Arowana - True Fossil Fish

Arowana - True Fossil Fish

Original article and photographs for Aquarticles
based on a translation of a museum display at Fur Island Museum, Denmark

Aquarticles.com

Editor's note: Most photographs can be enlarged by double-clicking your mouse over them.

Close up view of a 54 million year old fossilized skull of a marine Osteoglossidae,
a direct ancestor of today's Asian arowana (Scleropages formosus)

Being the Asian arowana-nut that I am (it is a big part of my aquarium hobby), I am always on the look-out for new information about them. It is not often in Denmark that new information comes my way. During a weekend get away a few weeks ago I decided to visit a local museum that is known to have a nice collection of fossilized fish. My kids bounded inside the building ahead of me while my wife and I stopped to pay our admission. Suddenly there was a huge commotion - my kids began shouting "Arowana! Arowana! Go show Daddy!". My wife and I looked at each other, just a bit puzzled. Being dragged into the next room by my youngest, sure enough, there is a stuffed Asian arowana displayed on the wall. Accompanying it was a display of a major fossil find in the local area some years back that focussed on - you guessed it - the ancestor of the Asian arowana. Well, my wife may have rolled her eyes a little, but suddenly I was pretty interested in what this museum had to offer.

With my wife's help I have been able to translate the Danish captions that accompany most of the items in the display.

A preserved Asian arowana (Scleropages formosus)

Map of aro range

Contemporary Arowana Range (Translation of text accompanying above map)

There are five types of fish in the family Osteoglossidae which exist today. The name Osteoglossidae means bone heavy fish. These are all freshwater fish and mainly live in tropical waters. When the Moler Ocean covered the western part of the Limfjorden, Denmark 54 million years ago, the climate was warm. At that time, the "bone heavy fish" lived in salt water. These fish today are as primitive as lung fish and are therefore of great interest to evolutionary biologists. The bone heavy fish are predators and have teeth on their tongue and on the roof of their mouth which they use to bite with. They also have teeth in their jaw.

The map diagram above shows their current distribution in the world today. In South America there are two species: Osteoglossum bicirrhosum (South American arowana) and Arapaima gigas, which are the world's biggest freshwater fish. In Africa lives Clupisudis niloticus (African arowana), which can be up to one meter long. In Australia and New Guinea, there are Steropages leichardtii (Australian arowana), which are really closely related to Scleropages formosus (Asian arowana) which live in Borneo, Sumatra, and Peninsular Malaya.

Some interesting notes about the Arapaima gigas are that it has a swimming bladder that is adjusted to atmostpheric oxygen. Grown fish surface to breathe about 5 times an hours. Strangely enough, the best bait to catch the Arapaima gigas is the south American arowana!

Whole cabinet display

Another view of the fossilized skull of a marine Osteoglossidae

Ancient-Arowna Osteoglossidae Jaw Bone

Ancient Osteoglossidae fossilised scales - apparently these are not too hard to
find in the surrounding area and so I spent a few hours looking...alas with no luck.

Ancient Osteoglossidae scales

Another photo of some ancient Osteoglossidae scales

Here is a photo of the fossilsed Osteoglossidae head - this being the top half of the fossil

Another full section photograph of the top half of the fossil

Making of a fossil diagram

Fossil Formation (translation of text accompanying above diagram)

A dead fish sinks to the bottom of the ocean where it is dark and has little oxygen. In the poisonous aerobic bottom water, there are no scavengers, which would destroy the fish body.

Over time, the fish is covered by new layers of sediment, (Mole-clay). The organic material of the fish disappears and only scales and bones are left.

Around the skeleton, calcium can leach out, settle around the bones, and form a kind of calcium-stone cast. Because of this, the bones are now safe from later destruction.

If the fish skeleton is not safely encased in a calcium-stone cast, the bones will over time be dissolved by the acidic water and only an imprint of the fish will survive.

History of the Giant Fossil Fish

On June 18, 1985, Fur Museum got a post card from a German tourist. He wrote that a huge fossil fish lay under a circle of stone on Fur's north beach. He had circled the area on a map on the postcard.

Photo of Fur Island, Denmark (top)
Photo of fossil recovery (bottom)

The museum director Erik Christiansen went out to the beach that night and he did find a fantastic fossil of a big fish in a lens of calcium-stone. The big calcium-stone was hauled out the next day.

Diagram of fossil being opened

The German tourist had already skilfully split the rock along its horizontal layers. And then he had taken the top part home- which was legal! - and the post card ensured that the museum got the other half.

Next year the German tourist contacted the Fur museum to talk about the fossil find. He offered the museum to borrow his top part of the fossil for five years if they fixed it up. The New Carlsburg Foundation paid for the restoration.

The enormous press coverage of the find and story, along with the efforts of the museum director Erik Christensen led to changes in the Danish laws governing ownership of artifacts of national historical value to Denmark. The new law was to secure new artifacts found for Denmark. The law came into effect in January 1990. So now, unusual rocks, fossils, etc., have to be delivered to national museums in Denmark (for example Fur Museum).

In the spring of 1992 the top part of the fossil was bought by a consortium of Danish museums "Statens Museums-nævn". Now the entire giant fish is back in Fur where it had lain for the last 55 million years.

Conclusion

For evolutionary biologists, the ancient fossils of Osteoglossidae found around the world are evidence that all of the world's continents were once joined together in one super-continent (Gondwanaland). Fossilised Osteoglossidae have been found in Europe, North America, Asia, India, Africa, and South America. The fact that the descendants of these truly ancient fish continue to inhabit many of these regions adds to their mistique. When it comes to the Asian arowana, it truly lays claim to being a living fossil fish.

Editor's note: For further reading on the subject of the arowana ancestors, I recommend you look at the following links:

Article on fossilised Osteoglossidae skull found in India

PDF article on Fossilized Scleropages remains found in Belgium

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