as the Ordovician, plant fossils appeared to be composed almost entirely of
algae, which probably formed stromatolite mounds and algal mats similar to
those on seashores today (Fig. 77).
The early seaweeds were soft and nonresistant. Generally, they did not
fossilize well. A seaweedlike plant grew half submerged in estuaries and rivers.
However, for plants to be truly shore bound, they had to reproduce entirely
out of water. The first land plants achieved this function with sacs of spores
attached to the ends of simple branches.When the spores matured, they were
cast into the air and carried by the wind to suitable sites where they could
grow into new plants.
The first complex plants livedin shallow waters just below the surface
probably as a protection against high levels of solar ultraviolet radiation.When
the atmospheric oxygen content rose to near present-day levels, the upper
stratospheric ozone layer screened out the deadly ultraviolet rays, enabling life
to flourish on Earth’s surface. Soon after plants crept ashore, the land was
sprawling with lush forests.
Before the invasion of the true plants, a slimy coating of photosynthe-
sizing cyanobacteria, or blue-green algae, might have inhabited the land. A
cover of algae accelerated the weathering of rock and the formation of soils
and nutrients required for advanced plant life. Prior to the emergence of land
plants, microbial soils were making Earth more hospitable for life out of water.
The microorganisms probably formed a dark, knobby soil, resembling lumpy
mounds of brown sugar spread over the landscape. In this manner, for about
half a billion years, simple plants paved the way for more advanced vegetation.
Prior to the arriv al of terrestrial microbes, the continents were much too
hot to support complex life-forms.The early organisms played an important role
in cooling the land surface by drawing down the atmosphere’s surplus carbon
dioxide for use in photosynthesis.The loss of this potent greenhouse gas cooled
the climate and allowed higher forms of life to populate the continents. TheFigure 77Algal mounds
rising toward the surface
from the surrounding
limestone bottom of a
barrier reef in Saipan,
Mariana Islands.
(Photo by P.T. Cloud,
courtesy USGS)
Historical Geology