bacteria helped resist erosion by binding the sediment grains together and soak-
ing up rainwater. Bacteria also provided nutrients for the early land plants.
Plant fossils of Ordovician age appear to be almost entirely composed of
algae similar to present-day algal mats found on seashores and the bottoms of
ponds (Fig. 66). Some marine algae lived in the intertidal zones and could
withstand only short periods out of the sea due to the risk of dehydration.
Even after developing protective measures to help the organisms survive out
of water for longer periods, they still depended on the sea for reproduction.Figure 66Rafts of blue-
green algae in a pool in
Indian Canyon, Duchesne
County, Utah.
(Photo by W. H. Bradley,
courtesy USGS)
Historical Geology