Eighth Edition
by Harold L. Levin

Chapter 12 - page 12

Life of the Paleozoic

Mass Extinctions in the Paleozoic

The Paleozoic was a time of adaptive radiations and extinctions.

Many of the geologic periods of the Paleozoic began with adaptive radiations, or times of rapid evolution of organisms.
Several of the Paleozoic periods ended with extinction events of varying severity.

The three most catastrophic extinctions in the Paleozoic Era were at the following times:

1. End of the Ordovician Period (443 m.y. ago)
2. End of the Devonian Period (359 m.y. ago)
3. End of the Permian Period (251 m.y. ago)

Of these, the Permian extinction was the most severe. In fact, the extinction event at the end of the Permian Period is considered to be the most catastrophic mass extinction in the history of life.

Diversity of marine animals, and extinction events over geologic time.

Late Ordovician Extinctions

Following a slight dip in diversity at the end of the Cambrian, the Ordovician was a time of renewed diversification. The number of genera increased rapidly, and the number of families increased from about 160 to 530. This increase was particularly dramatic among trilobites, brachiopods, bivalved molluscs, and gastropods. An extinction event at the end of the Ordovician led to an abrupt decline in diversity.

The extinction occurred in two phases.

1. First phase - affected planktonic and nektonic (floating and swimming) organisms such as graptolites, acritarchs, many nautiloids and conodonts, as well as benthic (bottom-dwelling) organisms such as trilobites, bryozoa, corals, and brachiopods.
2. Second phase - affected several trilobite groups, corals, conodonts, and bryozoans.

Both phases of the extinction event were related to global cooling and the growth of glaciers in Gondwana. Glaciation was coupled with the lowering of sea level and a reduction in shallow water habitat. As the climate cooled, tropical organisms showed the greatest decline.

As warming occurred and the glaciers began to melt, organisms which were adapted to the cooler conditions began to suffer extinction. This was the second phase of extinctions.

Late Devonian Extinctions

The Late Devonian extinctions occurred over a span of about 20 million years, and appear to have been the result of an ecological crisis in the seas, perhaps induced by changes occurring on the land.

The Devonian was the time of the appearance of trees and spread of land plants. This would have accelerated weathering rates, leading to large volumes of nutrients being washed into the seas.

Large quantities of nutrients in the water (such as phosphorus) causes algal blooms, or rapid algal growth. Bacteria breaking down large quantities of dead algae uses up all of the oxygen in the water, causing anoxic conditions (= "without oxygen"). This process is called eutrophication, and it occurs today in lakes, and causes massive "fish kills".

Extensive Devonian black shale deposits (for example, the Chattanooga Shale) suggest the widespread occurrence of anoxic conditions in the Devonian sea.

Glaciation may have been an additional contributing factor. By the Late Devonian, South America had drifted over the South Pole, and glaciations occurred.

Overall, 70% of marine invertebrate families went extinct in the Late Devonian.

Organisms most strongly affected (but not totally wiped out) by the Devonian extinction were:

1. Tabulate corals
2. Rugose corals
3. Stromatoporoids
4. Brachiopods
5. Goniatite ammonoids (cephalopod molluscs)
6. Trilobites
7. Conodonts
8. Placoderm fish

Late Permian Extinction Event

The Late Permian is marked by a catastrophic extinction event which resulted in the total disappearance of many animal groups. This was the largest extinction event in the history of life, exceeding even the extinction event at the end of the Cretaceous, which killed the dinosaurs.

More than 90% of all marine species that existed in the Permian disappeared or were severely reduced in number. Nearly half of the known families disappeared. Tropical forms experienced the most extensive losses.

The following marine organisms were extinct by the end of the Permian:

1. Fusulinid foraminifera
2. Rugose corals
3. Tabulate corals
4. Blastoids
5. Trilobites (which had become extinct somewhat earlier in the Permian)

Other groups of organisms were severely reduced in diversity, with some surviving species:

1. Brachiopods
2. Crinoids
3. Bryozoa
4. Ammonoids

Organisms which inhabited warm waters shifted their distributions toward the equator. Cool conditions prevented construction of reefs and the formation of limestones.

The Permian extinction also affected land dwellers. More than 70% of land animals disappeared or were severely reduced, including:

1. Amphibian families
2. Reptile families
3. Synapsids (which we used to call "mammal-like reptiles")

Among the plants, the spore-bearing plants that inhabited tropical coal swamps were replaced by seed-bearing gymnosperms, that could inhabit cooler, drier climatic conditions.

Many factors contributed (or may have contributed) to the Permian mass extinction, including:

1. Climatic change associated with assembly of Pangea (global cooling and drying, along with interruption of equatorial circulation)
2. Glaciation at both north and south ends of Pangea
3. Reduction in epeiric or epicontinental seas (habitat loss) as sea level dropped
4. Unusually active volcanism releasing CO₂ (flood basalts in Siberia), leading to global warming, which may have triggered release of large stores of methane gas frozen in sediments on the seafloor, causing increased global warming.
5. Possibility of an extraterrestrial impact, as indicated by spherical carbon molecules containing an extraterrestrial helium-3 isotope