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ScienceWise - Nov/Dec 2006

Gogo Fish Rewrites Evolutionary History

Article Illustration
The gogofish skull alongside the super high resolution 3D computer model created by the µCT scanner.
Article Illustration
Tim Sender and Gavin Young with the µCT scanner.

Using revolutionary micro CT scanner to perform three-dimensional X-ray scans

The Gogo formation in the Kimberley region of Western Australia presents a unique snapshot of an ancient tropical reef that existed back in the Devonian Period some 370 million years ago. Back then the Kimberley region was under the sea. Today it lies high and dry, the reefs forming rocky limestone hills separated by black soil plains. The plains represent areas of deeper water between the reefs, where thick layers of mud accumulated on the sea bottom.

The warm seawater back then was saturated with calcium, and concretions of limestone would form around any small object lying in the muddy sediments, including the remains of ancient marine animals that settled to the bottom. Over the centuries the limestone accreted around these shells and bones forming little balls of rock that protected their contents from being crushed as the sediments were compacted.

In more recent times erosion has released these spherical limestone nodules to the surface, where they litter the black soil plains. And this has become a palaeontologist’s lucky dip because around one in fifty of these nodules reveals a fossil when cracked open. Sometimes the fossils are common and well known. Sometimes they are truly remarkable.

When Dr Tim Senden from the ANU Department of Applied Maths, RSPhysSE accompanied Dr John Long, Head of Sciences at Museum Victoria and one of Australia’s most experienced palaeontologist, on a fossil fishing expedition to Gogo no-one was expecting what they would bring home – one of the most remarkable fossils in recent history.

Towards the end of the expedition, Dr Senden, who was on his first serious fossil trip, found the skull of a fish poking out from an eroded nodule. It was the remains of Gogonasus, one of the rarest fishes found at Gogo. Only three incomplete skulls of Gogonasus had previously been found (and that’s from thousands of fossil fish samples representing over 45 different fish species).

Gogonasus is the only known fish from this vicinity belonging to a major group called the tetrapodomorphs, an evolutionary branch that included early fish ancestors of the first four legged land animals, or ‘tetrapods’.

Further searching uncovered other nodules from the same animal providing what is the first complete tetrapodomorph skeleton ever discovered from Gogo. Tetrapodomorphs are interesting because they represent the very first steps that back-boned animals took to emerge from an aquatic existence onto dry land, ultimately evolving into all the amphibians, reptiles, birds and mammals. By studying the brain and sensory organs of these exquisitely preserved fossils, scientists can glean vital information about the evolution of vertebrate life on the planet.

One organ of special interest is the ear. Fish living in water have the semicircular canals of the inner ear, which are organs of balance, but no need for the middle ear. In a mammal the middle ear is formed as a series of tiny bones, called (after their shape) the hammer, stirrup and anvil bones, which transmit sound to the braincase from the outside. The first amphibians needed to adapt to the very different challenge of picking up sounds in the far less dense medium of air, by modifying some of the bones supporting the gill cover for this purpose.

By studying the ear structure of tetrapodomorphs, scientists can unravel the complex series of events that culminated not only in the development of modern mammalian ears, but also the complex brains that accompany them. The difficulty here is that the structures of the ear lay deep inside the bone of the animal’s skull. Two-dimensional X-ray radiographs are of limited value and conventional CT scanners such as those used in hospitals, don’t have anywhere near the spatial resolution to probe such tiny objects.

This is where the revolutionary micro CT scanner developed in the Department of Applied Maths really came into its own. This instrument is able to perform three-dimensional X-ray scans of objects with voxel (3D pixel) resolution of two micrometres. To put that into perspective, this is almost as fine as detail that can be seen in the optical microscopes. Using the CT scanner, Dr Senden and colleagues were able to build up a perfect three-dimensional model of the tetrapodomorph skull. This model reveals not only the middle ear, but nerves, blood vessels and various brain case structures vital to understanding the evolution of the complex brains required by land animals.

The 3D tomographical model produced by the CT scanner offers other exciting prospects too. Exact replicas can be directly printed into resin using rapid prototyping machines. This enables perfect replicas of the fossil to be created with both external and internal detail impossible to capture with conventional casting techniques. The computer model can also be used to create an animated creature. All the joints can be articulated in the virtual environment, enabling the scientists to test how the creature might have walked or swum, how its jaws moved and how it breathed.

The spectacular find and subsequent analysis has formed the basis of a recent article in the prestigious scientific journal Nature. It has also turned some aspects of accepted theory on evolutionary history on their heads. Because the Kimberley region can be so precisely dated, the group has been able to establish that the fish Gogonasus, with well developed bones inside the fleshy lobes of its fins, showed that precursors of the bones of the tetrapod limb were beginning to evolve some 30 million years earlier than had previously been thought.

The completeness of the skeleton has also enabled clear links to be drawn between this specimen and partial remains found as far away as Europe, Canada, and China. The wide distribution of these creatures lends weight to the possibility that amphibians may have emerged from the ocean, rather than evolving in localised fresh water deposits as some scientists had previously believed.

The Gogonasus research forms part of an ARC Discovery Project based at ANU and led by Dr Gavin Young from the Department of Earth and Marine Sciences. The CT Facility is also supported by the ARC and forms the central part of a large group working on computational interpretation of and simulation in 3D data of complex materials such as oil-bearing rocks, bone scaffolds and ink flow in paper.

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