THIS FISH WILL BLOW YOU OUT OF THE WATER…

Content: Dr. Brian Choo

Edited by: Susan Double

The art of the fossil preparator is an unseen, and perhaps underrated discipline. The fossils we see in museum and lab collections rarely come in from the field in a fit condition to display or study. Someone has to separate them from the sediment or rock they may be encased in, clean them, sometimes fix them if broken and finally harden the bone so that they may be preserved and protected into the future. This can take many weeks or months depending upon the process used.

            A fossil fish was recently brought into the prep lab by researcher Dr. Brian Choo (http://www.flinders.edu.au/people/brian.choo). Encased within a calcareous nodule, which had been cleaved neatly into part and counterpart in the field, it was barely recognisable, except by a fish expert, that is. The nodule was passed to our chief preparator, Carey Burke, to see if he could extract the fossil seen vaguely within. Thus began the long preparation process.

Above: The calcareous nodule containing the Moythomasia durgaringa as it arrived in the prep lab from the field (Photo: Carey Burke, 2016).

The fish is a marine Moythomasia  durgaringa  from the Upper Devonian Gogo Formation of Western Australia. This particular specimen was discovered by Diego Garcia-Bellido Capdevila (University of Adelaide) in 2015. While not a particularly rare specimen its state of preservation is remarkable given its age (~390 million years) – but we weren’t to know that until Carey had released it from its rocky prison.

            Carey used a process called the ‘Transfer Method’ which uses a clear plastic resin to preserve the specimen. Each half-nodule was set face down within transparent acrylic, then given repeated baths in a weak acid solution to remove the matrix. The end result is each half of the fish mounted on a clear slab.

Above: The two halves of the nodule encased in their clay moulds with plastic resin poured on top (Photo: Carey Burke, 2016)

MOYTHOMASIA

Actinopterygians, or ray-finned fishes, today comprise over 30,000 species, or the vast majority of animals we recognise as “fish”. Back in the Devonian, the ray-fins were not particularly diverse, with fewer than 30 species described worldwide.

            Moythomasia is one of the earliest and most primitive actinopterygian in the fossil record. It was also the first genus to attain an effectively global distribution, with different species known from Late Devonian marine deposits in Australia, Latvia, Germany and the USA. 

            The Australian species, Moythomasia durgaringa, thanks to the splendid preservation of the Gogo Formation, is the best known species in the genus, and one of the most completely known of all Palaeozoic ray-finned fishes. The earliest actinopterygians had delicate bodies, and are usually recovered as flattened whole fishes or isolated 3-dimension fragments. The Gogo ray-fins are unique in being complete and uncrushed.

 THE GOGO FORMATION

380 million years ago, during the Frasnian stage of the Late Devonian, a vast tropical reef system, over 1400 km in length, stretched along the coast of eastern Gondwana. While corals were present, the main reef builders were algae and massive calcareous sponges called stromatoporoids.

            Away from the vibrant shallows, the seabed dropped off into deep, dark anoxic basins.  The bodies of reef-dwellers which sank into these oxygen-poor areas were untouched by scavengers and became sealed within calcareous concretions. These deepwater sediments, with numerous limestone nodules, are today what we call the Gogo Formation. Over 50 Gogo fish species have been described, including four actinopterygians.

The end result of the painstaking preparation is breathtaking. Almost every anatomical detail of the fish is present, including tiny teeth and scales (see close-up photos below). It is a stunning example of the preparator’s art and Carey has to be congratulated on the result.

Above: The finished specimen                 (Photo: Brian Choo, 2016)

Above: Close-up of head – note teeth!        (Photo: Brian Choo, 2016)

Above: Close-up of scales.              (Photo: Brian Choo, 2016)