While recognition of the distinct Ordovician period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1906, when it was adopted as an official period of the Paleozoic era by the International Geological Congress.The Ordovician period started at a major extinction event called the Cambrian-Ordovician extinction events some time about 488.3 ± 1.7 Ma (million years ago), and lasted for about 44.6 million years. It ended with the Ordovician–Silurian extinction event, about 443.7 ± 1.5 Ma (ICS, 2004) that wiped out 60% of marine genera.
The boundary chosen for the beginning both of the Ordovician period and the Tremadocian stage is highly useful. Since it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species, the base of the Tremadocian allows scientists not only to relate these species to each other, but to species that occur with them in other areas as well. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.
Paleogeography and atmosphere
Sea levels were high during the Ordovician; in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved in the rocks.
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| Paleogeography during the Middle Ordovician Period, about 470 Ma (Picture from Wikipedia) |
During the Ordovician, the southern continents were collected into a single continent called Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents Laurentia (present-day North America), Siberia, and Baltica (present-day northern Europe) were still independent continents (since the break-up of the supercontinent Pannotia earlier), but Baltica began to move towards Laurentia later in the period, causing the Iapetus Ocean to shrink between them. The small continent Avalonia separated from Gondwana and began to head north towards Baltica and Laurentia. The Rheic Ocean between Gondwana and Avalonia was formed as a result.
A major mountain-building episode was the Taconic orogeny that was well under way in Cambrian times. In the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide into the atmosphere, turning the planet into a hothouse. These volcanic island arcs eventually collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician these volcanic emissions had stopped. Gondwana had by that time neared or approached the pole and was largely glaciated.
The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.
Climate
The Early Ordovician climate was thought to be quite warm, at least in the tropics. As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. The Panthalassic Ocean covered much of the northern hemisphere, and other minor oceans included Proto-Tethys, Paleo-Tethys, Khanty Ocean which was closed off by the Late Ordovician, Iapetus Ocean, and the new Rheic Ocean.
As the Ordovician progressed, we see evidence of glaciers on the land we now know as Africa and South America. At the time these land masses were sitting at the South Pole, and covered by ice caps.
Life
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| Large Nautiloids like Orthoceras were among
the largest predatory animals in the Ordovician. (Picture from
Wikipedia) |
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| Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee. (Picture from Wikipedia) |
For most of the Late Ordovician, life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected planktonic forms like conodonts, graptolites, and some groups of trilobites (Agnostida and Ptychopariida, which completely died out, and the Asaphida which were much reduced). Brachiopods, bryozoans and echinoderms were also heavily affected, and the endocerid cephalopods died out completely, except for possible rare Silurian forms. The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.
Fauna
Though less famous than the Cambrian explosion, the Ordovician featured an adaptive radiation, the Ordovician radiation, that was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those which would dominate for the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids; articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.
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| Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota. (Click for larger view. From Wikipedia) |
In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. The first Bryozoa appeared in the early Ordovician as did the first coral reefs, although solitary corals date back to at least the Cambrian.
Molluscs, which had appeared during the Cambrian or even the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods.
Now-extinct marine animals called graptolites thrived in the oceans. Some new cystoids and crinoids appeared.
It was long thought that the first true vertebrates (fish — Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian. The very first gnathostome (jawed fish) appeared in the Late Ordovician epoch.
During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution. It is marked by a sudden abundance of hard substrate trace fossils such as Trypanites, Palaeosabella and Petroxestes.
In the Early Ordovician, trilobites were joined by many new types of organisms, including tabulate corals, strophomenid, rhynchonellid, and many new orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first sea stars. Nevertheless the trilobites remained abundant, with all the Late Cambrian orders continuing, and being joined by the new group Phacopida. The first evidence of land plants also appeared; see Evolutionary history of life.
In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs and echinoderms all flourished, tabulate corals diversified and the first rugose corals appeared; trilobites were no longer predominant. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. Bioerosion became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensive carbonate hardgrounds which appear in abundance at this time. One of the earliest known armoured agnathan ("ostracoderm") vertebrate, Arandaspis, dates from the Middle Ordovician.
Trilobites in the Ordovician were very different than their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive sharks and nautiloids while other trilobites such as Aeglina prisca evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the trinucleids developed a broad pitted margin around their head shields. Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.
Flora
Marine fungi were abundant in the Ordovician seas to decompose animal carcasses, and other wastes.
Green algae were common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing in the form of tiny non-vascular mosses resembling liverworts. Fossil spores from land plants have been identified in uppermost Ordovician sediments.
Among the first land fungi may have been arbuscular mycorrhiza fungi (Glomerales), playing a crucial role in facilitating the colonization of land by plants through mycorrhizal symbiosis, which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores from the Ordovician of Wisconsin have been found with an age of about 460 million years ago, a time when the land flora most likely only consisted of plants similar to non-vascular bryophytes.
End of Period Extinction Events
The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct. The only larger one was the Permian-Triassic extinction event.
The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following Silurian Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were greatly reduced, along with many trilobite, conodont and graptolite families.
The most commonly accepted theory is that these events were triggered by the onset of most cold conditions in the late Katian, followed by an ice age, in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of the Ordovician.
The ice age was possibly not long-lasting, study of oxygen isotopes in fossil brachiopods showing that its duration could have been only 0.5 to 1.5 million years. Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.
The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm) which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.
Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change. Species limited to a single epicontinental sea on a given landmass were severely affected. Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.
Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.
At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders.
Melott et al. (2006) suggested a ten-second gamma ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly radiation, but most scientists agree that extinction events are complex with multiple causes.
End of Monday Reading
Orogeny refers to forces and events leading to a severe structural deformation of the earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts. The word "orogeny" comes from the Greek (oros for "mountain" plus genesis for "birth" or "origin"), and it is the primary mechanism by which mountains are built on continents. Orogens develop while a continental plate is crumpled and thickened to form mountain ranges, and involve a great range of geological processes collectively called orogenesis.
Formation of an orogen is accomplished in part by the tectonic processes of subduction, where a continent rides forcefully over an oceanic plate (noncollisional orogens), or convergence of two or more continents (collisional orogens).
Orogeny usually produces long arcuate (from arcuare, to bend like a bow) structures, known as orogenic belts. Generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with subduction zones, which consume crust, produce volcanoes, and build island arcs. The arcuate structure is attributed to the rigidity of the descending plate, and island arc cusps are related to tears in the descending lithosphere. These island arcs may be added to a continent during an orogenic event.
The processes of orogeny can take tens of millions of years and build mountains from plains or even the ocean floor. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by the dense underlying mantle.
Frequently, rock formations that undergo orogeny are severely deformed and undergo metamorphism. During orogeny, deeply buried rocks may be pushed to the surface. Sea bottom and near shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, the resulting mountains can be very high (see Himalaya).
An orogenic event may be studied as (a) a tectonic structural event, (b) as a geographical event, and (c) a chronological event. Orogenic events (a) cause distinctive structural phenomena related to tectonic activity, (b) affect rocks and crust in particular regions, and (c) happen within a specific period of time.
Orogenic cycle
Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including magmatization, metamorphism, crustal melting, and crustal thickening. Just what happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis.
In addition to orogeny, the orogen once formed is subject to other processes, such as sedimentation and erosion. The sequence of repeated cycles of sedimentation, deposition and erosion, followed by burial and metamorphism, and then by formation of granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle. For example, the Caledonian Orogeny refers to the Silurian and Devonian events that resulted from the collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle.
In simple summary, an orogeny is a long-lived deformational episode in which many geological phenomena play a role. The orogeny of an orogen is only part of the orogen's orogenic cycle. Among the other phases is erosion, described next.
Erosion
Erosion inevitably removes much of the mountains, exposing the core or mountain roots (metamorphic rocks brought to the surface from a depth of several kilometres). Such exhumation may be helped by isostatic movements balancing out the buoyancy of the evolving orogen. There is debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics). Thus, the final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and dip away from the orogenic core.
An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults. These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism.
Biology
The study of orogeny, coupled with biogeography (the study of the distribution and evolution of flora and fauna),[12] geography and mid ocean ridges in the 1950s and 1960s, contributed greatly to the theory of plate tectonics. Even at a very early stage, life played a significant role in the continued existence of oceans, by affecting the composition of the atmosphere. The existence of oceans is critical to sea-floor spreading and subduction.
Relationship to mountain building
Mountain formation
occurs through a number of mechanisms.
“Mountain complexes result from irregular successions of tectonic responses
due to sea-floor spreading, shifting lithosphere plates, transform faults,
and colliding, coupled and uncoupled continental margins.”
Large modern orogenies often lie on the margins of continents; the Alleghenian (Appalachian), Laramide, and Andean orogenies are examples of these in the Americas. Older, now inactive orogenies, such as the Penokean and Antler, are represented by deformed rocks and sedimentary basins further inland.
Areas that are rifting apart, such as the mid-ocean ridges and the East African Rift have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography. In strike-slip systems, such as the San Andreas Fault, restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that are not necessarily on tectonic plate boundaries.
Regions can also experience uplift as a result of
delamination of the lithosphere,
in which an unstable portion of cold
lithospheric root drips down into the
mantle, decreasing the density of the lithosphere and causing buoyant
uplift. An example is the
Sierra Nevada in
California. This
range of
fault-block mountains
experienced renewed uplift after a delamination of the lithosphere beneath
them.
Finally, uplift and erosion related to
epeirogenesis
(large-scale vertical motions of portions of continents without much
associated folding, metamorphism, or deformation) can create local
topographic highs.
End of Reading
Wednesday - Case Study - The Appalachians
The geology of the Appalachians dates back to the Ordovician, more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor - strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians and neighboring Anti-Atlas (now in Morocco) near the center. These mountain ranges were once higher than today's Himalaya mountain range, which was also formed by continental collision.
During the earliest Paleozoic Era, the continent that would later become North America straddled the equator. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal Plain Province. During this interval, the region was periodically submerged beneath shallow seas. Thick layers of sediment and carbonate rock were deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.
During the middle Ordovician Period (about 480-440 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event (Taconic orogeny) in North America.
Taconic Orogeny
The Taconic orogeny was a great mountain building period that perhaps had the greatest overall effect on the geologic structure of basement rocks within the New York Bight region. The effects of this orogeny are most apparent throughout New England, but the sediments derived from mountainous areas formed in the northeast can be traced throughout the Appalachians and midcontinental North America.
Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to grow progressively narrower. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to fold gradually downward. In this manner, shallow-water carbonate deposition that had persisted on the continental shelf margin through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. Sometime during this period a convergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along a subduction zone with an eastward-dipping orientation. Partial melting of the down-going plate produced magma that returned to the surface to form the offshore Taconic island arc. By the Late Ordovician, this island arc had collided with the North American continent. The sedimentary and igneous rock between the land masses were intensely folded and faulted, and were subjected to varying degrees of intense metamorphism. This was the final episode of the long-lasting mountain-building period referred to as the Taconic Orogeny.
When the Taconic Orogeny subsided in the New York Bight region during Late Ordovician time (about 440 million years ago), subduction ended, culminating in the accretion of the Iapetus Terrane onto the eastern margin of the continent. This resulted in the formation of a great mountain range throughout New England and eastern Canada, and perhaps to a lesser degree, southward along the region that is now the Piedmont of eastern North America. The newly expanded continental margin gradually stabilized. Erosion continued to strip away sediments from upland areas. Inland seas covering the Midcontinent gradually expanded eastward into the New York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically-quiet period persisted until the Late Devonian time (about 360 million years ago) when the next period of mountain-building began, the Acadian orogeny.
Acadian Orogeny
The Acadian orogeny is a middle Paleozoic mountain building event (orogeny), especially in the northern Appalachians, between New York and Newfoundland. The Acadian orogeny most greatly affected the Northern Appalachian region (New England northeastward into the Gaspé region of Canada). The Acadian orogeny should not be regarded as a single tectonic event, but rather as an orogenic era. It spanned a period of about 50 million years, from 375 to 325 million years ago. In Gaspé and adjacent areas, its climax is dated as early in the Late Devonian, but deformational, plutonic, and metamorphic events extended into Early Mississippian time. During the course of the orogeny, older rocks were deformed and metamorphosed, and new faults formed and older faults were reactivated.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of the Great Basin.
The cause of this great period of deformation is a result of the
plate-docking of a small continental landmass called
Avalonia (named after
the
Avalon Peninsula
of Newfoundland). The docking of Avalonia onto the composite margin of
Ganderia and Laurentia resulted in the closing of a portion of the
Rheic Ocean.
Avalonia was gradually torn apart as plate tectonic forces accreted the
landmass onto the edge of the larger North American continent. Today,
portions of the ancient Avalonia landmass occur in scattered outcrop belts
along the eastern margin of North America. One belt occurs in Newfoundland,
another forms the bedrock of much of the coastal region of New England from
eastern Connecticut to northern Maine
A period of
lithospheric
thinning that followed the Acadian orogeny created
volcanoes, such as the
large
Mount
Pleasant Caldera in southwestern
New Brunswick,
Canada.
Alleghenian (Appalachian) Orogeny
The Alleghenian orogeny or Appalachian orogeny is one of the geological mountain-forming events (orogeny) that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling "Alleghany Orogeny" (sic) originally proposed by H.P. Woodward (1957, 1958) is preferred usage. Approximately 350 million to 300 million years ago, in the Carboniferous period, when Gondwana (specifically what became Africa) and what became North America collided, forming Pangaea. This collision exerted massive stress on what is today the Eastern Seaboard of North America, resulting in a large-scale uplift of the entire region. Closer to the boundary between the colliding plates, tectonic stresses contributed to the metamorphosizing of the rock (i.e. the transformation of igneous and sedimentary rock into metamorphic rock). These stresses concurrently caused faults (mostly thrust faults and some strike-slip faults) as well as folding. The immense region involved in the continental collision, the vast temporal length of the orogeny and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians could have risen as high or perhaps even higher than the present-day Himalaya.
The Appalachian Orogeny is responsible for the creation of the mountains themselves and is not responsible for the topography that now typifies the Piedmont and coastal plain regions east of the mountain chain. The heavily-eroded hills of Piedmont are remnants of the sizeable mountain chain, while the coastal plain is made up of the material that was washed away in that process. Thus, the coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years ago to the present.
Evidence for the Appalachian orogeny stretches for many hundreds of miles on the surface from Alabama to New Jersey and can be traced further subsurface to the southwest. In the north it enters a region of confused topography associated with earlier orogenies, but clearly the Applachian deformation extends northeast to Newfoundland.
The mountains were once rugged and high, but in our time are now eroded into only a small remnant. Sediments that were carried eastward form part of the continental shelf. Sediments that were carried westward form the Allegheny and Cumberland Plateau, which in some areas are popularly called mountains, but are actually simply uplifted and eroded plateaus. Carbonates and fine sediments from these mountains were carried farther to form limey rocks in a shallow sea that was later uplifted and forms the bulk of Tennessee, Kentucky, Ohio, and Indiana.
A portion of the Alleghenian mountain system departed with Africa when Pangaea broke up and the Atlantic Ocean began to form. Today, this forms the Anti-Atlas mountains of Morocco. The Anti-Atlas have been geologically uplifted in relatively recent times, and are today much more rugged than their Alleghenian relatives.
Erosion
Pangea began to break up about 220 million years ago, in the Early Mesozoic Era (Late Triassic Period). As Pangea rifted apart a new passive tectonic margin was born and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and the mountains began to wear away.
By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present formed. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.
End of Reading
Thursday - Ordovician Species In-Depth - Bryozoa
The Bryozoa, also known as Ectoprocta, are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches, and others are found in polar waters. One class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. Over 4,000 living species are known. One genus is solitary and the rest colonial.
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 |
Fossils of about 15,000 bryozoan species have been found. Marine taxa with mineralized skeletons appear in rocks dating from the Arenigian stage of the Early Ordovician period, about 480. At this point all the modern orders of stenolaemates were present, and the ctenostome order of gymnolaemates had appeared by the Middle Ordovician, about 465. Other types of filter feeders appeared around the same time, which suggests that some change made the environment more favorable for this lifestyle. Fossils of cheilostomates, another order of gymnolaemates, first appear in the Mid Jurassic, about 172, and these have been the most abundant and diverse bryozoans from the Cretaceous to the present. Evidence compiled from the last 100 million years show that cheilostomates consistently grew over cyclostomates in territorial struggles, which may help to explain how cheilostomates replaced cyclostomates as the
 |
| Bryozoan fossils in an Upper Ordovician oil shale (kukersite), northern Estonia. |
dominant marine bryozoans. Marine fossils from the Paleozoic era, which ended 251, are mainly of erect forms, those from the Mesozoic are fairly equally divided by erect and encrusting forms, and more recent ones are predominantly encrusting. Fossils of the soft, freshwater phylactolaemates are very rare, appear in and after the Late Permian (which began about 260) and consist entirely of their durable statoblasts. There are no known fossils of freshwater members of other classes.
Since all the other phyla that have left fossils are found in Cambrian rocks, it is surprising that the earliest bryozoan fossil dates from the Ordovician, which immediately followed the Cambrian. This suggests that the first bryozoans appeared much earlier and were entirely soft-bodied, and the Ordovician fossils record the appearance of mineralized skeletons in this phylum. The Early Ordovician fossils may also represent forms that had already become significantly different from the original members of the phylum.
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