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Silurian Introduction
The Silurian period spans 443.7 million
years to 416 million years ago. Sir Roderick Impey Murchison (1792-1871),
a Scottish
geologist, worked to map the greywacke rocks underlying the
Old Red Sandstone in Wales. Many of these layers contained
fossils very different from those of the Cambrian. Murchison
published his work as The Silurian System in 1839. This period
was named for the Silures, a Celtic tribe living in Whales
during the Roman conquest (USGS).
Primary
Producers & Reefs
Evidence at the Ordovician/Silurian boundary indicates glaciation,
lowering of sea levels and decreased temperatures. At the beginning
of the Silurian reefs were virtually absent and the deposition of
marine limestones were reduced. An important warm water calcareous
alga also disappeared (Stanley, 1987, p. 75). Most phyla avoided
extinction and their surviving representatives would expand to create
new ecosystems during the Silurian period. These adaptive radiations
would not only occur in the marine environments but also now on the
land.
The dominant primary producers in the oceans continue to be cyanobacteria,
green and red algae (Knoll, Summons, Waldbauer, and Zumberge, 2007,
p. 148). As the Silurian unfolded, reef construction would be re-established.
In some locations, reefs made primarily of tabulate corals make their
first appearance in the Silurian (Kiessling, 2001, p. 44). Overall,
stromatoporoids and tabulate corals would be the primary builders of
reef systems in the Silurian. Rugose corals and bryozoans would play
only minor roles in reef building (Stanley, 1987, p. 75). In some environments
algae and stromatolites may have been important reef builders (Webb,
2001, p. 175).
Marine Invertebrates
Crinoids, and brachiopods continue to spread and diversify. Trilobites,
graptolites, stromatolites, and mollusks continue to be present
in large numbers. Sponges, corals, and bryozoans play important
roles in reef building. Calymene celebra is
a bottom-feeding trilobite that lived during the Silurian and is the
state fossil for
Wisconsin.Eurypterids
became major predators in shallow marine/estuary environments
during
the Silurian (Johnson and Stucky, 1995, p. 36).
The eurypterid is a chelicerate arthropod and looks like a
cross between a scorpion and lobster.
Bertie Waterlime
The
Bertie Waterlime is a fossil lagerstatten that provides excellent
examples of eurypterids and other rare arthropod fossils. Dolostones
in the Bertie Group can be used to make cement that cures while under
water, thus giving it the name waterlime. Fossils are collected from
formations associated with the Bertie and Roundabout groups throughout
New York and into Ontario, Canada. The Bertie Waterlime eurypterid
containing dolostones represent a hypersaline lagoon. The majority
of eurypterids found represent molts. Several species of eurypterids
are found in these deposits. Eurypterus remipes is the state
fossil for New York. A variety of marine life is preserved in the
Bertie
Waterlime dolostones. Primitive horseshoe crabs, aquatic scorpians,
phyllocarid crustaceans, trilobite-like arthropods, gastropods, orthocone
cephalopods, brachiopods, and stromatolites have been collected.
A few examples of the land plant Cooksonia have also
been recovered. A variety of eurypterid species have been found
in deposits
around
the word representing a time span of 100 million years (Ordovician
to Permian). One species of eurypterid reached a length of
2 meters, making it the largest known arthropod to have existed
on Earth
(Nudds & Selden, 2008, pp. 73-92).
Fish
Fish continue to diversify into new forms. Freshwater fish and fish
with jaws make their first appearance during the Silurian (Fisher,
1998).
A new group of Ostracoderms (jawless fish), the cephalaspids appear.
Cephalaspids were more advanced than heterostracans. Cephalaspids
or osteostracans (order Osteostraci) had a head shield made of
a single bone, which did not grow during the life of the fish.
Osteostracans had scale-covered extensions that acted as pectoral
fins, which improved their swimming ability. They also had a thin
layer of bone over their cartilage along with sensory organs on
the sides and tops of their heads.
The first vertebrates to evolve jaws are the acanthodians or spiny
sharks. Although these fish have a shark-shaped body with paired
fins and an upturned tail, they are not sharks. There is good reason
to believe that the jaws of acanthodians evolved from the first gill
arch found in their ancestral jawless fish (Dixon, D., 1988, p 32).
Acanthodians had skeletons made of cartilage, close fitting scales
in their skin made of bone, bone protecting their head and girdle,
and fins with a broad bony base that extended along the anterior
edge as a dentine spine. All of the fins had a reinforcing spine
except for the tail. Some acanthodians developed a bony operculum
covering their gill openings. Acanthodians lacked teeth, but instead
had gill-rakers used for suspension feeding. Acanthodians first appear
in the Silurian as marine fish, but evolve into many species including
freshwater
forms. The class Acanthodii goes extinct in the Permian. Placodermii,
a class of armored jawed fish also makes it first appearance in the
mid-Silurian; only fragments of placoderm fossils have been found
in Silurian deposits.
Land
Plants & Terrestrial
Ecosystems
Evidence
for land plants and primitive terrestrial ecosystems
becomes more common
in the
Silurian. The first land plants belonging to the
genus Cooksonia were stick-like with bifurcating
stems and no leaves or roots (Kenrick & Davis, p. 22).
The stems may have grown from rhizomes. The leafless, forked
stems of Cooksonia stood around 3
inches tall and where photoshythetic. Wind carried spores
were produced in rounded structures at the tips of stems
(Jonshon & Stucky,
1995, p. 44). Cooksonia is a representative of the
most primitive division of vascular plants, the Rhyniophyta.
Rhyniophytes are the first plants that possessed the three
evolutionary innovations for living on land, vascular tissue,
cuticles and stomata (Cleal & Thomas, 2009, p. 63).
Primitive land plants had no competition and attained
a worldwide distribution during the Silurian. The flora at
this time was low in diversity. Primitive terrestrial environments
containing millipedes, centipedes,
arachnids,
the first fungi,
bacteria,
worms, and simple
plants become established (Kazlev, Silurian Page). The Silurian
period would unfold without any large-scale biological disturbance
(Stanley,
1987, p. 75).
Cleal C.J. & Thomas,
B.A. (2009). Introduction to Plant Fossils. United Kingdom:
Cambridge University Press.
Dixon D., Cox, B., Savage,
R.J.G., & Gardiner, B. (1988).
The Macmillan Illustrated Encyclopedia of Dinosaurs and
Prehistoric Animals: A Visual Who’s Who of Prehistoric Life. New York:
Macmillan Publishing Company.
Falkowski, P.G. Knoll, A.H. (2007). Evolution
of Primary Producers in the Sea. China: Elsevier Academic Press.
Fisher
D., Liu T., Yip E., & Yu K. (1998). Silurian Life page
in The Web Geological Time Machine by UCMP Berkley. See: http://www.ucmp.berkeley.edu/help/timeform.html
Johnson,
K.R. & Stucky
R.K. (1995). Prehistoric Journey: A History of Life on Earth.
Boulder, Colorado: Roberts Rinehart Publishers.
Kazlev, M.A. (2002). Palaeos
Website. see: http://www.palaeos.com/Timescale/default.htm
Kenrick,
P. & Davis,
P. (2004). Fossil Plants. Washington: Smithsonian Books.
Kiessling, W. (2001). Phanerozoic Reef Trends Based on the Paleoreef
Database. In Stanley, G.D. Jr. [Ed] The History and Sedimentology
of Ancient Reef Systems (41-88). New York: Kluwer Academic/Plenum
Publishers.
Nudds
J.R. & Selden
P.A. (2008). Fossil Ecosystems of North America: A Guide
to the Sites and Their Extraordinary Biotas. Chicago:
The University of Chicago Press.
Stanley, S.M., (1987). Extinction. New York: Scientific American
Books.
USGS Publication: Major
Division of Geologic Time see: http://pubs.usgs.gov/gip/geotime/divisions.html
Webb,
G.E. (2001). Biologically Induced Carbonate Precipitation in
Reefs through Time. In Stanley, G.D. Jr. [Ed] The History
and Sedimentology of Ancient Reef Systems (159-203). New York:
Kluwer Academic/Plenum Publishers.
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