Gogofish paints new picture of early life

The Gogo formation in the Kimberley region of Western Australia attracts palaeontologists from around the globe due to its unique snapshot of an ancient tropical reef environment, but one that existed some 370 million years before our modern Great Barrier Reef. During the Devonian Period of the Earth's geological history the Kimberley region was under the sea. Today the entire reef system has been exhumed, the reefs forming rocky limestone hills and ridges, 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 was saturated with calcium, and created perfect conditions for the production of calcium carbonate, which could form as concretions of limestone around any small nucleation centre in the muddy bottom sediments. The remains of ancient animals that lived near the surface could sink to the bottom, and then cause the precipitation of calcium carbonate. Over the centuries layer upon layer of limestone was accreted around these shells and bones, rather like layers of nacre form around sand grains in oysters to create pearls onto the surface. Under this process, little balls of rock were created within the soft muddy sediments, and prevented crushing as the sediments were compacted. Today, erosion brings these spherical limestone nodules to the surface, where they litter the black soil plains. This is the paleontologist's lucky dip; about one in fifty nodules reveals a fossil when cracked open. It was on just such a fossil finding trip that three scientists discovered one of the most remarkable fossils in recent history.

Partially emerging from a naturally eroded nodule was the skull of one of the rarest fishes found at Gogo, called Gogonasus. Amongst thousands of fossil fish samples from Gogo, representing over 45 different fish species, only three incomplete skulls of Gogonasus had previously been found. On the other hand, it is one of the most interesting Gogo fossils, because it is the only one 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’. Searching the surrounding area uncovered other nodules from the same animal, to give 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 that occupy the land surface today. By studying the brain and sense organs of these exquisitely preserved fossils, scientists can glean vital information about this pivotal process in the evolution of 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, styrup 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. The gills of a fish were no longer needed for respiration, being replaced by air-breathing lungs in the first land animals. 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-rays 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 very tiny objects.

This is where the revolutionary micro µCT scanner developed at the Australian National University 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 micrometers. To put that into perspective, this is almost as fine as detail that can be seen in the most powerful optical microscopes. Using the µCT scanner, the team 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 that the µCT scanner produces, offers other exciting prospects too. Exact replicas can be directly printed directly into resin on rapid prototyping machines enabling perfect replicas of the fossil to be created with both external and internal detail impossible to capture with conventional casting techniques. The model can also be used to create a perfectly accurate animated creature. All the joints can be articulated in the virtual environment, which enables scientists to test, theories of 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 localized fresh water deposits as some scientists had previously believed.

Research Highlight Poster

 
 

Back Home