These researchers watched dead fish rot for 70 days—for science

Aurich Lawson/T. Clements et al.

Sometimes science can be a messy business—not to mention “disgusting and smelly.” Here’s how British researchers described their experiments monitoring the corpses of dead sea bass as they decayed over 70 days. In the process, they gained fascinating insight into how (and why) the soft tissues of internal organs can be selectively preserved in the fossil record, according to a new paper published in the journal Palaeontology.

Most fossils are bones, shells, teeth, and other forms of “hard” tissue, but sometimes rare fossils are found that have preserved soft tissue, such as skin, muscles, organs, or even the occasional eyeball. This can tell scientists a lot about aspects of the biology, ecology and evolution of such ancient organisms that skeletons alone cannot. For example, earlier this year, researchers created a highly detailed 3D model of a 365-million-year-old Jurassic ammonite fossil by combining advanced imaging techniques to reveal internal muscles that had never been seen before.

“One of the best ways to turn soft tissue into stone is to replace it with a mineral called calcium phosphate (sometimes called apatite),” said co-author Thomas Clements of the University of Birmingham. “Scientists have studied calcium phosphate for decades trying to understand how this process occurs, but one question we just don’t understand is why some internal organs seem more likely to be preserved than others.”

In particular, the muscles, stomach and intestines tend to “phosphate” much more often than other organs such as the kidneys and gonads. There are two common hypotheses to explain this. The first is that different organs break down at different rates, and the pH of certain organs falls below the critical threshold of 6.4. As these organs decompose, they create a distinct pH microenvironment that increases the likelihood of fossilization of these organs. Different minerals can form in different parts of the same carcass.

Examples of phosphatized soft tissues in fossils: (a) frog stomach with phosphatized cavity;  (b) micro-CT image of a Brazilian fish fossil with phosphated internal organs;  (c) Colubrid snake with phosphatized skin.
Increase / Examples of phosphatized soft tissues in fossils: (a) frog stomach with phosphatized cavity; (b) micro-CT image of a Brazilian fish fossil with phosphated internal organs; (c) Colubrid snake with phosphatized skin.

The second hypothesis is that tissue biochemistry plays a major role. In particular, a common pH environment is formed in the body cavity, which persists until the corpse decomposes.

According to Kliment etc., no previous studies have focused on documenting the pH gradients associated with the decay of specific anatomical features as a carcass decays in real time; past experiments have focused on recording pH fluctuations outside the carcass. So the team decided to address this gap and conduct experiments with rotting fish, documenting how the pH gradient changed over two and a half months.

First, they purchased several adult European sea bass from a local fish store as soon as possible after death (no more than 24-36 hours). Fish were kept on ice to slow decay but not frozen to avoid cell damage. They then inserted pH probes into different locations on each of the six sea bass carcasses to target specific organs: the stomach, liver, intestines and epaxial muscle. The fifth probe was used to monitor the pH of the environment at a distance of 1-2 mm from the carcass.

More examples of phosphatized soft tissues in fossils: (d) polychaete worm with phosphatized musculature;  (e) trilobite with phosphatized intestinal tract;  and (f) vampiropod octopus under UV light to show phosphatized tissues.
Increase / More examples of phosphatized soft tissues in fossils: (d) polychaete worm with phosphatized musculature; (e) trilobite with phosphatized intestinal tract; and (f) vampiropod octopus under UV light to show phosphatized tissues.

T. Clements et al., 2022

Leave a Comment

Your email address will not be published.