Introduction

The origin of a land flora during the Upper Ordovician–Lower Silurian border (± 440 millions years ago) represents a significant evolutionary event in the history of life. The subsequent evolution of a diverse auto trophic land flora created the conditions necessary for the diversification of a terrestrial hetero trophic fauna (Behrensmeyer et al. 1992). Today the land flora comprises roughly 300000 species distributed among three major groups, the bryophytes, the pteridophytes, and the seed plants. Considering the evolutionary significance of land plants, as well as their dominance and thus their ecological importance in today's biosphere, it is not surprising that evolutionary biologists are investing much time and resources in understanding the transition to land, and the relationships between major lineages of terrestrial plants. Elucidating the evolution of early land plants may allow for a better understanding on how these plants overcame new obstacles encountered in a terrestrial habitat. Character innovations that coincide with exposure to new selective forces can then be examined in a phylogenetic context for their evolutionary significance or potential adaptive value (Knoll et al. 1984, Knoll & Niklas 1987).