How did the expression dead fish come about?

PaleoanthropologyHow is a fossil made from a dead fish?

"How does this transition come about? How do we get from a living, existing animal to a fossil in the rock that is part of the rock itself?"

Fabian Gäb has been dealing with this actually quite simple question for a number of years. The postdoctoral fellow from the University of Bonn wants to fathom the processes that lead to an organism being preserved as a fossil for posterity. In old textbooks it is roughly stated that fish fossils are formed when the bodies sink rapidly after death, are covered by sediment, there is a lack of oxygen there, i.e. the stop of biological decay, and after a long time minerals are formed that the Preserved the shape of the animal - in short: the carcass petrified.

"If it were that easy, I'd guess we'd have much, much more fossils than we have right now."

(picture alliance / DB Atlantic Production Ltd.) Of fake fossils and dubious finds
Upright monkeys in the Allgäu and the lower jaw of an orangutan: in the history of paleanthropology there have also been misinterpretations, falsifications and dubious results. Three particularly spectacular cases.

In order to clarify what happens when fossilization occurs, the team from Bonn is researching the topic of fossilization as part of a DFG project. Fabian Gäb is trying to find out experimentally how a dead goldfish can be turned into a fossil. In a first experiment, he placed a carcass in salt water at room temperature and photographed the fish or what was left of it every day.

"We haven't made anything that looks like a fossil and what was left would never have been fossilized."

Most of the carcasses decomposed after two or three weeks, although there was hardly any oxygen, and neither waves nor temperature fluctuations. After all, the research team was able to refute the old thesis, according to which only a lack of oxygen can automatically lead to fossil formation. But if they don't do anything, they won't petrify.

The carcass only petrifies on the seabed

"What conditions can we change so that we can set something in motion that can be the initial spark for fossilization, namely: How do we first ensure that the fish, that is the carcass of an animal, is not completely decomposed?"

In the case of decomposition, gases are produced that expand the body and swim upwards with the belly and, in extreme cases, burst. That was to be prevented. So Fabian Gäb and his colleagues increased the pressure on the next goldfish carcasses - with the help of an autoclave, i.e. an airtight pressure vessel. This enabled them to simulate a pressure of up to eleven bar, which corresponds to a sea depth of up to 110 meters.

"If you let a fish lie and rot under otherwise equal conditions, it will not rupture at eight bar pressure, but it will with a bar. Well, these eight bars, we have simply found that they are enough to make a carcass burst open to be prevented by the resulting putrefaction gases. "

However, the carcass cannot petrify on the surface of the sea, it only succeeds in the sediment.

"We have to get this fish on the bottom, on the bottom of the sea. And then we found out: if you increase the pressure on a dead fish that swims, this fish sinks."

The higher the pressure, the faster the carcass sinks to the seabed. The temperature there is stable, there are few scavengers, hardly any movement, but plenty of sediment that can cover the body - necessary conditions for possible petrification. The next starting point for the experiments in Bonn was a higher salt content. Because: salt is also preserved.

"The higher the salt concentration, the better the fish that we have exposed to this seawater. And from a salt concentration of over ten percent you could no longer detect any noteworthy decay of these fish, at least externally."

The first weeks and months are decisive

During the experiments, Fabian Gäb and his colleagues observed another factor that surprisingly changed: the pH value.

"We found that the pH value went from slightly basic over the duration of the experiment, because sea water is overall slightly basic, has a pH value of 8.1, and always slips into the slightly acidic environment, always around pH six up to 6.5 according to the experiments. And so the idea arose that buffering the pH value under basic conditions could ensure maintenance. "

Further experiments confirmed this assumption. So it needs pressure to sink and maintain the carcass, as well as a decay-retarding influence from the salt content or alkaline conditions. Many factors have to come together for a carcass to have any chance of petrifying.

"Although fossilization is a process that potentially takes millions of years to complete, but in any case the results of which can be stable over millions to billions of years, it seems that the decisive course for this process was set in the first few days and Weeks after the death of the living being. "

This means that Fabian Gäb's team has only come to a point where it can preserve fish, but where they still consist mainly of organic substances. Around 30 animals have contributed to these findings and the end of the experiments is not yet in sight, because up to now they have not yet discovered any definite new formation of minerals in the goldfish - but that could change soon.

"The moment we talk, an experiment is running in my laboratory. In our experiments we try to force new mineral formation, which then also ensures that the physiological structures of such a fish are traced by these newly formed minerals. And that is the point at which we are right now and that is the right fossilization, where we actually talk about new mineral formation and then in the end we have a product that can really withstand the millions of years in a rock formations. "