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Middle Paleozoic
Silurian to Devonian time
440-417mya and 417-354mya
Silurian summary
Devonian summary
Not to know the events which
happened before one was
born,that is to remain always
a boy.
- Cicero
• The three mountainbuilding events (or
orogenies) in North
America during the
Paleozoic are:
1. Taconic orogeny
2. Acadian orogeny
3. Alleghanian orogeny
Ordovician
• Radiations of articulate brachiopods, gastropods
(snails), echinoderms (especially stalked crinoids
and blastoids).
• Decline of stromatolites: Probably due to more
specialized grazers (gastropods, echinoids, etc.).
• 1st tabulate-stromatoporoid reefs (more important in
middle Paleozoic). Fish diversity increases, but still
jawless. 1st evidence of terrestrial plants.
• Terminal Ordovician Extinctions:
• Disappearance of one third of all brachiopod and
bryozoan families, as well as many groups of
conodonts, trilobites, and graptolites
– Associated with massive Gondwanan ice age
Molasse and flysch deposits
• Flysch– deposited in basins behind
mountain chains
• Molasse– same type of sediments but
deposited on land (terrestrial)
• Thick build up of molasse is called a
clastic wedge
• Fig 9-14
Silurian 440-417mya
Paleogeography
• High sea levels worldwide following Late Ord. low
sea level
- why would this happen? (glaciers melting)
• Tippecanoe Sea flooded North America, deposited
St. Peter Sandstone
– pure, well-sorted, well-rounded quartz sandstone
– overlain by extensive limestone deposits, locally replaced
by dolomite.
• Eastern U.S. limestones are overlain by and
interbedded with shales along the periphery of the
Queenston delta or clastic wedge
– Niagara Falls area is a classic locality where these rocks
are exposed.
Laurentia collides with Baltica closing the northen branch of the Iapetus
Ocean and forming the "Old Red Sandstone" continent. Coral reefs
expand and land plants begin to colonize the barren continents
• St. Peter Sandstone
– Starved Rock SP
– Aquifer for many municipal
wells in Lake, McHenry,
Dekalb, Kane and other
counties
– Used for glass and well-filler;
mined in LaSalle County
• Niagara Falls from the Canadian side. The
Middle Silurian Lockport Dolomite forms the
resistant ledge at the top of the falls. The
Rochester Shale forms the slopes below. The
Upper Ordovician Queenston formation is at
the bottom of the falls.
Silurian marine life:
• Decline of the Cambrian fauna: trilobites survive the
terminal Ordovician extinctions, but at reduced
diversity
• Increase in the abundance and distribution of
tabulate-stromatoporoid reefs
• More advanced jawless fish: development of paired
fins as stabilizers
• Towards the end of the Silurian, the earliest jawed
fish– acanthodians (spiny fishes)
• Radiation of bivalves (clams)
Silurian life
Invasion of the land by vascular plants
(plants with water-conducting tissues, as opposed to non-vascular plants like
mosses)
--Why is this significant?
• waxy outer coating to prevent water loss
• pores for gas exchange
• reproductive structures that could function on land
• complex water circulation system
Psilophytes - small Middle Silurian plants with horizontal stalks just below the surface
of the ground, with vertical stems bearing spore sacs.
• Colonization by plants builds up the food web to later allow colonization by animals
(allows animals to move onto land too!)
Psilophyton, a psilophyte.
This specimen is 370 my and is
from
the Devonian of Quebec, Canada.
Smithsonian Institution,
Museum of Natural History
Washington, D.C
Silurian marine life:
• New taxa:
Eurypterids:
• "Sea scorpions", although found in brackish (and
possibly fresh water) deposits
• Large arthropod predators (up to 3 m long)
• Some capable of short duration travel out of water
http://www.ucmp.berkeley.edu/arth
ropoda/chelicerata/eurypterida.html
Paleogeography and Geology of the Devonian:
• Devonian—age of fishes
• Extremely warm conditions worldwide: evaporites
common North and South of 30º.
• Continued widespread tabulate-stromatoporoid
reef complexes; grew to huge proportions
compared to Silurian reefs
• In eastern Laurentia, the Taconic Appalachians
had worn all the way down by the beginning of the
Devonian.
– No molasse deposits; instead once again stable
platform deposits
By the Devonian the early Paleozoic oceans were closing, forming a "prePangea". Freshwater fish were able to migrate from the southern
hemisphere continents to North America and Europe. Forests grew for the
first time in the equatorial regions of Artic Canada.
Evaporite formation
• Isopach map
showing the
thickness and
areal extent of
Silurian evaporite
basins in the Great
Lakes area. The
Michigan Basin is
on the left.
Paleogeography and Geology of the Devonian:
• Collision of Laurentia and Baltica to form
Laurussia (sometimes called Euramerica):
• Iapetus Ocean closed in the North, called the
Caledonian Orogeny (Silurian-Devonian
boundary)
• Rolls southward, compressing Avalonian
"peninsula" of Laurentia between Laurentia and
Baltica, called the Acadian Orogeny (second
phase of Appalachian Orogenies: Devonian)
• Molasse of the Caledonian-Acadian Orogeny is
called Old Red Sandstone in Europe, and the
Catskill Clastic Wedge in North America.
• Cycle of deposition for Acadian Orogeny was
like that of the Taconic orogeny—1.passive
margin-- 2. Flysch– 3. molasse
Devonian life
• Still have our friends brachiopods, trilobites
• Radiation of fish during Devonian
– Ostracoderms:bony skin,no jaws
– Placoderms:platey skin
– Bony fish; two kinds
– Ray-finned fish; dominant fishes of the modern
world.
– Lobe-finned fish;The lungfish
(lungfish live today in freshwater) and the
crossopterygians
• important group of lobe-finned fish because it gave rise
to the amphibians during the Devonian.
Late Devonian life
•
•
•
•
Amphibians invade the land
First trees
First seeds– non-flowering plants
Mass extinction just before the end of the
Devonian.
–
–
–
–
tabulate-stromatoporoid reefs disappear
many fish extinct
many extinctions among floating and swimming animals
many freshwater forms extinct
• Devonian fossils
Thornton Quarry
• Limestone quarry—ancient reef complex
• Silurian Age limestone—Niagaran
formation
• Fig 14-22
• Do you want to go on a tour?
http://www.thornton60476.com/quarry.ht
ml
http://ebeltz.net/fieldtrips/tq-ft.html

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