EARLY PALEOZOIC ERA

CAMBRIAN PERIOD: 542 to 488 million years ago

ORDOVICIAN PERIOD: 488 to 444 million years ago

CONTINENTS

During the Cambrian Period, ancestral North America was positioned close to the equator and about 30% of its mass was covered by shallow seas (including one along the eastern border which would include parts of New York and Pennsylvania) were covered by a shallow sea. The ancient Iapetus Ocean which separated North America and Europe began its gradual closure as North America and Europe collided. The Iapetus was narrower than the modern North Atlantic and, given that South America and Africa were fused, there was no equivalent of the South Atlantic. The great Tethys seaway separated the northern continents and the southern continents. Warm shallow seas covered much of the interior of Ancestral North America (and other continents) during the Cambrian and Early Ordovician. By the end of the Cambrian, more than half of the continental landmasses were covered by shallow seas. The sites of the future Appalachian, Caledonian, and Ural mountains began their uplifting (Stokes, 1982; Seyfert, 1979; Dunbar, 1969).

By the Late Ordovician, Ancestral North America and Ancestral Europe were fused. The Appalachian mountains were rising. Thrusting and tectonic activity was occurring in Eastern PA. and Southeastern New York. The "slate belt" of New Jersey and Pennsylvania represents Ordovician rock. Petroleum reserves which were discovered (and exhausted) from sites such as Ohio, Oklahoma, and Texas dated from the Ordovician. Seaways retreated and returned to cover continental landmasses (including about half of North America). A shallow seaway probably connected Northwestern Europe to the Southwestern Appalachian mountains, given the great number of shared species between sites as far as Scotland and the American Southwest. A global cooling resulted in the formation of continental glaciers across the regions of South America and Africa (such as the modern Sahara Desert) which were close to the South Pole (Seyfert, 1979).

LIFE ON LAND:

Although terrestrial environments seem to have hosted early plants and a few types of arthropod, there were no trees, ferns, flowers, insects, or vertebrates.


MARINE LIFE:

sea life


In the Cambrian, reefs were composed of algae and archaeocyathans. A number of indicators (such as the existence of reefs far from the equator) suggests that the climate was warmer than normal. In the Ordovician, corals diversified and the first coral reefs were known. Marine life became much more complex, and included a diversity of shelled brachiopods and squidlike nautiloids encased in a hard shell.

brachiopodnautiloid

Arthropods diversified into a variety of groups, including trilobites.

trilobitestrilobitestrilobites

The earliest jawless fish diversified into a variety of lineages.

fishfish

conodont

EXTINCTIONS

Archaeocyanthans were an ancient group of sponges which left no living descendants. They were extremely successful in the early Cambrian but became extinct by Middle Cambrian for unknown reasons. With their loss, many organisms which depended on the reef habitats they provided were also affected (Prothero, 1998).Towards the end of the Ordovician, it is estimated that 57% of marine genera became extinct. The diversity of trilobites was greatly reduced. Many types of echinoderms, brachiopods (more than half species), bryozoans (more than half), early corals, cephalopods, acritarchs, and conodonts became extinct. Global cooling might have caused this mass extinction since the animals of warmer water seem to have been affected most. Because it lasted 2 million years, no one single event (like an asteroid) is likely to have caused it.