Tuesday, 11 November 2014

A Tectonic Trigger For The Cambrian Explosion

Waptia fieldensis - just one of the thousands of
species produced by the Cambrian Explosion
Different hypotheses have been put forward to explain the Cambrian Explosion from increases in oxygen levels in the atmosphere to the proliferation of the homeobox - a group of genes responsible for shaping a microscopic cluster of cells into a fully formed creature.

Now another possible factor has been put forward to explain the greatest explosion of life in the Earth's history: the fragmentation of a super-continent roughly 530 million years ago.

The distribution of the Earth's landmasses has a profound effect on its inhabitants. Antarctica exists at the South Pole. The gaps separating it from the southern coasts of the other continents allows a complete circum-planetary current to flow, meaning that no warm currents reach the coast of Antarctica. Combined with its polar position, freezing temperatures have been apparent for the past few tens of millions of years, forcing its inhabitants to adapt to these harsh conditions: penguins have feathers, polar bears have fur, seals and whales, blubber.

The concept of continental distribution affecting evolution may also hold a clue as to the trigger of the Cambrian Explosion. 'At the boundary between the Precambrian and Cambrian periods, something big happened tectonically that triggered the spreading of shallow ocean water across the continents, which is clearly tied in time and space to the sudden explosion of multicellular, hard-shelled life on the planet,' said Ian Dalziel from the University of Texas at Austin's Jackson School of Geosciences.

Dalziel proposes that present-day North America was still attached to the southern continents until sometime into the Cambrian period. Current reconstructions of the globe's geography during the early Cambrian show the ancient continent of Laurentia - the ancestral core of North America - as already having separated from the super-continent Gondwanaland. In contrast, Dalziel suggests the development of a deep oceanic gateway between the Pacific and Iapetus oceans isolated Laurentia in the early Cambrian, a geographic makeover that immediately preceded the global sea level rise and apparent explosion of life.

The continental distribution during the Cambrian.
'The reason people didn't make this connection before was because they hadn't looked at all the rock records on the different present-day continents,' he said. The rock record in Antarctica, for example, comes from the very remote Ellsworth Mountains. 'People have wondered for a long time what rifted off there, and I think it was probably North America, opening up this deep seaway, It appears ancient North America was initially attached to Antarctica and part of South America, not to Europe and Africa, as has been widely believed.'

Upon examining the diagram of the continental distribution, I noticed that Namibia, Mexico and Uruguay would have existed on the edges of the widening gulf between North America, South America and Africa.

Ediacaran communities have been found in all three countries and whilst they are of different ages, I wonder if the initial stages of fragmentation may have contributed to the proliferation of the Ediacara biota during the last days of the Precambrian, as well as playing a part in the Cambrian Explosion. The implications of this continental distribution may extend further than Dalziel suspects. What is certain is that continental distribution will have an ever widening role in understanding the evolution of life.