How fish dodged extinction’s bullet

LIFE on Earth was nearly extinguished 251 million years ago. Some 96 per cent of marine species went extinct. The world’s forests disappeared, and for about a million years no plant grew higher than a half a metre. It took 100 million years for biodiversity to return to pre-extinction levels. The global ecosystem was shattered and the history of our planet took an abrupt turn, laying the basis for the rise of dinosaurs, mammals and flowering plants.

The cause of this catastrophe was a series of immense volcanic eruptions that produced the vast stepped layers of igneous rock known as the Siberian traps (the word “trap” comes from the Dutch for “stair”). This period of hellish volcanic activity spewed hundreds of thousands of gigatonnes of carbon dioxide and other greenhouse gases into the atmosphere, and covered huge areas of land with thousands of cubic kilometres of lava.

In the aptly titled The Worst of Times, Paul Wignall, professor of palaeoenvironments at the University of Leeds, explores the cataclysm that precipitated the end-Permian mass extinction. His analysis covers a staggering 80 million years, from the Triassic to the beginning of the Jurassic. He believes that a series of lesser extinction events in that period had similar volcanic causes.

The book’s central claim – unaccountably left until the closing pages – is that the ecological effects of the eruptions were exacerbated because, during this period, the world’s entire land mass had fused into a single supercontinent called Pangaea. This had consequences for the environment – for example, a single continent would have led to less rainfall per unit area, which may have meant that some parts were uninhabitable.

In a relaxed style, Wignall describes the significance of recent measurements and discoveries, and introduces us to framboids, rudists, crurotarsans, gorgonopsians and many other obscure scientific terms. There are no illustrations here, so have Google to hand as you read.

Wignall’s excellent introduction to the latest thinking about this key period in Earth’s history could profitably be read alongside Michael Benton’s end-Permian-focused When Life Nearly Died, recently released in a new edition.

The Worst of Times reveals how little we understand about past ecologies. It is baffling that some taxa, including two major animal groups – fish and insects – breezed through the catastrophe without too much damage.

The profound atmospheric and temperature changes at this time would have led to acidified and oxygen-depleted seas – yet fish, at the top of the food web, survived. Many plants died – but plant-eating and detritus-feeding insects were largely unaffected.

There is something amiss here: either we do not have an accurate picture of the extinction process, or we do not understand Permian-Triassic ecology. Or both.

CULTURELAB

7 October 2015

Source: How fish dodged extinction’s bullet | New Scientist

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