Late Paleozoic Seas

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Late Paleozoic Seas
EPSC 233 Earth & Life History (Fall 2002)
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.
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.
Most Cambrian-Ordovician echinoderms were filter
feeders with short stalks…
Gogia, a filterfeeding eocrinoid
with long arms,
used as baffles.
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.
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…
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).
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.
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).
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.
Sponge Gyrtocoelia (branching strings of beads)
and spiny brachiopods (foreground, center).
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.
Nautiloids: shelly mollusks related to
the modern Nautilus, which is possibly
the only surviving genus of the entire
order.
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.
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.
Nautiloids were gradually replaced by coiled shelly
cephalopods, ammonoids, during the late Paleozoic.
Ammonoids are excellent index fossils.
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.
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.

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