Having evolved, it is the ultimate fate of all species to go extinct.

There are four principle patterns shown by extinction events,

which may or may not form a continuum.

Background extinctions are local in nature, happening continuously, and

lead to the demise of one, or a few taxa.

Mass extinctions are episodic,

geologically fast, global in extent, and affect numerous taxa.

Regional extinctions, are similar to mass extinctions, but

are only regional in influence.

For example, the terrestrial form of Jamaica, was completely wiped out and

the island was drowned, 50 million years ago.

Taxon extinctions are global in extent, but only affect one group of organisms.

For example,

such events occurred repeatedly during the geological history of the aminoids.

Any evolutionary system can be divided into two parts, the pattern, and

the process.

That I mention of geological time is important in studying both patterns, and

processes of extinctions.

Thus, the process, or processes of background extinction, driven by physical,

or biological environment, can

only be meaningfully assessed by reference to the pattern seen in the fossil record.

In determining the processes which drive mass extinctions,

the following inferences are important.

Mass extinctions are driven by different environmental forces to background

extinctions.

Further, mass extinctions have different driving mechanisms discernible for

different events.

Finally, mass extinctions are most probably driven by multiple

causes that may, or may not be interrelated.

There are five major mass extinctions that can be recognized in the rock record,

amongst many other less significant peaks of extinction.

The late Ordovician extinction, about 445 million years ago,

coincided with a period of major glaciation.

Followed after half a million years, by a deglaciation.

The glaciation drove such environmental changes as a global drop in temperature.

An unassociated drop in sea level.

The latter had the effect of greatly reducing the areas of shelf seas,

where much of the world's biomass was then concentrated,

at a time when terrestrial organisms were a minor component of the biosphere.

The deglaciation, apart from indicating a warming event,

also led to sea levels being elevated to greater than pre-extinction depths.

Effectively drowning shallow shelf areas.

The extinction was just delivered as a one two punch to the marine biota.

The late Devonian extinction, about 375 million years ago, was again,

temperature related.

Global calling being associated with widespread marine anoxia.

It did not occur at the end of the Devonian, but

rather around the boundary between the Frasnian and

Succeedian Famennian stages, sub-divisions of the late Devonian.

Again, it was probably not a geologically sudden event, and different groups,

such as reef building organisms, and brachiopods,

went extinct at different times about the Frasnian-Famennian boundary.

That this event was temperature related, is suggested by it causing widespread

extinction in tropical reefs and peri-reefal ecosystems.

The end Permian extinction, about 250 million years ago,

is the largest mass extinction recognized in the fossil record.

It may have led to the demise of 96% of marine species.

At least nine different driving mechanisms have been suggested.

Almost certainly, it was the result of the coincidence of two or more of these.

This extinction coincided with the formation of a supercontinent,

Pangaea, which stretched between the poles.

This amalgamation of all continental blocks, would have greatly reduced

the length of shelf seas available to shallow water organisms.

Further, many Permo-Triassic deposits are desert redbeds,

in which pale-ontological evidence, either body, or trace fossils is rare,

thus making recognition of the precise pattern of extinction difficult.

The late Triassic extinction, between about 220 and

210 million years ago, is again difficult to precisely identify, and

seems to be in a period of high extinction rates.

Again, the continents, although beginning to separate was

still in close association and redbeds are widespread.

A wide variety of marine invertebrates may have suffered extinction in the Carnian,

more so than the Norian.

Many are associated with environmental changes indicated

by changes in sedimentary fasces.

Good data suggests a widespread increase in rainfall in the mid-Carnean.

Other features indicate that there was a major marine regression,

and decrease of habitable shelf area.

The End Cretaceous Extinction,

about 65 million years ago, is the only major mass extinction for

which multiple lines of evidence of the Bolide Impact is available.

Such as a peak in abundance of platinum group elements, including iridium.

These are otherwise much rarer in the earth's crust,

than elsewhere in the solar system.

The search for iridium anomalies at other extinction horizons,

has failed to produce equally conclusive evidence of impacts.

A definite pattern of the occurrence of mass extinctions,

was first suggested in the late 1970's.

However, it did not gain statistical support until early in 1984,

in a paper published by David Raup and Jack Sepkoski.

Both of the University of Chicago.

Raup and Sepkoski analyzed the last appearance data,

that is extinctions of marine families, and

found strong statistical indications of a periodicity of mass extinctions.

With a frequency of about every 26 million years since the end of the Permium.

Suggested driving mechanisms included,

isolation about the galactic plane, a rogue planet, planet X, or

nemesis, a postulated sister star to the sun with a highly elliptical orbit.

However, other analysis questioned the validity of periodicity.

Hoffman considered that the 26 million year periodicity of mass extinctions,

was an artifact generated by arbitrary decisions,

such as the geological time scale used, and how maxima extinctions are defined.

Patterson and Smith, leading experts on fossil fishes and dicynodonts,

analyze the quality and accuracy of this database for their areas of expertise.

They found that only 25% of the families used by Raup and

Sepkoski were correctly dated monophyletic groups.

When the 75% of the data belonging to groups that

were not monophyletic, and or incorrectly dated was removed,

and corrected from the graph of extinctions since end of the Permian.

Some of the apparent peaks of extinction disappeared.

This suggests that the 26 million year periodicity,

may be an artifact based on noise in the data set.

Most critically, Stigler and Wagner tested the statistical basis for

the periodicity of extinctions, by examining the distribution of

the stratigraphic units, mostly stages, used to arrive at the pattern.

They noted that the stages were of differing duration's.

And they randomize the order of these strategraphic units repeatedly.

Stages in which major extinctions were considered to occur were identified, and

for each randomized strategraphic sequence, a best fit for

periodicity of extinctions was determined.

This was like shuffling the rock record, something you can't really do.

The results of these tests were surprising,

in that 27% of the best fit analyses gave a periodicity of 26 million years.

Which they concluded was an artifact of the duration's of the stages.

Thus, Raup and Sepkosky's periodicity, may be a statistical artifact.

What happened on our planet during mass extinctions?

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