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.

Gogia, a filter-feeding eocrinoid with long arms, used as baffles.

Most Cambrian-Ordovician echinoderms were filter feeders with short stalks…

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.