Late Paleozoic Seas

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Late Paleozoic Seas
EPSC 233 Earth Life History (Fall 2002)
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Recommended reading STANLEY Earth System
History Chapter 15, pp. 402-410.
Keywords Carboniferous and Permian periods,
calcite seas vs. aragonite seas (see also pp.
278-282), fusulinids, ammonoids, spiny
brachiopods, coal swamps, Joggins, lycopod trees,
seed ferns, primitive insects.
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The middle Paleozoic reef builders (tabulate
corals and stromatoporoids) no longer play a
major role after the late Devonian
extinction. Trilobites continued to
dwindle. Brachiopods recovered and developed
new, increasingly specialized shell
shapes. Among echinoderms, crinoids (sea lilies)
become very abundant, forming thick underwater
fields.
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Most Cambrian-Ordovician echinoderms were filter
feeders with short stalks
Gogia, a filter-feeding eocrinoid with long arms,
used as baffles.
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During the late Paleozoic, crinoids (sea lilies),
anchored to the sea floor, developed short,
medium or longer stems, and with fewer or more
branching arms. They became a very abundant group.
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Most crinoids fell to pieces when they died,
because they lived in shallow water (lots of wave
action). Their ossicles (mostly donut-shaped
pieces of calcite) make up widespread thick beds
of crinoidal limestone during the
Mississipian. They must have formed crinoid
meadows (imagine underwater fields of sea
lilies, waving back and forth) in the shallow
waters that had been occupied by reef-builders
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Seawater chemistry had changed since the middle
Paleozoic. The Mg/Ca ratio of seawater increased
worldwide during the late Paleozoic. This
appears to happen during periods where the
worldwide volume of mid-ocean ridges decreases
(less seafloor spreading means slower production
of new oceanic crust).
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Chemical reactions take place where seawater is
in contact with hot igneous rock, through
fractures in the young oceanic crust.
These reactions tend to lower the Mg/Ca ratio in
seawater.
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The Mg/Ca ratio decreases when mid-ocean ridges
are more active. Seawater gives up Mg2 ions and
carries away Ca2 ions when it comes in contact
with hot igneous rocks. This activity is most
intense when ocean basins are expanding rapidly.
This also influences global sealevel. The new
crust near mid-ocean ridge is buoyant because it
it hotter (less dense). The ocean floor rises
higher towards the mid-ocean ridges, pushing sea
level higher worldwide. This had been the case
during the Middle Paleozoic (Silurian-Devonian
periods).
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Mg2, a major ion in seawater, slows down the
formation of calcite (the CaCO3 mineral in
brachiopod shells). It does not affect the
precipitation of aragonite (a different form of
CaCO3) that modern tropical corals
secrete. During the late Paleozoic, higher Mg/Ca
ratio in seawater favoured aragonite-forming
animals rather than those building shells out of
calcite. This may have stopped tabulate corals
and stromatoporoids from coming back. Late
Paleozoic reefs were smaller and largely built by
aragonitic algae and sponges.
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Sponge Gyrtocoelia (branching strings of beads)
and spiny brachiopods (foreground, center).
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Nautiloids are cephalopods, a type of
mollusk. Mollusks are a very diverse phylum. -
aplacophorans (one-shelled mollusks) - pelecypods
(bivalves clams, oysters) - gastropods
(snails, slugs) - cephalopods (octopi, squids,
nautilus) Most mollusks have in common a
specialized raspy tongue (radula), a muscular
foot and a characteristic type of gills.
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Nautiloids shelly mollusks related to the modern
Nautilus, which is possibly the only surviving
genus of the entire order.
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Nautiloids build chambered shells (visible in the
shell, cut in half, to the right) that they use
to control their buoyancy by filling them with
gas or water.
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Like Nautilus, Ordovician nautiloids were
probably squid-like animals hidden in conical
shells. The earliest nautiloid shells were either
straight or gently curved, usually with very
thick walls.
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Nautiloids were gradually replaced by coiled
shelly cephalopods, ammonoids, during the late
Paleozoic. Ammonoids are excellent index fossils.
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Fusulinids are relatively large (mm to cm)
foraminifera (amoeba-like eukaryotes that built a
shell). They had an enormous adaptive radiation
during the Late Carboniferous and Permian (5000
species in the Permian alone). They were bottom
dwellers, but are relatively good index fossils
for the late Paleozoic.
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More profound changes were taking place in
terrestrial than in marine ecosystems. The
Carboniferous system (i.e. the group of rocks
deposited in that time interval) got its name
from the coal beds which preserve fossilized
remains of plants. Coal was highly coveted as
the Industrial Revolution was underway The
Carboniferous system of rocks was described
before the Devonian.