SPEER, Brian R., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
Studies of the origin of land plants have traditionally focused on early vascular plants and their unique adaptations to the terrestrial environment. However, accumulating evidence strongly supports a non-vascular (bryophytic) origin of the land flora. Phylogenetic analyses using both molecular and morphological characters place the bryophytes as a paraphyletic grade in which mosses are sister to the vascular plants, and liverworts are sister to all other land plants.
In this light, a reinterpretation of the earliest plant fossils emphasizes the role of liverworts as ecological models and potential members of the early land flora. Given the great diversity of extant liverworts (more than 6,000 species), only one of the two liverwort classes (Marchantiopsida) will be considered.
Once a framework phylogeny has been generated, it is possible to map characters onto clades, and to map clades onto stratigraphic data. In the case of the earliest plant fossils, the available characters are those of spore structure, shape, and dispersal. Spores of the taxa included in the parsimony analysis were mounted on microscopy stubs and photographed in UCMP's environmental scanning electron microscope (E-SEM). Characters present in the spores may then be mapped onto the existing phylogeny, and the resulting diagram used to interpret the fossil stratigraphy. Results compare favorably, with basal taxa producing spore tetrads as found in Ordovician deposits while separate trilete spores appear later in the Silurian. The congruence of the fossils and spore characters suggest that liverworts may have already undergone an initial radiation by the Silurian. The liverwort spore images serve a second useful purpose. Spore and pollen data can be an important source for reconstructing paleoclimate. There are collected databases and atlases of spore morphology for seed plants, ferns, and mosses, but no such information is yet available for liverworts. The spore images collected for this study will be published both in print and electronically, providing a much needed reference.