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VIZUALIZING EARTH HISTORY
By Loren E. Babcock
Chapter 10
Paleozoic World
Early Paleozoic
Cambrian overview
At the start of the Paleozoic Era, Earth’s tectonic plates
were in a transitional stage between the breakup of
Pannotia and reassembly as Pangea. Major cratonic
pieces were Laurentia, Baltica, Siberia, and Gondwana.
Epeiric sea - Shallow sea formed when marine water covers
part of the continental crust. Also called epicontinental sea.
Global sea level was relatively high through most of the
Paleozoic Era. As a result, thick successions of marine
sediments were deposited on continental shelves.
Early Paleozoic
Cambrian overview
Early Paleozoic
The Ediacaran-Cambrian transition.
Evolution’s Big Bang
The early evolutionary radiation of multicellular organisms is
commonly called the “Cambrian explosion.” Not only did
numerous organisms emerge during this time, but a
variety of animals evolved resistant coverings or skeletons
that could be preserved as fossils.
Cambrian explosion - The appearance of numerous taxonomic
groups in the Cambrian. It reflects a widespread evolutionary
diversification especially among multicellular animals.
Early Paleozoic
Evolution’s Big Bang/Major life forms of the Cambrian
Trilobite - A Paleozoic marine arthropod characterized by a
calcified exoskeleton divided lengthwise into three lobes.
Cephalopods - Swimming carnivorous mollusks, which had
shelly skeletons, first appeared in the fossil record.
Conodont - An extinct early chordate that secreted phosphatic
tooth-like structures along the pharynx.
Chordate - An animal possessing a notochord and
pharyngeal gill slits.
Graptolite - An extinct hemichordate animal, or chordate
relative, that had an organic skeleton.
Early Paleozoic
Exceptional preservation of fossils and
Cambrian biodiversity
A large and important share of the preserved record of
Cambrian biodiversity came from organisms lacking
biomineralized skeletons.
Coinciding with the first appearance of trilobites are some
of the earliest Phanerozoic deposits containing
exceptionally preserved fossils.
Cambrian strata include an unusually large number of such
deposits, like the the Burgess Shale of British Columbia,
Canada, and the Chengjiang deposit of Yunnan, China.
Early Paleozoic
Exceptional preservation of fossils and
Cambrian biodiversity
Early Paleozoic
Explain how exceptional preservation of fossils
has increased our understanding of ancient life.
Most Cambrian organisms had non-biomineralized external
coverings. Most animals had exoskeletons made of chitin.
Although chitin affords some protection, it is made of sugar
and makes a good meal for many predators.
Other non-biomineralizing life forms —the organisms without
hard parts— comprised 90% or more of the fossils in some
Cambrian deposits. This implies that where only hard
part-bearing fossils are preserved, most of the original
biodiversity has been filtered out
of the fossil record.
Early Paleozoic
Cambrian Predatory-prey System
Early Paleozoic
Ordovician sea level and tectonics
During the Ordovician, global sea levels reached some of the
highest positions ever. The sea level rise of the
Ediacaran-Cambrian continued into the earliest Ordovician,
but before the end of the Early Ordovician it dropped
precipitously.
Shifting plates delivered big changes to the Ordovician world.
Closure of the Iapetus Ocean closed the distance between
Laurentia and Avalonia, and brought tectonism to
eastern Laurentia and environs.
Iapetus - The proto-Atlantic Ocean.
Early Paleozoic
Paleozoic tectonic events.
Orogenesis, or mountain building, on the Laurentian mainland
resulted in the rise of the Taconic Mountains. This event, the
Taconic orogeny, was the first of three mountain building
episodes along the Appalachian margin of Laurentia
during the Paleozoic.
Plate collision caused the thrusting of large wedges of flysch
over shallow carbonate deposits. In places, slices of basaltic
ocean floor, or ophiolites, were thrusted onto the craton as well.
Closure of Iapetus caused the collision of island volcanoes and
other crustal pieces, including Avalonia, with eastern Laurentia.
Early Paleozoic
Ordovician marine life
The Ordovician Period marks the appearance of an assemblage
of marine life characteristic of the post-Cambrian Paleozoic.
Extinctions in the late part of the Cambrian took a toll on animal
groups, but the recovery was unexceptional. Trilobites,
mollusks, and brachiopods diversified at a fairly slow rate.
Few niche spaces were newly reopened to be exploited.
Cephalopods became the top predators in marine
environments around the beginning of the Ordovician.
Early Paleozoic
Ordovician marine life
Early Paleozoic
Ordovician marine life
Fishes were a threat to marine life of the Cambrian and
Ordovician, although the extent of their impact is hard to assess.
Graptolites, stalked crinoids (or sea lilies), bryozoan colonies,
and mollusks made some evolutionary gains in the Ordovician
but they were setting the groundwork for more spectacular
developments later.
By the end of the Ordovician, cyanobacterial mounds were a
rarity in normal marine environments.
Early Paleozoic
Early forays onto the land
Live organisms were washed ashore early in Earth’s history.
Trace fossils from beach sandstones suggest that animals
occasionally crawled on dry land as early as the Cambrian.
They may have left the water by accident or even deliberately
— perhaps the shore was a place to bury eggs.
The first evidence of plants conquering the land comes
from the Cambrian. These early land plants were
probably from a marine sister group, the green algae.
Spores are the only known evidence of terrestrial plants older
than the Silurian, when stems of land plants became fossilized.
Early Paleozoic
Ordovician glaciation and extinction
At the end of the Ordovician, global temperatures dropped
and the ice cap in southern Gondwana expanded. In
Gondwana, tillites, dropstones, and striated pavements
record the glacial event.
The effect of climate change on the biosphere was quick and
catastrophic: it precipitated one of the most severe mass
extinctions in Earth history. As glaciation reached a
maximum, brachiopods, bryozoans, corals, trilobites,
conodonts, and nautiloids disappeared in large numbers.
Middle Paleozoic
Silurian-Devonian tectonics and sea level
By the early part of the Silurian Period, Taconic mountain
building had ceased in eastern Laurentia, and erosion left
few remnants of the mountains.
In the mid-Silurian, renewed subduction and closure of Iapetus
caused Baltica to collide with Laurentia, principally along what is
now eastern Greenland and Norway-Scotland-Ireland.
Middle Paleozoic
Middle Paleozoic
Silurian-Devonian tectonics and sea level
Euramerica - A composite continent formed by collision of
Laurentia with Avalonia and Baltica during the Devonian Period.
Caledonian orogeny - Silurian-Devonian orogenic activity
that affected western Europe from the British Isles
through Scandinavia.
Acadian orogeny - Orogenic activity during the Devonian
along the Appalachian margin of Laurentia.
Middle Paleozoic
Explain the tectonosedimentary cycle.
Tectonosedimentary cycle - The record of continental collision
in basinal sedimentation patterns. It comprises passive margin
sedimentation followed by deep water (foreland basin) deposits
and then shallow water to nonmarine deltaic deposits.
Rivers carried detrital sediments eroded from the Acadian
Mountains to their mouths along the shoreline of the
Appalachian Basin, then deposited their loads in an
ever-thickening clastic wedge called the Catskill Delta.
Middle Paleozoic
Mid-Paleozoic marine life
Glaciation at the end of the Ordovician decimated many shallow
marine invertebrates, but recovery came rather soon as marine
water flooded shelf areas, opening new habitats.
Trilobites were still important in marine ecosystems, but they
never were as diverse after the Ordovician extinction
as they were before it.
Corals, especially the massive, fast-growing tabulate corals,
and coralline sponges, both refilled vacant niches
and diversified into unfilled niche space.
Middle Paleozoic
Mid-Paleozoic marine life
Graptolites, almost disappeared at the end of the Ordovician,
increased their numbers greatly during the Silurian Period.
During the Silurian, marine life underwent ecological
reorganization, a new ecological “arms race” between
predators and prey ensued.
The most notable evolutionary event of the mid-Paleozoic
was the rise of jawed fishes. Jaws made it possible for fishes
to bite into prey, and even to crush shells or skeletons.
The Devonian is called
the “Age of Fishes.”
Middle Paleozoic
The invasion of land by plants and animals.
Plants and animals appeared on land in several phases during
the Paleozoic. A complete transition to land occurred by the
end of the Silurian, and adaptation was completed in
the Devonian Period.
Life on land has certain basic requirements that are different
from the requirements for life in water. The primary problems
associated with the transition to land are:
1. gas exchange with the outside environment;
2. internal fluid flow and maintaining osmotic balance;
3. maintaining support for the body;
4. internal temperature regulation and prevention of desiccation;
and 5. protection of developing embryos.
Middle Paleozoic
The invasion of land by plants and animals
Insect - An arthropod having three pairs of legs and wings,
at least primitively.
The evolution of winged insects from non-wing-bearing
Terrestrial hexapods around the Middle Devonian marks
the beginning of evolution’s greatest animal success story.
By the end of the Carboniferous Period, insects had
undergone an almost unimaginably large adaptive radiation,
and a high reproductive rate leading to rapid speciation.
Middle Paleozoic
Devonian glaciation and biotic crisis
Late in the Devonian the polar ice cap in Gondwana expanded
during a time when Earth was entering a cooling phase.
At the same time, marine organisms declined in species
diversity. More than 50% of genera known from fossils
disappeared in a time span of perhaps 3 million years.
Brachiopods, trilobites, conodonts, ammonoids, and
jawless fishes all suffered major losses in the Late Devonian.
Late Paleozoic
Carboniferous-Permian tectonic, climate, and sea level
The late Paleozoic was a time of important tectonic change,
and tectonic events influenced global climatic and sea level
changes.
Each of the mountain building events (the Taconic orogeny
in the Ordovician Period, the Acadian orogeny in the
Devonian Period, and the Alleghanian orogeny in the
Carboniferous and Permian periods) was followed by the
opening of an ocean basin.
The cyclic recurrence of plate-tectonic spreading and basin
closure through subduction has been referred to as a
Wilson cycle.
Late Paleozoic
Carboniferous glacial-interglacial cycles
Climatic conditions of the Carboniferous were a study in
contrasts. A greenhouse world in the early half of the period
(the Mississippian) gave way to a series of rapid glacial
cycles, reflected in sea level fluctuations, in the later
half (the Pennsylvanian).
Milankovitch cycles - Longer intervals (41,000-year cycles)
resulted from the tilt of the Earth’s axis, which affects where and
how much sunlight falls on the globe. The longest cycles,
100,000 to 120,000 years, are related to the deviation over time
in the Earth’s elliptical orbit around the Sun. Less of the Sun’s
energy arrives on Earth when the Earth is at a distant point in its
orbital path than when it is closer to the Sun.
Late Paleozoic
Carboniferous glacial-interglacial cycles and their
effects on sedimentation patterns globally.
Cyclothems show a change upsection from continental
deposits including redbeds, fluvial sediments and coal
beds, through marginal-marine and shallow marine
sandstones, siltstones, and limestones, to deeper water
gray and black shales. The shales mark an inflection point
as sea level was shifting from a transgressive (deepening)
phase to a regressive (shallowing) phase.
Late Peleozoic marine life
Marine life of the Carboniferous was similar to that of the
Devonian. The seas were rich in brachiopods, bryozoans,
and mollusks. Trilobites were no longer diverse, but some
species were locally abundant.
In the Carboniferous (Mississippian) deposits of crinoid-rich
limestones or encrinites were abundant. Crinoids and other
stalked echinoderms formed impressive “meadows” stretching
for hundreds of kilometers across the warm, shallow,
epeiric seas.
Late Paleozoic terrestrial life
The most conspicuous terrestrial life forms of the Carboniferous
and Permian were plants. During warmer intervals, especially
in the late part of the Carboniferous, forests flourished in
extensive coastal wetlands of the tropics. These areas of
massive vegetative production are sometimes referred to as
“coal swamps.”
The world’s coal reserves come from thick peat deposits, which
were lithified to coal seams, deposited in these settings. It was
the prominent coal beds of Britain that gave rise to the name
“Carboniferous.”
Late Paleozoic
Understand the major marine and terrestrial life forms
of the Carboniferous-Permian.
Reptiles appeared in the late Carboniferous, and in the
Permian, they began diversifying into the groups we know
as squamates (lizards and snakes), archosaurs (crocodiles,
dinosaurs, and flying reptiles), and others.
Big evolutionary advantages they had over their amphibian
ancestors were internal fertilization and an amniotic egg.
The synapsids first appeared in the late Carboniferous and
diversified in the Permian. Synapsids have only
one jaw bone in each side of the lower jaw.
Late Paleozoic
Permian extinction
More than 80% of all marine species became extinct in the
waning stages of the Permian Period.
The causes of the Permian biotic crisis were a combination of
changes that dealt the lethal blow to so many species.
The fusing of the continents into the supercontinent Pangea,
reduced the area of shallow epeiric seas. Adding to the crisis,
sea level dropped roughly 100 meters during the last 2 million
years of the Permian Period.
Large continental fissure eruptions in Siberia and China may
have blocked sunlight from reaching the Earth or added CO2
and water vapor to the atmosphere, enhancing
the greenhouse effect.