those that went extinct were many trilobite species. Before the extinction,
trilobites accounted for about two-thirds of all species but only a third there-
after. One dominant group of trilobites called the Ibex fauna vanished at the
end-Ordovician mass extinction possibly due to widespread glaciation. They
were succeeded by the Whiterock fauna, which tripled their genera and sailed
through the extinction practically unscathed.
Graptolites (Fig. 65) were colonies of cupped organisms resembling a
conglomeration of floating stems and leaves. Certain groups went extinct at
the end of the Ordovician. Graptolites were thought to have suffered total
extinction in the late Carboniferous about 300 million years ago. However,
the discovery of living pterobranchs, possible modern counterparts of grapto-
lites, suggest these might be living fossils.
Near the end of the Ordovician 450 million years ago, the concentra-
tion of atmospheric oxygen generated sufficient levels of ozone in the upper
stratosphere to shield Earth from the Sun’s deadly untraviolet rays. Thus, for
the first time, plants began to come ashore to populate the land. When the
early plants first left the oceans and lakes for a home on dry land, they were
greeted by a harsh environment. Ultraviolet radiation, desert conditions, and
lack of nutrition made life difficult. First to greet the land plants were soil bac-
teria that churned sediments into lumpy brown mounds. The presence of
these bacteria helped speed weathering processes, without which hot bare
rock would have covered most of the landscape and land plants would have
had little success gaining a roothold.
Cyanobacteria, formerly called blue-green algae, might have been prepar-
ing the soil for the land invasion as early as 3 billion years ago. Ancient
cyanobacteria, which were resistant to high levels of ultraviolet radiation, first
lived in shallow tide pools, from which they eventually colonized the conti-
nents. They might have improved the terrestrial climate for a life out of water
by drawing downatmospheric carbon dioxide, thereby countering the green-
house effect, which prev ents thermal energy from escaping the planet.The soil
Figure 65Graptolites
appeared to havegone
extinct in the late
Carboniferous.ORDOVICIAN VERTEBRATESNemaPeridermTheca