Geology behind mass extermination

November 30, 2011 - 05:42

New study challenges the established view that a sudden climate change caused a sharp reduction in the number of animal species.

Collecting fossils in northern Greenland. Christian Mac Ørum Rasmussen and colleagues are left in the middle of nowhere in an area rich in brachiopod fossils. (Photo: Jisuo Jin)

Up to now, a sudden period of global cooling has been seen as causing the destruction of 85 percent of all species on Earth 440 million years ago.

But now Danish researchers say the cause was entirely different – geological processes underground that joined a number of microcontinents together with a larger continent.

“Our study shows that it was not climate change but the location of the continents that determined the number of animal species on Earth,” says Christian Mac Ørum Rasmussen, a postdoc at the Natural History Museum of Denmark (NHM), who carried out the study. “This gives us a completely new view of the current reduction in species, which may very well be due to a similar geological mechanism rather than climate change.”

Mussels tell a story of prehistoric catastrophe

The event Rasmussen refers to occurred at the end of the so-called Ordovician period in the Paleozoic geological era.

The researchers have formed an impression of what happened by mapping the occurrence of one of the most widespread living creatures of the age – a mussel-like animal called brachiopod.

About 440 million years ago, a number of islands – microcontinents – fused together with a large continent. This was a catastrophe for many animal species, which died out. This reconstruction, which shows the general movement of the continents and not the movement of the microcontinents, shows a large change in the continents’ relative location to each other in the time before the mass extinction of animal species. (Map: Ron Blakey)

“In the Paleozoic geological era, brachiopods were very prevalent on the ocean beds,” says Rasmussen. “Changes in the number of species of this animal group tell us about the biodiversity that existed at this time in the history of the Earth.”

They found plenty of brachiopod fossils during field work in Greenland, Russia and Scandinavia. Since brachiopods have changed their appearance over time, and many brachiopod fossils have been collected, dated and described in scientific literature, researchers can easily determine when a brachiopod fossil lived.

The fossils collected by Rasmussen and his colleagues were entered in a detailed scientific database that covers the period from 15 million years before the sharp reduction in the number of animal species to 15 million years afterwards.

Islands a wonderland for life

When they started to analyse the many fossils in the database, the researchers discovered to their surprise that the fall in biodiversity was concentrated on small islands (microcontinents) that existed off the coast of a continent called Laurentia – today’s North America – and occurred as geological processes pushed the islands towards and joined them with Laurentia.

The coasts of the microcontinents offered many good habitats for animal life, and the great richness of animals in this wonderland attracted many predators. But these habitats disappeared when the geological forces fused the islands with Laurentia.

Although there is much talk of global warming today, we are actually in an ice age geologically
Christian Mac Ørum Rasmussen

The animal life in the microcontinent habitats fought and lost out to the animal life in the habitats on the continental coasts.

“The standard theory was that the acute fall in animal species occurred across the whole globe at that time,” says Rasmussen. “But we can see that the fall took place primarily on islands and this indicates that it was caused by a local geological process rather than a global climate event.”

He  adds that a few species adapted to their new living conditions, while most species died out.

“It has taken millions of years for biodiversity to flourish again,” he says.

History repeats itself

During the past 50,000 years, the Earth has seen the sixth-largest fall in the number of species and this catastrophe has many similarities to the mass extinction in the Ordovician period.

Facts

The Paleozoic geological era comprises the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and the Permian periods.

It lasted about 300 million years. Dating of the earliest limit has changed in recent years and is now given as 545 million years ago.

The Ordovician period was named after the Celtic tribe of the Ordovices, and covers the approximate period 495-443 million years ago.

The second-oldest period in the Paleozoic era after the Cambrian Period, the Ordovician period comprises six sub-divisions and saw a powerful expansion in most marine animal groups.

The new knowledge is causing the Danish researchers to question whether the fall in the number of animal species that we see today is really due to human activity, as many researchers argue, or to geological processes that are moving the continents.

In the research article, published in Palaeogeography, Palaeoclimatology, Palaeoecology, Rasmussen and his colleague at the NHM, Professor David A. T. Harper, draw parallels to the current situation and say that the conditions for the two catastrophes are the same.

Today there are many continents in the northern hemisphere – just as there were 440 million years ago.

“Although there is much talk of global warming today, we are actually in an ice age geologically,” says Rasmussen. “We believe the positions of the continents during the ice age may have played a large role in the extinction of the species – a development that was accelerated later by human activity.”

Read the article in Danish at videnskab.dk

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Translated by
Michael de Laine