<P> Fossil evidence for cyanobacteria also comes from the presence of stromatolites in the fossil record deep into the Precambrian . Stromatolites are layered structures formed by the trapping, binding, and cementation of sedimentary grains by microbial biofilms, such as those produced by cyanobacteria . The direct evidence for cyanobacteria is less certain than the evidence for their presence as primary producers of atmospheric oxygen . Modern stromatolites containing cyanobacteria can be found on the west coast of Australia . </P> <P> Early plants were small, unicellular or filamentous, with simple branching . The identification of plant fossils in Cambrian strata is an uncertain area in the evolutionary history of plants because of the small and soft - bodied nature of these plants . It is also difficult in a fossil of this age to distinguish among various similar appearing groups with simple branching patterns, and not all of these groups are plants . One exception to the uncertainty of fossils from this age is the group of calcareous green algae, Dasycladales found in the fossil record since the middle Cambrian . These algae do not belong to the lineage that is ancestral to the land plants . Other major groups of green algae had been established by this time, but there were no land plants with vascular tissues until the mid-Silurian . </P> <P> The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance of spores in the fossil record (Cambrian spores have been found, also). The first terrestrial plants were probably in the form of tiny plants resembling liverworts when, around the Middle Ordovician, evidence for the beginning of the terrestrialization of the land is found in the form of tetrads of spores with resistant polymers in their outer walls . These early plants did not have conducting tissues, severely limiting their size . They were, in effect, tied to wet terrestrial environments by their inability to conduct water, like extant liverworts, hornworts, and mosses, although they reproduced with spores, important dispersal units that have hard protective outer coatings, allowing for their preservation in the fossil record, in addition to protecting the future offspring against the desiccating environment of life on land . With spores, plants on land could have sent out large numbers of spores that could grow into an adult plant when sufficient environmental moisture was present . </P> <P> The first fossil records of vascular plants, that is, land plants with vascular tissues, appeared in the Silurian period . The earliest known representatives of this group (mostly from the northern hemisphere) are placed in the genus Cooksonia . They had very simple branching patterns, with the branches terminated by flattened sporangia . By the end of the Silurian much more complex vascular plants, the zosterophylls, had diversified and primitive lycopods, such as Baragwanathia (originally discovered in Silurian deposits in Victoria, Australia), had become widespread . </P>

When do bryophytes (including mosses liverworts and hornworts) first appear in the fossil record