Skeletal remains of conodonts are
providing new evidence of ocean
conditions 470 million years ago.
Image provided by Dr Julie Trotter.

An international research team led by Australian palaeontologist Dr Julie Trotter claims to have discovered the likely trigger that caused a major surge in the biodiversity of ancient oceans.

"The timing and extent of biodiversification was already well recognised by palaeontologists, but we have provided the first plausible explanation for why it occurred," says Dr Trotter, who works at the Australian National University’s Research School of Earth Sciences.

However, the discovery has plunged the team into a long-running scientific controversy over what really happened on Earth 470 million years ago during the so-called Ordovician period.

Just six months ago, another international team led by geologist Dr Birger Schmitz of Sweden's University of Lund reported in Nature Geoscience that their research showed the cause of the Great Ordovician Biodiversity Event (GOBE) was Earth being bombarded by kilometre-wide meteorites from the Asteroid Belt.

Writing in Science, Dr Trotter argues that it was not meteorites but a steady cooling of the world's oceans over a period of 20 million years that created conditions for "one of the greatest radiations of life in Earth's history".

Dr Trotter and her team made the claim after finding new evidence, some of it drawn from the ancient sea that once swept Western Australia's Canning Basin, that over the 20 million year period ocean temperatures cooled from around 42 degrees Celsius to a much more life-stimulating 30 degrees and lower.

"That put it within our present day range of equatorial sea surface temperatures," Dr Trotter says.

The Ordovician Earth was nothing like today's world. Supercontinents like Gondwanaland were all but lifeless and the oceans were home to simple life forms.

The coming of GOBE changed all that. According to Dr Trotter's research, it was those favourable temperatures that triggered an explosion of new life forms and diversity across the oceans.

"It was then that plankton and more complex macro organisms become plentiful," Dr Trotter says. "The first real corals appeared and the number of species of known marine vertebrates more than quadrupled."

Discovering the Ordovician oceans were much cooler than previously thought has been the real breakthrough for Dr Trotter and her team, with the skeletal remains of a tiny extinct sea creature, known as a conodont, providing the evidence.

The geological age of the conodont used in the study was already known but the team analysed a mineral found in its skeleton for changes in the ratio of two oxygen isotopes, which is dependent on the temperature of the water in which the creatures lived.

Dr Trotter says it is still uncertain what specifically caused the oceans to cool, but says climate is partly driven by natural geological processes.

"Fundamental earth processes, such as tectonics, promotes continental weathering that can draw down CO2, which in turn cools the climate," she says.

"Movements of continents and the opening of seaways also influence ocean circulation and heat distribution, thereby affecting climate."