Middle PaleozoicSilurian 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 mountain-building 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 lifeInvasion 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 HistoryWashington, 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/arthropoda/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 "pre-Pangea".  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.html

http://ebeltz.net/fieldtrips/tq-ft.html