Sea levels rose again and deposited a new transgres-sive sequence, known as the Tippecanoe sequence, from 490 to 410 million years ago during the Ordo-vician and Silurian Periods. Life in the Ordovician changed drastically from what it was in the Cambrian, and a great number of organisms flourished in the Ordovician. This may be attributed to the large amount of continental submergence (under the shallow seas), which created a large number of ecological niches, and also from the mild, steady climate of the times. Trilobites were not as numerous in the Ordovician as they were in the Cambrian, and they were less abundant than the Brachiopods and Bryo-zoans. Other organisms saw major changes in the Ordovician. The phosphatic shells of brachiopods were beginning to be gradually replaced by calcareous shells, and mollusks (snails) experienced rapid evolution, with new groups appearing, including the cephalopod (Nautiloids), which appeared and went through a very rapid evolution, including the appearance of giant forms reaching 30-40 feet (9-12 m). Since the cephalopods were also common, they make excellent index fossils for the Ordovician. Bryozoans appeared for the first time, making hard skeletal structures out of calcium carbonate, and various types of corals appeared including the solitary rugose coral, and more colonial forms that gradually developed prismatic forms that allowed them to grow more closely together. Graptolites are distinctive index fossils since they were widely distributed and evolved quickly. Graptolites were probably floating colonies that resembled seaweed but are generally preserved as single blades about a quarter inch to inch (0.5-3 cm) long. The Ordovician also saw the development of the first vertebrates, with fish containing armored plates found in a few fossil locations.
The shallow Ordovician seas had diverse ecosystems, and many of the modern ecosystem types such as continental shelves and reefs were firmly established in the Ordovician. At this time most life on Earth was based on the seafloor, between the shoreline and the deep abyssal plains. These included a number of different types of organisms, such as infaunal organisms that live within the bottom sediments, and epifaunal organisms that live on the bottom surface. Sessile benthic dwellers are those that are attached to some object on the bottom, whereas burrowers move through the sediment. Vagrant ben-thic organisms move around on the bottom, whereas deposit feeders eat small organic particles in the bottom sediments. suspension or filter feeders capture and eat other organisms that float in the water, whereas planktonic organisms are passive floaters that live above the bottom. Nektons are actively swimming organisms, most of whom live in the photic zone, through which light can penetrate in the oceans.
Like the Sauk Sequence, the base of the Tippeca-noe sequence is marked by a pure quartz sandstone or quartzite, known as the St. Peter sandstone, forming a volume of quartz of some 4,800 cubic miles (20,000 km3). The base of the St. Peter sandstone has slightly different ages in different places, being slightly older at the outer edges of the craton than in the interior of the continent, showing that unconformities can be time-transgressive, having different ages in different places.
Life in the Tippecanoe Sea was quite different from that in the Sauk Sea of the early Ordovician. The Tippecanoe ecosystems included diverse types of corals, stromatoporoids, bryozoan cephalopods, brachiopods, and armored fish. Upper Ordovician limestones are typically very shelly. The upper Ordo-vician Seas reached all-time highs, representing the most complete flooding of continent ever in the geological past.
Middle to Late Orodovician deposition on the eastern side of the North American craton changed from limestone- to graptolite-bearing black shales, indicating a deepening of the water conditions. This deepening reflects a drastic and important change related to the approach of the Taconic island arc that would soon collide with the then North American craton in the Middle Ordovician. Other evidence also suggests that mountains were being uplifted on the eastern side of the craton during the Middle to Late ordovician. These include the presence of volcanic ashes interbedded with the shales, and the fact that the black shales are succeeded eastward and upward by a clastic wedge, containing sandstone, conglomerate shale, and so on eroded from mountains in the east. This clastic wedge represents a foreland basin, where many layers of sandstone, shale, and conglomerate were deposited by rivers that flowed out of the rising mountain range in the east, then redeposited by turbidity currents in deeper water environments during active deformation of the mountain range in a sequence of rocks known as flysch. The flysch, deposited during active deformation of the mountain range in the east is succeeded upward by deposits of molasse, which is nonmarine irregularly stratified conglomerate, sandstone, shale, and coal deposited in the late stages of mountain building.
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