1. Figure 11.16: The Later Ordovician (490-440 m.y. ago) was a time when North America was almost completely covered by a warm, shallow sea following the widespread Tippecanoe transgression. In fact, all continents were almost completely covered with water during this time.

2. The relatively archaic animals of the Cambrian seafloor were replaced in the Late Cambrian and Ordovician by a great diversity of different fauna consisting of corals, brachiopods, cephalopods, bryozoans and crinoids. This complex array included swimmers, floaters, attached filter feeders, burrowers, victims and predators.

3. Figure 11.4: Life in the Ordovician became organized into a complex food chain where primary producers (algae) where fed upon by snails. Microscopic plankton constituted the food for filter feeders such as sponges, stromatoporoids, echinoderms, bryozoans, brachiopods and corals.

4. Trilobites were scavengers of the seafloor while straight-shelled nautiloids (which grew up to two meters in length) became the top predators.

5. After these organisms died, they underwent bacterial decay and their nutrients were recycled back into the food chain.

6. Figure 11.5: During the Cambrian, most invertebrates (sponges and archaeocyathids) fed only a few cm above the sea floor while trilobites scavenged the seafloor. The Ordovician was a time when multiple feeding levels began to develop both above and below the sea floor. Brachiopods attached to the substrate fed on food particles only a few centimeters above the seafloor while long-stalked crinoids extended up to 3 meters.

1. Figure 11.6: The most common fossils in Ordovician rocks are the brachiopods. Although Brachiopod larvae swim about freely, the adults are frequently anchored or cemented to objects on the sea floor by a fleshy stalke (pedicle) or by spines.

2. Brachiopods in some ways resemble clams but differ from clams in shell symmetry. Brachiopod valves are symmetric on either side of the midline but the two valves differ from each other in size and shape.

3. Articulate varieties had teeth and sockets in their hinge area in order to connect their shells whereas inarticulate brachiopods held their shells together mainly by muscles.

4. Articulate brachiopods covered the Ordovician sea floor in tremendous numbers filtering out food particles from the water.

5. Figure 11.4: Two groups of brachiopods were characteristic of the Ordovician: The orthids were simple forms with finely ribbed shells. The strophomenids were more abundant and typically were shaped like a "D". Some strophomenids were extremely flat while others had bowl-shaped shells.

1. Figure 11.6: Bryozoans are colonial animals that form coral-like skeletons with thousands of tiny holes. Ordovician bryozoans ranged from branching varieties to massive colonies having irregular, lumpy shapes.

2. Each hole houses a tiny filter-feeding animal (zooid) with a mouth surrounded by a tentacled feeding organ called the lophophore.

3. The earliest unquestioned occurence of bryozoans is from Lower Ordovician strata of the Baltic, but they did not become abundant until Middle Ordovician and Silurian times.

4. Like the corals, bryozoans contribute to the framework of reefs.

1. Figure 11.5: Crinoids "sea lilies" are echinoderms related to starfish and sea urchins.

2. Crinoids consist of long stalks rooted to the seafloor with arms extended into filter-feeding fans. Some crinoid stalks were long enough for the tentacles to reach several meters above the seafloor.

3. The arms bear ciliated food grooves that serve to move food particles towards the mouth.

1. The Ordovician was a time when the first true coral reefs appeared, some exceeding 100 meters in length and 6-7 meters in height.

2. Corals are members of the phylum Cnidaria that also includes sea anemones and jellyfish.

3. Figure 11.6: Two groups of Paleozoic corals became the dominant reef builders; the rugosids "horn corals" and the tabulate "honeycomb" corals.

4. Corals and sea anemones are generally in the polyp form while jellyfish are in the Medusa form. The polyp secretes a calcareous cup in which it lives.

5. An extinct group of sponges, the stromatoporoids, were also important Ordovician reef builders and typically formed fibrous, calcarious skeletons of pillars stacked into thick layered structures similar to Precambrian cyanobacterial mats (stromatolites).

6. A fourth Ordovician reef builder were the receptaculitids resembled the head of sunflowers and therefore are also known as "sunflower corals". Receptaculitids were actually not corals at all but were probably formed by algae.

1. Figure 11.2: Ordovician Nautiloids were hard-shelled predators related to the modern squid and octopus.

2. Nautiloids reached 10 meters in length and were typically encased in a long conical shell. Their huge heads contained long, grasping tentacles that snared fauna and crushed their shells using strong, parrot-like beaks.

Starfish were also important predators in the Ordovician seas and used the suckers on their tube feet to pull apart the shells of brachiopods and clams in order to devour the soft tissue inside.

1. Figure 11.6: Trilobites were important scavengers in the Cambrian and continued to be during the Ordovician.

2. Trilobites were swimming or crawling arthropods. Their skeleton was composed of chitin (hard outer covering of insects) strengthened by calcium carbonate. As in many other arthropods, growth was accomplished by molting. Although many trilobites were sightless, the majority had either single-lens eyes or compound eyes composed of a large number of discrete visual bodies.

3. Trilobites were most abundant in the Late Cambrian and remained fairly abundant throughout the Ordovician and Silurian, but declined abruptly afterwards. Trilobites were extinct by the end of the Paleozoic Era (250 m.y. ago).

1. Gastropods (snails) first appeared in Lower Cambrian strata.

2. Ordovician snails had coiled shells taking on a variety of forms and orientations that housed the soft parts of the animal. The soft parts (as in modern forms) consist of a distinct head, mouth, eyes, tentacles and a flattened foot to provide for gliding movement.

3. Some Ordovician snails were predatory and drilled through the shells of brachiopods in order to to feed on the soft parts inside. Other gastropods were herbiverous.

4. The advent of herbiverous gastropods led to the final decline of stromatolites, which were virtually wiped out by the end of the Ordovician. Today, stromatolites only exist in a few refuges (e.g. salt water or areas of strong currents) that are inhospitable to grazing snails.

1. Figure 11.7: For a long time, scientists puzzled over the origin of mysterious carbonized streaks that looked like pencil marks on black shale. These strange markings eventually were recognized to be the fossils of graptolites.

2. Graptolites were colonial animals with multiple cups on a long central rod, each cup housing a tiny filter-feeding animal.

3. Graptolites arose in the Late Cambrian and became abundant in the Ordovician and Silurian seas.

4. Most graptolites floated on the sea surface and were distributed worldwide, thus making them important index fossils. When they sank to the bottom in deep waters, they were preserved in stagnant mud that eventually lithified into black shale.

5. Graptolites were thought to have become extinct during the late Paleozoic. Recently, however, living creatures resembling graptolites were found in sediment dredged from the deep sea off New Caledonia.

1. Conodonts are microscopic, tooth-like fossils that were long thought to represent the teeth of some extinct vertebrate. Recent studies, however, have led some scientists to consider conodonts as part of a worm-like or eel-like organism and were used to support some kind of grasping or breathing structure in the mouth or throat region.

2. Conodont elements arranged in a natural assemblage.

3. Several fossils of conodont animals have been reported over the past three decades, but the most informative fossil was found in 1995 in Ordovician shale in South Africa. This fossil resembled an elongate, eel-like body about 40 cm long and similar to the living cephalochordate Branchiostoma. The exact function of the conodont elements, however, is still debatable.

4. Conodonts diversified in the Ordovician and were distributed worldwide during the Paleozoic, but became extinct by Triassic time.

5. Conodonts are useful as good index fossils. Color alteration in conodonts can be used to estimate the depth of burial and maximum temperature attained by the enclosing strata.

1. Figure 11.8: Ostracodes are microscopic, shrimplike crustaeans that lived inside bean-shaped shells that were hinged over their backs. Ostracodes are suspension feeders and live within the surface plankton or on the bottom of marine and fresh waters.

2. Ostracods first appeared in the Cambrian and evolved during the Ordovician, some reaching almost a cm in length. Modern ostracods, however, are microscopic in size.

1. Figure 11.9: The earliest vertebrates were the jawless fish which first appeared in the Ordovician. Modern examples of jawless fish include the hagfish and lamprey.

2. Early Paleozoic jawless fish are collectively termed ostracoderms which means "shell skin" and refers to the bony exterior that was a distinctive trait of many of these fishes, although some were unarmored. Jawless fish contained an internal skeleton made of cartilage. The jawless fish had a simple ringlike mouth opening but could not bite and probably filtered food either out of the mud on the sea floor or directly from seawater.

3. Figure 11.9: The fossil remains of Ordovician fish mainly consist of isolated plates and scales in marine rocks, although small bony scales resembling fish scales were recently found in Upper Cambrian sandstones in Wyoming.

1. Figure 11.10: Fauna that dominated the Cambrian (trilobites, archaeocyathids, archaic brachiopods) mostly became extinct by the late Paleozoic. The Ordovician radiation, however, gave rise to the "Paleozoic fauna" that proliferated until the great Permian extinction. They continue to survive in lesser numbers today.

2. No one knows for sure what caused the great Ordovician radiation of fauna but several suggestions have been made:

(a) The late Cambrian and Ordovician was a time when every continent was almost completely flooded, forming enormous shallow seas where marine life could diversify.

(b) Oxygen levels in the atmosphere, which gradually increased throughout the Precambrian, finally reached modern levels during the Ordovician. This, in combination with warm waters, made CaCO3 readily available for shell formation.

(c) The appearance of efficient predators with crushing jaws put pressure on archaic animals to either specialize or go extinct, thus leading to greater diversity.

1. The end of the Ordovician marked a major extinction event when over 100 families of marine animals worldwide were wiped out. In North America, more than half the species of brachiopods and bryozoans disappeared. Nautiloids were decimated and the number of trilobites significantly declined.

2. Late Ordovician extinctions occurred mostly among tropical animals whereas survivors and replacements were adapted either to deep waters or colder waters from higher latitudes. This suggests that a severe, worldwide cooling event was responsible for the Late Ordovician extinctions.

3. This worldwide cooling event is evident in Late Ordovician glacial deposits found in North Africa which, during Late Ordovician time, was located near the south pole.

Figure 11.13: The six major sequences designated by L.L. Sloss. Note the Tippecanoe Sequence.

Figure 11.14: Every continent contains rock sequences that are preserved over vast areas and these sequences are separated from adjacent strata by major unconformities. The ages of the extensively preserved strata are often similar on different continents. If these large areas of preserved strata are indicative of major transgressions, the age similarities of these strata for different continents suggests that the transgressions affected all continents simultaneously and are probably related to global (eustatic) sea-level rise. Conversely, similarities in the ages of major unconformities on different continents suggest global sea-level falls.

1. Figure 11.12: During the Late Cambrian and Early Ordovician, the Sauk transgression led to flooding of the North American continent. Following deposition of dolomite across vast areas of the continent during the Early Ordovician, the sea retreated and the interior of the continent was again exposed to weathering and erosion.

2. The North American continent underwent extensive erosion and removal of previously deposited rock sequences resulting in a widespread unconformity.

3. Subsequent rise of sea level during the Ordovician Tippecanoe transgression deposited the St. Peter Sandstone directly above the unconformity. The St Peter Sandstone therefore defines the base of the Tippecanoe Sequence.

4. The St. Peter Sandstone consists of sediments derived from previous Cambrian sandstone that were eroded during exposure of the continental interior. Much of the St. Peter was dispersed across exposed portions of the craton by wind and rivers during continental exposure and was further reworked and re-deposited during the subsequent Tippecanoe transgression. Much of the St. Peter sandstone therefore represents near-shore, beach deposits although dune and river deposits are also preserved.

5. Sediments comprising the St Peters sandstone were derived through erosion of Cambrian sandstone followed by extensive reworking. The St. Peter sandstone is therefore very well sorted and the individual grains are well rounded and frosted, indicating extensive abrasion.

1. Figure 11.16: Following deposition of the St. Peter Sandstone, marine carbonates containing abundant fossils formed throughout the craton in the warm, shallow sea. These limestone deposits indicate that the Late Ordovician epieric sea was teaming with life and the continent was still tropical. Late Ordovician sediments also included hummocky stratification (deposition by shallow water currents), hard-ground surfaces (indicating occasional subareal exposure) and deep basin deposits suggesting that the continent had attained greater relief since the Cambrian.

2. Figure 11.15: Upper Ordovician sedimentation patterns throughout the cration provides evidence to the widespread extent of the Late Ordovician sea. The absence of Ordovician strata over some areas indicate subsequent warping and erosion.

1. In contrast to the limestone deposition that occurred over most of the North American craton during the Late Ordovician, sedimentation within the Appalachian Region involved deposition of dark mudstone, sporadic graywacke and chert.

2. Graptolitic shale deposits are also found along the eastern portion of the craton and indicate thickening eastward towards some former sediment source area. East of the graptolitic shale deposits, volcanic lava flows and ash deposits hint at the existence of a nearby volcanic arc.

3. These sedimentary deposits hint at a major tectonic disturbance in the area during Ordovician time. This major tectonic disturbance is termed the Taconian Orogeny.

1. Figure 11.21: There is further evidence that a major mountain-building event occurred along eastern North America during the Ordovician.

2. In the northern Appalachian region, conglomerates with fragments of mixed Cambrian and Ordovician formations occur locally within black shale sequences and possibly represent submarine avalanches of debris derived from shallower marine environments. These deep-water conglomerate deposits suggest earthquake disturbances.

3. Widespread volcanic ash and dark, graptolite-bearing mud reflect structural unrest and crustal uplift somewhere towards the east.

4. Local red shale in the southern Appalachians indicate crustal uplift accompanied by subareal erosion.

5. In northeastern U.S. and southeastern Canada, black shale is succeeded eastward by Upper Ordovician red shale that was deposited in non-marine river and deltaic environments. The shale probably comprises a clastic wedge consisting of debris eroded from rising mountains to the east and deposited in an adjacent foreland basin.

6. In New England and adjacent Canada, the presence of Late Ordovician and Early Silurian granites indicate igneous activity associated with a major tectonic disturbance.

7. Long, narrow bands of ultramafic rocks define a suture zone, the site of collision between the North American continent and some sort of terrain.

8. Associated heterogeneous sandstone and conglomerates indicate erosion of complexly deformed rocks of many different kinds and ages.

9. Major unconformities separate Lower- to Middle Ordovician rocks from overlying younger strata.

10. Figure 11.18: Summary of restored cross section showing the relations of Ordovician facies to the Taconian land.

1. Figure 11.22: More than a century ago, J. D. Dana tried to explain the geology of eastern North American by proposing that a large, uplifted area of Precambrian igneous and metamorphic rocks bordered the eastern North American shelf region throughout much of Phanerozoic time. This Precambrian borderland supposedly comprised the eastern source region for clastic-wedge sediments now found in the Appalachian region of eastern North America and Canada.

2. Figure 11.23: The concept of borderlands has since been revised in light of plate tectonic theory so that sedimentary source regions are now explained in terms of orogenic belts.

3. Prior to the beginning of mountain building in mid-Ordovician time, the Appalachian region was a broad, flat, passive continental shelf receiving clastic sediments only from the craton. The transformation of eastern North America into an orogenic belt introduced additional sedimentary sources.

1. Cratonic source consists of westerly-derived mature quartz sand.

2. Volcanic source of volcanic-rich clastic sediments is associated with lava and volcanic ash.

3. Tectonic land source consists of immature feldspathic and lithic graywacke. These vast clastic deposits coarsened towards the east where the source area (composed chiefly of older, partly metamorphosed sedimentary and igneous rocks) was located.

1. Figure 11.28: In Newfoundland, remnants of oceanic lithosphere are overlain by a succession of Ordovician rocks that record a progression from marine to non-marine conditions.

(a) Ophiolite suite consists of oceanic lithosphere.

(b) Flysch facies indicate a marine environment of deposition. Evenly stratified graywacke (with graded bedding) alternate with dark shale, similar to the Ordovician graptolitic shale-graywacke sequence of the Appalachian region. This flysch was probably deposited by turbidity currents within an unstable tectonic setting in close proximity to a tectonically and volcanically active source region.

(c) Molasse facies defined as non-marine deposition. Irregularly stratified coarse sandstone (often cross-bedded) consisting of conglomerate, minor shale, coal, and red beds. This facies reflects erosion above sea level during final uplift of mountains or highlands.

2. The Ordovician rock record of the Appalachian belt represent a progression from stable continental shelf to deep marine conditions, followed by a gradual change to nonmarine conditions. The rock record therefore records the tectonic evolution of eastern North America from passive continental margin to active margin, culminating in collision with a volcanic arc or microcontinent.

3. Figure 11.33: The Early Paleozoic rock record of eastern North America records three different stages of evolution.

(a) Stage One: A Cambrian to early Ordovician passive continental margin with deposition of passive-margin sandstone (cratonic source area) and limestone.

(b) Stage Two: Late Ordovician arc collision and deposition of flysch within a foreland basin.

(c) Stage Three: Uplift and thrusting of the Taconian Mountains resulting in westward deposition of non-marine, molasse-type deposits.

1. Figure 7.37: Beginning in mid-Ordovician time, the eastern passive margin of North America became tectonically active. Eastward subduction of the North American Plate beneath a volcanic arc or continent resulted in gradual closing of the intervening ocean basin.

2. Figure 7.36: The replacement of shallow-marine shelf carbonates by graptolite-bearing black shale and graywacke indicated a deepening of the shelf and formation of a foreland basin due to thrust loading of the continental margin by the westward obducting arc complex.

3. Figure 11.34 illustrates the relationship of Taconian collision to thrust loading of the continental margin, deposition of clastic wedges into foreland basins, creation of the Cincinnati Arch as a foreland bulge and the Michigan Basin as an intracratonic basin.

1. Figure 11.36: Newfoundland, Nova Scotia, eastern Massachusetts and areas of the southern Appalachians were once part of the separate land fragment during the Ordovician known as the Avalonia microcontinent.

2. The Avalonia microcontinent may have rifted from northwestern African during the Early Ordovician and was carried towards North America behind the Taconian volcanic arc.

3. The Taconian volcanic arc collided with North America during the Late Ordovician.

4. Later, Avalonia collided with North America during the Devonian Acadian orogeny.

5. Figure 11.37: By Late Devonian time, the Appalachian region was comprised of a series of exotic terranes consisting of island arcs and microcontinents.