The ORDOVICIAN ( /ɔːrdəˈvɪʃən/ ) is a geologic period and
system , the second of six periods of the
Paleozoic Era . The
Ordovician spans 41.2 million years from the end of the Cambrian
Period 485.4 million years ago (Mya) to the start of the Silurian
Period 443.8 Mya.
The Ordovician, named after the Celtic tribe of the
Ordovices , was
Charles Lapworth in 1879 to resolve a dispute between
Adam Sedgwick and
Roderick Murchison , who were placing
the same rock beds in northern Wales into the
Cambrian and Silurian
periods, respectively. Lapworth recognized that the fossil fauna in
the disputed strata were different from those of either the Cambrian
Silurian periods, and placed them in a period of their own. It
received international sanction in 1960, when it was adopted as an
official period of the
Paleozoic Era by the International Geological
Life continued to flourish during the
Ordovician as it did in the
Cambrian period, although the end of the period was marked by
Silurian extinction event . Invertebrates, namely
molluscs and arthropods , dominated the oceans. The Great Ordovician
Biodiversification Event considerably increased the diversity of life.
Fish , the world's first true vertebrates , continued to evolve, and
those with jaws may have first appeared late in the period. Life had
yet to diversify on land. About 100 times as many meteorites struck
the Earth during the
Ordovician compared with today.
* 1 Dating: extinction events
* 2 Subdivisions
* 2.1 British stages
* 3 Paleogeography
Ordovician meteor event
* 4 Geochemistry
* 5 Climate and sea level
* 6 Life
* 6.1 Fauna
* 6.2 Flora
* 7 End of the period
* 8 References
* 9 External links
DATING: EXTINCTION EVENTS
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Ordovician Period began with a major extinction called the
Cambrian–Ordovician extinction event , about 485.4 Mya (million
years ago). It lasted for about 42 million years and ended with the
Silurian extinction event , about 443.8 Mya (ICS, 2004)
which wiped out 60% of marine genera . The dates given are recent
radiometric dates and vary slightly from those found in other sources.
This second period of the
Paleozoic era created abundant fossils that
became major petroleum and gas reservoirs.
The boundary chosen for the beginning of both the
Tremadocian stage is highly significant. It correlates well
with the occurrence of widespread graptolite , conodont , and
trilobite species. The base (start) of the
scientists to relate these species not only to each other, but also to
species that occur with them in other areas. This makes it easier to
place many more species in time relative to the beginning of the
A number of regional terms have been used to subdivide the
Ordovician Period. In 2008, the ICS erected a formal international
system of subdivisions, illustrated to the right. There exist
Baltoscandic, British, Siberian, North American, Australian, Chinese
Arenig continues until the early Darriwilian,
subsuming the Dapingian. The Llanvirn occupies the rest of the
Darriwilian, and terminates with it at the base of the Late
Sandbian represents the first half of the Caradoc; the
Caradoc ends in the mid-Katian, and the Ashgill represents the last
half of the Katian, plus the Hirnantian.
During the Ordovician, the southern continents were collected into
Gondwana started the period in equatorial latitudes and, as
the period progressed, drifted toward the
South Pole .
Early in the Ordovician, the continents of
Laurentia (in present-day
North America ), Siberia , and
Baltica (present-day northern Europe)
were still independent continents (since the break-up of the
Pannotia earlier), but
Baltica began to move towards
Laurentia later in the period, causing the
Iapetus Ocean between them
to shrink. The small continent
Avalonia separated from
began to move north towards
Baltica and Laurentia, opening the Rheic
Gondwana and Avalonia.
Taconic orogeny , a major mountain-building episode, was well
under way in
Cambrian times. In the early and middle Ordovician,
temperatures were mild, but at 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, a greenhouse gas, into the
atmosphere, turning the planet into a hothouse.
Initially, sea levels were high, but as
Gondwana moved south, ice
accumulated into glaciers and sea levels dropped. At first, low-lying
sea beds increased diversity, but later glaciation led to mass
extinctions as the seas drained and continental shelves became dry
land. During the Ordovician, in fact during the Tremadocian, marine
transgressions worldwide were the greatest for which evidence is
These volcanic island arcs eventually collided with proto North
America to form the Appalachian mountains. By the end of the Late
Ordovician the volcanic emissions had stopped.
Gondwana had by that
time neared the south pole and was largely glaciated .
ORDOVICIAN METEOR EVENT
Ordovician meteor event is a proposed shower of meteors that
occurred during the
Middle Ordovician period, roughly 470 million
years ago. It is not associated with any major extinction event.
External mold of
Ordovician bivalve showing that the original
aragonite shell dissolved on the sea floor, leaving a cemented mold
for biological encrustation (
Waynesville Formation of Franklin County,
Ordovician was a time of calcite sea geochemistry in which
low-magnesium calcite was the primary inorganic marine precipitate of
Carbonate hardgrounds were thus very common, along
with calcitic ooids , calcitic cements, and invertebrate faunas with
dominantly calcitic skeletons. Biogenic aragonite , like that
composing the shells of most mollusks , dissolved rapidly on the sea
floor after death.
Cambrian times, when calcite production was dominated by
microbial and non-biological processes, animals (and macroalgae)
became a dominant source of calcareous material in Ordovician
CLIMATE AND SEA LEVEL
Ordovician saw the highest sea levels of the
Paleozoic , and the
low relief of the continents led to many shelf deposits being formed
under hundreds of metres of water. The sea level rose more or less
continuously throughout the
Early Ordovician , leveling off somewhat
during the middle of the period. Locally, some regressions occurred,
but sea level rise continued in the beginning of the
Late Ordovician .
Sea levels fell steadily in accord with the cooling temperatures for
~30 million years leading up to the
Hirnantian glaciation. During this
icy stage, sea level seems to have risen and dropped somewhat, but
despite much study the details remain unresolved.
At the beginning of the period, around 485.4 million years ago, the
climate was very hot due to high concentration of CO2 (4200 ppm) in
the atmosphere, which gave a strong greenhouse effect. By contrast,
today the concentration is above 400 ppm. Marine water temperatures
are assumed to have averaged 45 °C (113 °F), which restricted the
diversification of complex multi-cellular organisms. But over time,
the climate became cooler, and around 460 million years ago, the ocean
temperatures became comparable to those of present-day equatorial
North America and
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
Khanty Ocean , which was closed
off by the Late Ordovician,
Iapetus Ocean , and the new
Rheic Ocean .
Ordovician progressed, we see evidence of glaciers on the land
we now know as
South America , which were, at the time near
South Pole , and covered by ice caps .
A diorama depicting
Ordovician flora and fauna.
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 and graptolites .
Ptychopariida completely died out, and
Asaphida 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
Silurian Extinction Events may have been caused by an ice
age that occurred at the end of the
Ordovician period, due to the
expansion of the first terrestrial plants , as the end of the Late
Ordovician was one of the coldest times in the last 600 million years
of earth history.
_ Nautiloids like
Orthoceras _ were among the largest predators
in the Ordovician. Fossiliferous limestone slab from the
Liberty Formation (Upper Ordovician) of Caesar Creek State Park near
On the whole, the fauna that emerged in the
Ordovician were the
template for the remainder of the Palaeozoic. The fauna was dominated
by tiered communities of suspension feeders, mainly with short food
chains. The ecological system reached a new grade of complexity far
beyond that of the
Cambrian fauna, which has persisted until the
Though less famous than the
Cambrian explosion , the Ordovician
radiation was no less remarkable; marine faunal genera increased
fourfold, resulting in 12% of all known
Phanerozoic marine fauna.
Another change in the fauna was the strong increase in filter-feeding
organisms. The trilobite , inarticulate brachiopod , archaeocyathid ,
and eocrinoid faunas of the
Cambrian were succeeded by those that
dominated 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.
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. Although solitary corals date back
to at least the
Cambrian , reef -forming corals appeared in the early
Ordovician, corresponding to an increase in the stability of carbonate
and thus a new abundance of calcifying animals.
Molluscs , which appeared during the
Cambrian or even the
became common and varied, especially bivalves , gastropods , and
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
Middle Ordovician there was a large increase in the
intensity and diversity of bioeroding organisms. This is known as the
Bioerosion Revolution. It is marked by a sudden abundance
of hard substrate trace fossils such as _
Petroxestes _ and _
Osprioneides _. Several groups of
endobiotic symbionts appeared in the Ordovician.
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, all the Late Cambrian
orders continued, and were 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 , cornulitids , tentaculitids 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
Bioerosion became an important process, particularly
in the thick calcitic skeletons of corals, bryozoans and brachiopods,
and on the extensive carbonate hardgrounds that 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 from their
predecessors in the
Cambrian . Many trilobites developed bizarre
spines and nodules to defend against predators such as primitive
eurypterids 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. Molecular clock analyses suggest that early arachnids
started living on land by the end of the Ordovician.
Ordovician edrioasteroid _Cystaster stellatus_ on a cobble
from the Kope Formation in northern Kentucky. In the background is the
cyclostome bryozoan _Corynotrypa_.
Fossil Mountain, west-central Utah;
Middle Ordovician fossiliferous
shales and limestones in the lower half.
Outcrop of Upper
Ordovician rubbly limestone and shale, southern
College of Wooster students.
Outcrop of Upper
Ordovician limestone and minor shale, central
College of Wooster students.
Trypanites _ borings in an
Ordovician hardground , southeastern
Petroxestes _ borings in an
Ordovician hardground , southern Ohio.
Ordovician kukersite oil shale , northern
Bryozoan fossils in
Ordovician kukersite oil shale, northern Estonia
Brachiopods and bryozoans in an
Ordovician limestone, southern
_Vinlandostrophia ponderosa_, Maysvillian (Upper Ordovician) near
Madison, Indiana. Scale bar is 5.0 mm.
Ordovician cystoid _
Echinosphaerites _ (an extinct echinoderm )
from northeastern Estonia; approximately 5 cm in diameter.
_Prasopora_, a trepostome bryozoan from the
Ordovician of Iowa.
Ordovician strophomenid brachiopod with encrusting inarticulate
brachiopods and a bryozoan.
The heliolitid coral _Protaraea richmondensis_ encrusting a
gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana.
_Zygospira modesta_, atrypid brachiopods, preserved in their original
positions on a trepostome bryozoan; Cincinnatian (Upper Ordovician) of
Graptolites (_Amplexograptus_) from the
Ordovician near Caney
Green algae were common in the Late
Cambrian (perhaps earlier) and in
the Ordovician. Terrestrial plants probably evolved from green algae,
first appearing as tiny non-vascular forms resembling liverworts .
Fossil spores from land plants have been identified in uppermost
Ordovician sediments. The green algae were similar to today's sea moss
. Colonization of land would have been limited to shorelines
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 THE PERIOD
Main article: Ordovician–
Silurian extinction events
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 became 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 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.
Oxygen isotopes in fossil
brachiopods show its duration may 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.
Ordovician glaciation event was preceded by a fall in
atmospheric carbon dioxide (from 7000 ppm to 4400 ppm). The dip was
triggered by a burst of volcanic activity that deposited new silicate
rocks, which draw CO2 out of the air as they erode. This selectively
affected the shallow seas where most organisms lived. As the southern
Gondwana drifted over the South Pole, ice caps formed
on it, which have been detected in Upper
Ordovician rock strata of
Africa and then-adjacent northeastern South America, which were
south-polar locations at the time.
As glaciers grew, the sea level dropped, and the vast shallow
Ordovician seas withdrew, which eliminated many
ecological niches. When they returned, they carried diminished founder
populations that lacked many whole families of organisms. They then
withdrew again with the next pulse of glaciation, eliminating
biological diversity with 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
Those species able to adapt to the changing conditions survived to
fill 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.
An alternate extinction hypothesis suggested that a ten-second
gamma-ray burst could have destroyed the ozone layer and exposed
terrestrial and marine surface-dwelling life to deadly ultraviolet
radiation and initiated global cooling.
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