Ordovician
Era 570 mya
When the Cambrian period (dated back to
approximately 445 million years ago) ended by a mass extinction, the Ordovician
period arose. This was approximately 510 million years ago and lasted until 438
million years ago. The land in the northern region was almost completely
submerged in water, while the land in the southern region formed the world's
super-continent called Gondwana, and a few, small additional islands that
surrounded Gondwana.
Events
marking the beginning and end of the geologic period
Beginning
of period
ÿ
The Ordovician period began
when most of the Cambrian trilobites died out. The trilobites were replaced by
various groups of unknown origin during the Ordovician.
ÿ
A boundary marking the end of
the Cambrian and the beginning of the Ordovician is the appearance of planktic
dictyonemid graptolites.
ÿ
The graptolites and
archeocyathids, which are two important groups throughout the Cambrian, become
extinct in the Ordovician.
ÿ
North America finished its
slow immersion under the waters in the Sauk Transgression of Late Cambrian to
the Early Ordovician.
ÿ
The Ordovician could appear
to be an expansion of the Cambrian since the sea was still filled with
invertebrates.ÝÝ However, during the Ordovician
the first animals with backbones arose. These animals were the jawless fishes
with bony skeletons, and scientists call them Agnatha.
End
of Period
Gondwana continually moved south, causing much of the land to be submerged underwater.Ý Consequently, the southern region of Gondwana became immersed in ice as it moved over the South Pole.ÝÝ As a result, the water temperature continued to cool forming ice sheets, which then resulted in massive glaciers, shallow seas beginning to drain, and sea levels dropping.ÝÝ This created the third largest extinction ever.ÝÝ The extinction was 90% at the generic level and 12 % at the family level, resulting in the Silurian period with a uniform, abridged fauna.
http://a300.g.akamaitech.net/7/300/792/1998-10-19/www.britannica.com/eb/image?id=802
Climate/ecological
communities
Climate
Early in the Ordovician
period, the climate was tropical and mildly warm, with an atmosphere containing
an excess of moisture.Ý The moisture was
due to the continents equatorial position; where the equator ran straight
across the middle of continent.ÝÝ On the
contrary, the end of the Ordovician was possibly the coldest time in the
Earthís history due to the glaciation events.
Ecological Communities
Many of the organisms
present during the Cambrian era, extended into the Ordovician period (their
habitat continuing to remain restricted to water).ÝÝ Even though Ordovician communities had a superior ecological complexity
compared to the Cambrian communities, the Cambrian contained organisms that are
more diverse.ÝÝ The Ordovician contained
the highest amount of change at the phylum, class, and order levels.
~For
example, shallow water fauna varied latitudinaly and each new fauna was being
established from Gondwana sources.
This evolution is
partially due to the filling of new ecological niches, and partially due to
frequent extinctions and subsequent radiations which occurred during the
Cambrian.
ÿ
New varieties flourished,
including cephalopods, corals (including rugose and tabulate forms), bryozoans,
crinoids, graptolites, gastropods, and bivalves.
ÿ
ÝThe marine animals operated in conjunction with fish, algae,
cephalopods, coral, crinoids, and gastropods to form a livable community.
ÿ The ecological communities had an increase in predators, such as nautiloids and starfish.
http://www.kheper.auz.com/gaia/Paleozoic/Ordovician/ord04b.gif
Animal
Life
ÿ
In the beginning of the
Ordovician period bushy graptolites formed a foundation for the rapidly
evolving planktonic graptolites.
ÿ
This foundation contained
various marine invertebrates, including graptolites, trilobites, brachiopods
and some early vertebrates, such as the conodonts and jawless fish.
ÿ The jawless fish, called agnathan, occurred first in shallow marine environments in Boliva, Australia.
http://www.phacops.com/images/g.gif
ÿ
During the Ordovician period the
first kinds of creatures would present themselves that later would lead to
fishís evolution.
ÿ
Even though stromatolites
were abundant throughout the Ordovician, animal life radiation slowly led to
their deterioration.
Plant
Life
Land plants, containing
tetrahedral spores, first appeared and extended into the tropics during the
middle of the Ordovician and continued throughout the late Ordovician. The fossil
record shows that the plants consisted of only cuticle layers and spores.
Continental Drift
Most of the world's land
was collected into the super-continent, Gondwana and the tropics were almost
entirely covered by ocean.Ý Gondwana
consisted of Southern Europe, Africa, South America, Antarctica, and
Australia.Ý The climate during this time
was mild and warm.ÝÝ North America was
almost completely under water, and the equator ran down the center. As Gondwana
slowly submerged it settled on the South Pole and glaciers formed causing the
seas to drain and the sea levels to drop.Ý
The changes in sea level caused the shoreline to shift east and west
about (0.6 - 3.1 miles) every thousand years.ÝÝ
As Europe moved towards North America, the climatic fluctuations of
bitter cold became severe.
http://www.scotese.com/images/458.jpg
Adaptive radiations
At the end of the
Cambrian period, after the mass extinction of trilobites and nautiloids, there
was a period of adaptive radiation of numerous faunal and phyla groups.
~For example, there was
decline in the number of stromatolites that almost completely wiped them
out.Ý This was due to new and diverse
snails that had begun to graze on them.
Radiation originated in
shallow water near the coast and spread to deeper waters later in the
Paleozoic. Another cause of adaptive radiation was having a significant locale;
which provided an advantage over other competitors.
~For example, the great
radiation that occurred in the invertebrates was do to the presence of durable
parts emerging in other organisms.Ý
These parts increased the speed of locomotion in trilobites, or
increased the efficiency of filtration of plankton in other animals.
Extinctions
A mass extinction
occurred at the end of the Ordovician era.Ý
During that time the second most devastating extinction in earth history
occurred, destroying 60% of all marine invertebrate genera and 25% of all
families and plants and tropical marine faunas. Approximately one third of all
the brachiopod and bryozoan families, conodonts, trilobites, and graptolites
were terminated.Ý This extinction was
between 440-450 million years ago.
Two extinctions occurred
during the Ordovician period.Ý The first
extinction took place when the extensive ice age began and effected the species
that lived in environments that were destroyed by the sea level dropping.Ý The second extinction occurred as the ice
age ended leading into the Silurian period and effected the cold water marine
species as the sea level and temperature began to rise.
Caused by
The glaciating of the
Gondwana is the most probable cause of the mass extinctions (which also denotes
the end of the Ordovician).
ÿ
Due to the ecospace on the
continental shelves space becoming limited, because of the cooling createdÝ by the glaciations, could have also drove
the mass extinction.
ÿ
Rugose Corals- the lowered
sea levels in the continental sea region, caused a decline in the masses and
range of the rugose corals, and finally lead to their disappearance.Ý The taxa from the continental margin
continue to have advantages such as withstanding relatively cool waters and
having a large geographic range, allowing them to survive the extinction.
ÿ
ÝReef taxa- True sponges played a minor role in Cambrian faunas,
but in the Ordovician, they became important reef builders.Ý The disintegration of the reefs led to the
cold waters flooding the lands.
ÿ
Different landmasses that
made up Gondwana had several volcanic arcs.ÝÝ
As one land area moved towards another a basin, containing exotic
trilobites and graptolites moved westward as the collision advanced.
http://vishnu.glg.nau.edu/rcb/Ordglobe.jpg
Evidence for Glaciation event
ÿ
Glacial deposits are present
in the Saharan Desert.
ÿ
Rock Magnetism and glacial
deposit data.
ÿ
Iapetus Ocean (proto-Atlantic)
was blocked, thus eliminating numerous habitats.
ÿ
The collapsing of the
Cambro-Ord platform, where the ocean waters divided the continents of Gondwan.
ÿ
Deposits of Burgess shale
have been found that confirms an anoxic environment without decaying bacteria
and scavenging animals. The passive margins of the continent have extensive
ÿ Anticosti Island- This area near Quebec contains benthic fauna, preserved in mixed clastics, and carbonate strata, but no black shaleís and rarely a graptolite.
Fossil resources
ÿ
In the late Ordovician the
first terrestrial fossil occurred in the redbeds of Pennsylvania.ÝÝ Since the planktonic grapholites rapidly
evolved, they became the major fossils for the Ordovician.
ÿ
Currently the oldest state
fossil, (living during the Ordovician Period) is probably Ohio's trilobite
Isotelus.
ÿ
Most fossils during the
Ordovician are marine invertebrates that can be found in Whiterock Formation,
located in Utah and Nevada, and the Cincinnati Series in Ohio, Indiana, and
Kentucky.
ÿ
Cincinnati was built on land
where the Earth's crust has been bent upwards, creating what is called the
"Cincinnati Arch." Erosion and glacial action have exposed the
remains of animals that lived 450 million years ago.
ÿ
Kentucky Bluegrass Region is
composed of limestones and shale from the Ordovician Period.Ý They are exposed along the Palisades of the
Kentucky River. This limestone is quarried and use in construction and to
produce natural bottled spring water.
ÿ
The Taconic Organy-The
continents of North America, Greenland, Europe collided forming the Appalachian
Mountains.
ÿ
ÿ
http://www.toyen.uio.no/palmus/galleri/montre/m_ordovicium.jpg
Alternative Hypothesis
ÿ
Ones who do not believe in
the ìmass extinctioî pose a theory of how ecological crowding of marine
ecosystem could have lead to the decrease in diversity.
ÿ
As more fossils are collected
from various regions of the world, this "mass extinction" may
transpire into being a somewhat insignificant event.
Glossary
of terms
Adaptive Radiation-organisms
rapidly fill new ecological niches and increase in both numbers and diversity.
Bivalve-a mollusk,
such as an oyster or a clam that has a shell consisting of two hinged valves.
Brachiopods- Were extremely abundant, until a mass extinction. They
are diversified in numerous morphologies and participated in the build-up of
ancient reefs.
Bryozoan- Any of various small aquatic animals of the phylum
Bryozoa that reproduce by budding and form colonies attached to stone or
seaweed.
Cephalopods-Any various marine and lusks of the class Cephalopoda,
such as the squid, having a large head, large eyes, prehensile tentacles, and
often an ink sac used for protection of defense.
Conodont-A Paleozoic microfossil usu, having a tooth like shape and
considered to be the remains of an extinct marine organism.
Crinoids-any various echinoderms of the class Crinoidea,
characterized by a cup-shaped body, feathery radiation arms and either a stalk or
claw like base.
Gastropods-any of various mollusks of the class Gastropoda, such as
the snail, characteristically having a single, usu, coiled shell or no shell at
all, a ventral muscular foot, and eyes and feelers.
Gondwana-The super continent of the Southern Hemisphere, a
hypothetical landmass that according to the theory of plate tectonics broke up
into India, Australia, Antarctica, Africa, and South America.
Graptolites- (zooplankton) Now considered a third class of
hemichordates, the graptolithina or graptolites. Graptolites are common fossils
in Ordovician and Silurian rocks, and most fossil graptolites look like
nothing.Ý They are tubular in
cross-section, with the "teeth of the saw" formed by short open
branches from the main tube.
Mass Extinction- extinction of significant component of global faunas and
relatively sudden, which is over a few million years.
Nautiloids- swimmers rested on sea floor, feeding on benthic
animals.
Ostracod- any of various minute, chiefly freshwater crustaceans of
the subclass Ostracoda, having a bivalve carapace.
Plankton-The collection of small or microscopic organism, including
algae, that float or drift in great number in fresh or salt water, especially
at or near the surface and serve as food for larger organisms.
Regose- having a rough wrinkled surface.
Stromatolites-a sedimentary structure if laminated carbonate or silicate
rocks, produced over geologic time by the trapping of sediment by groups of
microorganism, especially cyanobacteria.
Taconic
Organy-The continents of North America,
Greenland, Europe collided forming the Appalachian Mountains.
Trilobites-any of numerous extinct marine arthropods of the class
Trilobita of the Paleozoic Era, having a segmented body divided into three
vertical lobes.