Life of the Past

The Paleozoic Era

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 (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 (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.

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