Tiny tooth-like fossils called conodonts
have helped scientists study the
temperatures of ancient oceans.
Photo courtesy of ANU.

Microscopic tooth-like fossils have helped an international team led by scientists from Australian National University (ANU) to demonstrate for the first time a link between a cooling climate and increasing biodiversity in ancient oceans almost 500 million years ago.

The team, headed by Dr Julie Trotter of the ANU Research School of Earth Sciences, measured the oxygen isotope ratio of the fossils, less than two millimetres long, from extinct eel-shaped sea creatures called conodonts. The oxygen isotope ratio in a conodont depends on the temperature of the water in which the animal was living. Their findings were published recently in Science.

The fossils, made from similar material to fish teeth (calcium phosphate), preserve the oxygen over geological timescales much better than the calcium carbonate from fossil seashells used in earlier studies. The findings from the study have shed new light on an ancient era and provide clues to oceanic life responses to climate change.

“Our research shows that over the period from about 490 to 470 million years ago, the equatorial sea surface cooled from about 40ºC to temperatures similar to the tropics of today,” said Dr Trotter. “This new moderate temperature regime persisted for around 25 million years, during which there was an explosion in marine biodiversity – it’s recognised as one of the greatest evolutionary radiations in history. Oceans then cooled to glacial conditions, when there were major extinctions. Change the climate, change the life on Earth.”

To unlock this temperature record from the fossils, the team used a room-sized instrument called a ‘SHRIMP’.  The SHRIMP II Sensitive High Resolution Ion MicroProbe (Mark II) was invented by scientists from the ANU Research School of Earth Sciences. It measures isotopes from samples as small as five microns in diameter, or about one tenth the diameter of a human hair.

“Using the SHRIMP to measure oxygen from phosphate microfossils is a real breakthrough,” said Dr Trotter. “The SHRIMP will let us track climate change over hundreds of millions of years more easily and reliably, and therefore help us to understand how life might respond to future climate change.”

The SHRIMP instruments were developed at the ANU, have been commercialised by Australian Scientific Instruments Pty Limited (ASI), and exported to seven countries. They were originally built for the dating of rock samples at the microscopic level, but are now also used for the study of the stable isotopes of light elements such as oxygen, carbon and sulphur.