Life of the Past
The Cambrian Period
The Cambrian Period (600 million years ago, first
period of the Paleozoic) is named from Wales (Latin,
Cam bria), where rocks of this age were first
studied. In the Lower Cambrian, the first common and
widespread fossils occur: algae, arthropods,
brachiopods, sponges, coelenterates, worms, mollusks
and echinoderms. All lived in the sea. It is
surprising to find so many relatively complex groups
in the oldest fossil-bearing rocks. But in many ways
Cambrian animals are primitive. Brachiopods are
represented by the inarticulate forms (p. 82) and
the echinoderms by primitive edrioasteroids.
Most trilobites were large, but a few (the
agnostids and eodiscids, pp. 94-95) were among the
smallest and least ornamented. The first ostracods
appeared in the Lower Cambrian. Mollusks were mostly
represented by tiny sea snails (gastropods), but
bivalves (pelecypOds) appeared in the Upper Cambrian
algae were very similar to their very simple
PreCambrian ancestors.
Middle Cambrian Burgess Shale in British Columbia
contains remarkable fossils —including softbodied
worms and sea cucumbers. The seas in which these
creatures lived occupied two great subsiding belts
(geosynclines) in North America. Cambrian rocks have
a thickness of over 12,000 ft. in parts of the Rocky
Mountains.
THE ORDOVICIAN PERIOD The Ordovician
Period (425 to 500
million years ago) was named for the Ordovices, an
ancient Celtic
tribe. The Ordovician Period saw the rise of new
animal
groups of great importance. Bony fragments from the
Middle Ordovician of Colorado and Wyoming are
evidence of the oldest vertebrates, but we do not
yet know much about these fish-like creatures.
Tetra corals,
graptolites, echinoids, asteroids, crinoids and
bryozoans all appeared for the first time, while the
articulate brachiopods far outnumbered the
inarticulate. Most of the trilobites were different
from those of the Cambrian. Some cephalopods
reached a length of 13 feet..
In parts of North America and Europe, Ordovician
seas covered areas that had been land during
Cambrian times. Volcanoes belched lava locally.
Uplift and mountain building occurred in eastern
North America. Not all the rocks laid down in those ancient seas contain the same
fossils. Limestone and shales around Cincinnati, Ohio,
contain beautifully preserved brachiopods, corals, bryozoans, mollusks, trilobites and crinoids. Black
shale of
the same age in New York, Quebec and Wales contain
graptolites and occasional trilobites. Different Ordovician
environments enabled different animals to prosper in
each region. Most common were shallow-water lime and
mud deposits noted for their well-preserved fossils. The repeated and widespread invasion of North America
by Ordovician seas has produced extensive Ordovician
sediments. Outcrops of these rocks occur widely over
much of the continent. Some Ordovician sediments are
important oil producers, and Ordovician slates are quarried in Vermont. The
Silurian period (for Silures, an ancient tribe of
the Welsh borderland) lasted from 425 to 405 million
years ago. Its faunas differ from those of the
Ordovician in the presence of new families and
genera, rather than in the appearance of completely
new groups of animals. In fact, the most important
newcomers are not animals, but plants. Fossils of
the oldest land plants come from the Upper Silurian
of Australia (p. 151). Fragments of what may be
still earlier land plants have recently been found
in the Ordovician of Poland and southeastern United
States. Some of the best Silurian fossils
(including algae, corals, stromatoporoids,
brachiopods, crinoids and trilobites) come from the
ancient reefs in Silurian limestone, such as those
near Chicago. Scorpion-like eurypterids, some nine
feet long, lived in estuaries and lagoons. Several
types of fish are well preserved in parts of the
Upper Silurian. Silurian volcanic activity occurred
in many areas, and in Scandinavia and Britain
mountain building took place at the close of the
Period. In other places, deserts and land-locked
seas were present in which salt deposits
accumulated, such as those of New York, Ohio and
Michigan. These deposits are still worked as
important commercial sources of salt. Outcrops of
Silurian rocks are common in eastern North America.
As in the Ordovician Period, the character of the
rocks and of the fossils is evidence of widespread
and generally shallow seas.
The Devonian period
The Devonian Period began about 405 million years ago and ended about 60
million years later. It saw the great expansion of
fishes, land plants, and the first land animals,
primitive amphibians. The fishes (pp. 132- 138)
included several kinds of jawless fish (ostracoderms),
plate-skinned fish (placoderms), sharks and the
first bony fishes (osteichthyes). From one group,
the lobe-fin fishes (crossopterygians), the first
amphibia (ichthyostegids) arose. These show a
mixture of fish and amphibian characters. These
unusual fossils, from a warm, moist environment,
were found in mountains of Greenland. The oldest
spiders, millipedes and insects appear in the
Devonian, as do fresh-water clams. Early land plants
were simple, lacking true roots and leaves, but with
the vascular or conducting system found in all later
land plants. Late in the Devonian, great forests of
scale trees and seed ferns were widespread.
Devonian coral reefs include large cup corals two
feet high and compound corals eight feet across.
Horn corals were numerous and varied. Brachiopods
and mollusks continued to flourish; the first common
ammonites appeared, but true graptolites were
already extinct and trilobites were greatly reduced
in numbers. In many continental areas thick deposits
of red sands and muds accumulated.
The Mississippian Period
The Mississippian period is named for the limestone
bluffs along the Mississippi River where typical
outcrops occur. It was a period (345 to 310 million
years ago) of shallow, warm seas, in which corals,
brachiopods, crinoids, blastoids, bryozoans and
foraminifera flourished. In places these fossils are
so abundant that they make up most of the rocks. On
land, amphibia continued to develop, while land
plants spread in all moist areas and anticipated the
great coal swamp forests of the Pennsylvanian. Much
of North America except the far west and the east
coast was under water during Mississippian time.
The Pennsylvanian Period
The Pennsylvanian period (310 to 280 million years
ago) was named after the great coal-bearing strata
of Pennsylvania. It saw the development of lowlands,
great swamps, and deltas surrounded and often
covered by shallow seas. Some of the land was barren
sand deserts (English Midlands), or salt basins
(Colorado). Great trees, some 150 feet high, formed
the coal forests in low swampy land that was often
flooded. Most common were the scale trees (lycopods),
seed ferns (pteridosperms), horsetails and cordaites.
Here lived giant “dragonflies,” with a 30-inch
wingspan, and many kinds of amphibians.
Pennsylvanian rivers and deltas were inhabited by
countless clams, other shellfish and fishes. This
period also saw the emergence of the reptiles from
amphibian ancestors. In addition to Tuditanus
(above), fossils representing four groups of
primitive reptiles have been found in the shales of
Kansas. The seas continued to support rich
invertebrate life, which included abundant
spindle-shaped foraminifera (fusulinids), corals,
brachiopods, mollusks, bryozoans, crinoids,
ostracods and a few trilobites. THE PERMIAN PERIOD
The Permian Period (280 to 230 million years ago) began with typical
coal-forest plants, which were later replaced by
primitive conifers, especially in semi-arid upland
regions. In parts of the Southern Hemisphere the
most common plants were a distinctive group of
tongue ferns (Glossopteris). Many new insects
appeared, including beetles and true dragonflies.
Streams and ponds contained a variety of fishes.
Amphibians flourished along their banks, but were
overshadowed by newer, more active reptiles. Early
reptiles differed from amphibia only in details of
the skull and vertebrae. Seymouriamorphs were
squat, lumbering reptiles about two feet long, with
flat, massive heads. Fossil eggs from the Lower
Permian of Texas, the oldest land eggs known, may
belong to them. Other reptiles were quite different.
Drmetrodon, the sail-backed lizard, was a savage
carniyore, about 10 feet long. Edaphosaurus, a
vegetarian, was also a sailback. The purpose of
these sails is obscure. They may have served as
primitive temperature controls.
Other Permian reptiles included mesosaurs, small,
longsnouted, aquatic creatures, and other species
similar, but unrelated, to modern lizards. Another
group, the theriodonts (beast teeth), known from
South Africa and Russia, were small, agile
carnivores, from which mammals are descended.
Cynognathus was a typical theriodont, about 6 feet
long, with a doglike skull, and differentiated
teeth. Its legs, placed below the body, lifted it
clear of the ground. This was a better adaptation to
a more active life than the sprawling legs of
amphibians and primitive reptiles. The close of
the Permian marked the end of the Paleozoic Era—the
first great chapter in the recorded history of life.
By then, many animals and plants which had dominated
the Paleozoic scene had become extinct. Fusulinid
foraminifera, various bryozoans, rugose corals,
productid brachiopods, trilobites and blastoids all
vanished, as well as many crinoids and cephalopods.
Giant scale trees dwindled in numbers. Most
horsetails and many ferns became extinct.
Amphibia and some fish underwent a drastic
reduction. Why this happened is not clear, but
it may have been connected with extreme climatic
changes during the late Permian when seas were
very restricted and large, high continents
emerged. In many areas, coral reefs fringed the
shores of deserts and vast inland salt lakes
formed. Extensive glaciers covered parts of the
Southern Hemisphere. New mountain chains slowly
rose, the Appalachians and the Urals among them. |
