Looking more like a sculpted work of art than a fossil, this fish belongs to the remarkable Santana Formation, found in Brazil’s Araripe Geopark. Once picked clear of the surrounding matrix, every detail of this Cretaceous fish can clearly be seen, a palaeontologist’s dream. Possibly the most amazing thing about this superb specimen is that it isn’t an unusual sight for this deposit. This is thanks to the rare conditions of the riverbed at the time these fish were alive. Described by Dr. David Martill as a ‘Soup-ground’, the riverbed was exceptionally soft, allowing the fish to penetrate deep below the surface of the bed when they sank after their death. This protected them from scavanger’s and also sharply limited the amount of oxygen available in their immediate surroundings. This anoxic environment kept decay to a minimum, allowing the fossilization process to take place undisturbed.
The soupy nature of the riverbed meant that the fish sank deep into the ground on death, falling into it at a sharp angle, nose-first. Once buried, the decomposition gases that would otherwise have escaped into the water react with the surrounding sediment. This causes the formation of a concretion around the fish, protecting the carcass from further damage by the weight of sediment above it as well as sealing any further gasses within the body. It’s thanks to these concretions that palaeontologists have been able to calculate the angle at which the fish have sunk into the sediment, the laminations preserved beautifully next to the fossil.
Such is the sheer quality of preservation in these specimens that, occasionally, even details as delicate as the animal’s gills can be found to be intact. Barely more than a few cells thick, a fish’s gills are often the first part to decay beyond all recognition. Indeed, the only part of these fossils that is often absent is the body cavity. The gases of decomposition provide an environment for decomposition to occur, though only briefly. The loss of the organs, whilst a pity from a palaeontological viewpoint, allows another breathtaking phenomenon to occur. Reminiscent of a geode, many of the fossil fish at this location have had their body cavities filled with crystal growth.
Image Credit - http://www.annsus.com/index_en.html
Read More - http://www.geolsoc.org.uk/Geoscientist/Archive/November-2007/Out-of-Eden - http://www.palaeontologyonline.com/articles/2013/patterns-in-palaeontology-exceptional-preservation-of-fossils-in-concretions/ - http://www.paulselden.net/uploads/7/5/3/2/7532217/geologytodaycorrect.pdf
