Scientists Put Forward New Theory for Cambrian Extinction Event

The history of life on planet Earth has been punctuated with a series of extinction events.  More than fifty major extinctions have been identified over the last 500 million years or so.  Life on Earth has had to get used to these setbacks, however, extinctions have permitted new organisms to evolve and they have helped to “spur on” evolution.  A new paper has just been published in the scientific journal “Nature” that sheds light on one of the first major extinction events recorded in the fossil record – the Late Cambrian extinction.

Following the Cambrian explosion that saw the recording of all the known animal phyla in the fossil record from approximately 540 million years ago, life was thriving in the shallow seas of the Cambrian Period.  Porifera (sponges) were widespread, along with the Brachiopods and Molluscs. Although there is evidence of the first Chordates (back-boned animals or animals with a notochord), the most advanced creatures were the Arthropods, creatures such as Trilobites that had already evolved into many different families.

An Illustration of a Typical Trilobite

Picture Credit: Everything Dinosaur

However, from approximately 520 million years to 490 million years ago, a series of swift extinction events took place, radically altering the types and quantity of marine invertebrate genera present.  A team of geologists have completed a study of rock strata laid down in the middle of this geological period and their data showing high levels of sulphur and carbon indicate that changes to the atmosphere may be the cause for the dying out of many types of organism.

According to their study, reported in this weeks edition of “Nature” the ocean’s oxygen levels fell sharply and sulphur levels rose very quickly, killing off genera that could not adapt.

Benjamin Gill, one of the authors of the report (Post-doctoral Fellow and research Fellow assistant at Harvard University), commented:

“Around 499 million years ago, large portions of the ocean were oxygen deficient and also contained hydrogen sulphide.”

The geologists studied a specific subset of Cambrian extinctions that began approximately 499 million years ago and lasted for two to four million years.  Low oxygen levels had been postulated as putting the brake on the advancement of life forms, but until now there was little supporting evidence for this theory.

The chemical analysis of the strata shows that from the six locations studied; there were unexpectedly high levels of various isotopes of carbon and sulphur.  In modern oceans, these mix of elements only occur in oceans which lack oxygen, such as parts of the Black sea in the Crimea.

There has been some work done previously to show that there is a band of iridium deposited in some parts of the world, in rocks dating from around 500 million years ago.  This rare Earth element has in this instance been associated with intense volcanic activity.  The high levels of carbon and sulphur found in this study, may reinforce the theory that volcanic activity gave rise to a series of extinctions that led to something like 30% of all marine genera becoming extinct.

Gill and his colleagues remain unsure as to what caused the oxygen levels to collapse, however, anoxia (lack of oxygen) would have been devastating to life in the oceans at the time.

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