550 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
from borings in shells suggests that this form
of predation was an important part of the
ecosystem. The rapid diversifi cation of
armored and protective strategies was a
feature of the Cambrian radiation. It is prob-
able that predation had an important infl u-
ence on evolutionary processes by the
coevolution of predator–prey systems, where
prey and predator organisms evolve ever-
better defensive and offensive strategies,
respectively. Such “arms races” became inten-
sifi ed several times during the Phanerozoic,
most notably in the Cretaceous when new
effi cient predators such as crabs, hole-boring
gastropods and teleost fi shes caused the evo-
lution of dramatic changes in the lifestyles of
their prey (see Chapters 14 and 16).
Adaptation 6: biological reefs
Biological reefs are the marine equivalents of
tropical rain forests. Modern reefs are highly
diverse, colorful frameworks of brain, horn
and staghorn corals, together with organ
pipes, sea fans and sea whips, providing food
and accommodation for thousands of species
from the mantis shrimp to the carpet shark.
These large carbonate structures form the
basis of many types of tropical islands, from
barrier reefs to fringing atolls. The origin of
reefs was a major event.
Throughout the Phanerozoic, the main reef
builders have changed, with major changes
punctuated by mass extinctions. The fi rst
reefal frameworks appeared during the Early
Cambrian, constructed fi rst by solitary, clus-
tered polychaete worm tubes, and then by
archaeocyathans (see Chapter 10). Ordovi-
cian reefs were dominated by algae, bryozo-
ans, stromatoporoids and rugose and tabulate
corals (see Chapters 10 and 11). Tabulate
corals and stromatoporoids, together with
algae and bryozoans, dominated reefs from
the Silurian until near the end of the Devo-
nian. Carboniferous and Permian reefs were
made from bryozoans, algae and calcareous
sphinctozoan sponges. During the Mid Trias-
sic, frameworks of algae, scleractinian corals,
bryozoans and sphinctozoan sponges devel-
oped. Jurassic and Cretaceous reefs were con-
structed by scleractinian corals, lithothamnian
algae and siliceous and sphinctozoan sponges,
to which were added rudist bivalves in the
Late Cretaceous. During the Tertiary, sclerac-
tinian corals expanded to their present diver-
sity, where they now dominate biological
frameworks.
Adaptation 7: terrestrialization
The colonization of the land added major new
environments to those previously occupied by
life. It is hard to date the fi rst move of life on
to land. Soils have been reported from Mid
Precambrian sequences, and microbial life
may have extended a greenish scum around
the water’s edge. Ordovician soils suggest that
larger plants and animals had moved onto
land. By the Silurian, small vascular plants
such as Cooksonia, with a well-developed
vascular system, stomata, a waxy covering
and trilete spores, were well established (see
Chapter 18). The new land plants relied on
the soil for some of their nutrients, but they
also generated modern-style soils over land-
scapes that had previously been bare rock.
This stabilization of the land by plant growth
slowed down the rate of erosion, and it was
one of the most dramatic effects that life has
had on the physical nature of the Earth.
Colonizing invertebrates were faced with
problems of dehydration, respiration and, to
a lesser extent, support. These problems were
overcome by the development of waterproof
skins, lungs and skeletal support. Although
hydrostatic skeletons, such as those of slugs,
have been successful, the toughened exoskel-
eton of arthropods was an ideal protective
covering, providing support and attachment
for the soft parts. By the Early Devonian, the
low green vegetation was inhabited by myria-
pods, insects and possibly arachnids. During
the Carboniferous, these faunas were supple-
mented by oligochaete worms and scorpions,
together with both prosobranch and pulmo-
nate gastropods (see Chapters 12 and 14).
Vertebrates moved onto the land during the
Devonian, presumably to exploit the new
sources of plant and invertebrate food, and
full terrestrialization occurred with the rep-
tiles in the mid-Carboniferous, when the
amniotic egg evolved (see Chapter 16).
Adaptation 8: trees and forests
The next major expansion of living space on
land took place during the Carboniferous,
with the development of forests. The fi rst tree-