unlike in vascular plants, phytolith deposition in bryophytes has not been extensively studied, although previous studies have listed them as silica accumulators and phytoliths have been observed in the moss Polytrichum stictum. In this study, we test to what extent phytolith deposition occurs in the bryophytes, whether this deposition is ancestral within each phylum, and what the resulting patterns say about the likelihood of phytolith deposition in early land plants. We also evaluate whether phytoliths can be used to trace the Paleozoic emergence of plants on land and/or emergence of bryophyte lineages. To do this we sampled multiple species of liverworts, mosses, and hornworts from the Burke Herbarium at the University of Washington and numerous other herbaria, and extracted phytoliths from this material. Phytoliths were observed in multiple lineages within all three bryophyte phyla. However, ancestral state reconstruction in mosses and liverworts suggest that significant phytolith deposition was not ancestral in these phyla; rather, it is a derived feature with certain clades of liverworts and mosses. It remains unclear whether rare/light phytolith deposition was plesiomorphic within land plants. The presence of phytoliths from water conducting cells in the simple thalloid liverwort clade Pallaviciniales suggests that phytolith production may have been occurring in bryophytes by at least the Devonian given the similarities between modern Pallaviciniales species and the Devonian fossil Pallaviciniites. The clades of bryophytes with the greatest observed phytolith deposition include the complex thalloid liverworts and the acrocarpous moss orders Polytrichales and Bryales. Given that these mosses have more highly developed water conducting tissue relative to most other mosses, we hyopothesize a relationship between increased water conducting capabilities and an increase in phytolith deposition.