Geology Reference
In-Depth Information
Chapter Summary
Table 21.4 (page 591) summarizes the major evolutionary
and geologic events of the Paleozoic Era and shows their
relationships to one another.
Multicelled organisms presumably had a long Precam-
brian history, during which they lacked hard parts.
Invertebrates with hard parts suddenly appeared dur-
ing the Early Cambrian in what is called the “Cambrian
explosion.” Skeletons provided such advantages as protec-
tion against predators and support for muscles, enabling
organisms to grow large and increase locomotor effi -
ciency. Hard parts probably evolved as a result of various
geologic and biologic factors rather than from a single
cause.
Marine organisms are classifi ed as plankton if they are
fl oaters, nekton if they swim, and benthos if they live on
or in the seafl oor.
Marine organisms are divided into four basic feeding
groups: suspension feeders, which consume microscopic
plants and animals as well as dissolved nutrients from
water; herbivores, which are plant eaters; carnivore-
scavengers, which are meat eaters; and sediment-deposit
feeders, which ingest sediment and extract nutrients
from it.
The marine ecosystem consists of various trophic levels
of food production and consumption. At the base are pri-
mary producers, on which all other organisms are depen-
dent. Feeding on the primary producers are the primary
consumers, which in turn are fed on by higher levels of
consumers. The decomposers are bacteria that break
down the complex organic compounds of dead organ-
isms and recycle them within the ecosystem.
The Cambrian invertebrate community was dominated
by three major groups: the trilobites, brachiopods, and
archaeocyathids. Little specialization existed among the
invertebrates, and most phyla were represented by only a
few species.
The Middle Cambrian Burgess Shale contains one of the
fi nest examples of a well-preserved soft-bodied biota in
the world.
The Ordovician marine invertebrate community marked
the beginning of the dominance of the shelly fauna
and the start of large-scale reef building. The end of the
Ordovician Period was a time of major extinctions of
many invertebrate phyla.
The Silurian and Devonian periods were times of diverse
faunas dominated by reef-building animals, whereas the
Carboniferous and Permian periods saw a great decline
in invertebrate diversity.
Chordates are characterized by a notochord, dorsal hol-
low nerve cord, and gill slits. The earliest chordates were
soft-bodied organisms that were rarely fossilized. Verte-
brates are a subphylum of the chordates.
Fish are the earliest known vertebrates, with their fi rst
fossil occurrence in Upper Cambrian rocks. They have
had a long and varied history, including jawless and
jawed armored forms (ostracoderms and placoderms),
cartilaginous forms, and bony forms. It is from the lobe-
fi nned fi sh that amphibians evolved.
The link between crossopterygian lobe-fi nned fi sh and
the earliest amphibians is convincing and includes a
close similarity of bone and tooth structures. The transi-
tion from fi sh to amphibians occurred during the Late
Devonian. During the Carboniferous, the labyrintho-
dont amphibians were the dominant terrestrial verte-
brate animals.
The earliest fossil record of reptiles is from the Late
Mississippian. The evolution of an amniote egg was
the critical factor that allowed reptiles to completely
colonize the land.
Pelycosaurs were the dominant reptile group during
the Early Permian, whereas therapsids dominated the
landscape for the rest of the Permian Period.
In making the transition from water to land, plants
had to overcome the same basic problems as animals—
namely, desiccation, reproduction, and gravity.
The earliest fossil record of land plants is from Middle
to Upper Ordovician rocks. These plants were probably
small and bryophyte-like in their overall organization.
The evolution of vascular tissue was an important event
in plant evolution as it allowed food and water to be
transported throughout the plant and provided the plant
with additional support.
The ancestor of terrestrial vascular plants was probably
some type of green algae based on such similarities as
pigmentation, metabolic enzymes, and the same type
of reproductive cycle.
The earliest seedless vascular plants were small, leafl ess
stalks with spore-producing structures on their tips.
From this simple beginning, plants evolved many
of the major structural features characteristic of
today's plants.
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
Search WWH ::
Custom Search