Chapter11 – The Early-to-Middle Paleozoic World
11.1 Beginnings of the Phanerozoic Eon
 Continents drifted into warm, tropical latitides
 Sea level was much higher than today
o Warm, shallow epicontinental seas spread across the continents
 As the continents moved, some of them collided, forming mountain chains
11.2 Tectonic Cycle: Impacts on the Hydrosphere, Atmosphere, and Rock Cycle
 Sea Level, CO2, and Sedimentary Facies
o The Paleozoic tectonic cycle began with one or more supercontinents
present
o The rifting and continued movement of continents cause seafloor
spreading centers to displace large amounts of water out of the ocean
basins, producing epicontinental seas and decreasing Earth’s albedo
o Rifting, subduction and volcanism produced large amounts of CO2; no
plants yet existed on land to draw it down
o The Transcontinental Arch and other broad structural highs and basins
appeared at various times during the Phanerozoic, in response to tectonic
activity or plate movements
 Ocean Circulation and Chemistry
o The widespeard seas, low albedo, high CO2 levels, and the circulation of
surface waters at low latitudes contributed to warming the Earth
o Circulation within the deep oceans was relatively sluggish, which implies
that the oceans held relatively little oxygen; at times the oxygen minimum
zone spread into epicontinental seas
o Widespread limestone deposition in the shallow epeiric seas suggests high
levels of calcium and carbonate ions in the seawater; their abundance
resulted from extensive continental weathering due to high levels of CO2
in the atmosphere
o The CCD may have shallowed well up onto the continental slope due to
limestone deposition in shallow water starving the deep sea of CaCO3
o Hard parts of marine organisms were more prone to consist of the mineral
calcite (“i.e., “calcite seas” prevailed)
11.3 Tectonic Cycle and Orogeny
 Physiographic Provinces of the Appalachian Mountains
o The origenic episodes of the southern margin of Laurentia are primarily
responsible for the modern physiographic provinces of the eastern portion
of North America, including the Appalachian Mountains
 Orogenic Episodes
o Taconic Orogeny – involved the uplift of a volcanic arc as the Iapetus
Ocean began to close
o Acadian Orogeny – during this event the microcontinent Avalonia was
trapped between plates as Iapetus continued closing
o Antler Orogeny – terranes were accreted to what is now the western
margin of North America
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o Ellesmere Orogeny – may have involved the collision of Laurentia with an
island arc
11.4 Impact of Orogeny on Earth Systems
 The orogenic phases account for sea-level regression during the first half of the
Paleozoic tectonic cycle
o Changes in ocean circulation also resulted
 By the Late Ordovician, Gondwana lay over the south pole and became glaciated
o This enhanced deep ocean circulation, oxygenation of the deep ocean, and
marine photosynthesis (by stimulating the upwelling of nurtrients)
11.5 Diversification of the Marine Biosphere
 Plankton and Microfossils
o Acritarchs rediversified and were responsible for much of the
photosynthesis in the ocean during the Early-to-Middle Paleozoic
o Major zooplankton groups include radiolarians, graptolites, conodonts,
and certain trilobite species
 Benthic Ecosystems
o The Cambrian Fauna was dominated by invertebrate trilobites and
inarticulate brachiopods
o The Paleozoic Fauna included articulate brachiopods, nautiloid and
ammonoid cephalopods, bryozoans, crinoids, and blastoids
o Communities exhibited tiering (suspension feeding at different levels
above and below the seafloor), demonstrating that food chains were
lengthening and food webs becoming more complex
 Reefs
o Reefs are biogenic, wave-resistant structures; they are one of the most
diverse ecosystems on Earth
o The organisms that form reefs has changed through time, a phenomenon
called ecological replacement
o Cambrian reefs made by stromatolites were later replaced by
archaeocyathid reefs
o Stromatoporoids became the dominant reef builders of the Middle
Paleozoic; rugose and tabulate corals were also present
11.6 Marine Realm Invades the Terrestrial Biosphere
 Invertebrates
o Arthropods expanded their range into marginal marine, freshwater and
terrestrial habitats
 Fish
o Fish are the most primitive group of vertebrates; they appeared in the
Cambrian in nearshore marine environments
o Jawless ostracoderms were later replaced by jawed acanthodians,
placoderms and more modern groups such as sharks and early bony fish
o One group of bony fish, the lobe-finned fish, are thought to be ancestral to
the amphibians
 Amphibians and the Invasion of Land
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o Paleozoic amphibians were different from their modern counterparts
(frogs, toads, and salamanders); they ranged greatly in size, shape, and
niche
o Early amphibians conquered come barriers to the invasion of land (gravity
and air-breathing), although they remained tied to water for reproduction
 Land Plants and the “Greening” of the Continents
o During the Cambrian, only cyanobacterial films, fungi and lichens existed
on land
o The earliest land plants probably had to remain close to water for
reproduction
o Plant fragments have been found from Ordovician deposits; by the
Silurian, more advanced land plants definitely appeared, containing
vascular tissue
o Spore-bearing ferns, sphenopsids, and lycopsids appeared in the
Devonian, along with the first seed plants (seed ferns)
o The evolution and diversification of land plants was probably crucial for
the invasion of land by vertebrates and their subsequent evolution
11.7 Extinction
 Life suffered a series of setbacks during extinctions in the Cambrian, Late
Ordovician and Late Devonian
o The Late Ordovician extinction is thought to have resulted from glaciation
and global cooling
o The spread of terrestrial forests may have contributed to the Late
Devonian extinction through indirectly causing ocean anoxia, although
other factors were probably also involved
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