Chapter11 the Early-To-Middle Paleozoic World

Chapter11 – The Early-to-Middle Paleozoic World

11.1 Beginnings of the Phanerozoic Eon

11.2 Tectonic Cycle: Impacts on the Hydrosphere, Atmosphere, and Rock Cycle

Sea Level, CO2, and Sedimentary Facies
The Paleozoic tectonic cycle began with one or more supercontinents present
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
Rifting, subduction and volcanism produced large amounts of CO2; no plants yet existed on land to draw it down
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
The widespeard seas, low albedo, high CO2 levels, and the circulation of surface waters at low latitudes contributed to warming the Earth
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
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
The CCD may have shallowed well up onto the continental slope due to limestone deposition in shallow water starving the deep sea of CaCO3
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
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
Taconic Orogeny – involved the uplift of a volcanic arc as the Iapetus Ocean began to close
Acadian Orogeny – during this event the microcontinent Avalonia was trapped between plates as Iapetus continued closing
Antler Orogeny – terranes were accreted to what is now the western margin of North America
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
Changes in ocean circulation also resulted
By the Late Ordovician, Gondwana lay over the south pole and became glaciated
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
Acritarchs rediversified and were responsible for much of the photosynthesis in the ocean during the Early-to-Middle Paleozoic
Major zooplankton groups include radiolarians, graptolites, conodonts, and certain trilobite species
Benthic Ecosystems
The Cambrian Fauna was dominated by invertebrate trilobites and inarticulate brachiopods
The Paleozoic Fauna included articulate brachiopods, nautiloid and ammonoid cephalopods, bryozoans, crinoids, and blastoids
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
Reefs are biogenic, wave-resistant structures; they are one of the most diverse ecosystems on Earth
The organisms that form reefs has changed through time, a phenomenon called ecological replacement
Cambrian reefs made by stromatolites were later replaced by archaeocyathid reefs
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
Arthropods expanded their range into marginal marine, freshwater and terrestrial habitats
Fish
Fish are the most primitive group of vertebrates; they appeared in the Cambrian in nearshore marine environments
Jawless ostracoderms were later replaced by jawed acanthodians, placoderms and more modern groups such as sharks and early bony fish
One group of bony fish, the lobe-finned fish, are thought to be ancestral to the amphibians
Amphibians and the Invasion of Land
Paleozoic amphibians were different from their modern counterparts (frogs, toads, and salamanders); they ranged greatly in size, shape, and niche
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
During the Cambrian, only cyanobacterial films, fungi and lichens existed on land
The earliest land plants probably had to remain close to water for reproduction
Plant fragments have been found from Ordovician deposits; by the Silurian, more advanced land plants definitely appeared, containing vascular tissue
Spore-bearing ferns, sphenopsids, and lycopsids appeared in the Devonian, along with the first seed plants (seed ferns)
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
The Late Ordovician extinction is thought to have resulted from glaciation and global cooling
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