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

Sometimes science can be a messy job – not to mention “disgusting and smelly”. That’s how British researchers describe their experiments monitoring dead sea bass carcasses as they decomposed within 70 days. In the process, they gained some interesting insights into how (and why) the soft tissue of internal organs was selectively preserved in the fossil record, according to a new paper published in the journal Paleontology.

Most fossils are bones, shells, teeth, and other forms of ‘hard’ tissue, but occasionally preserved soft tissues such as skin, muscles, organs, and even eyeballs are found Rare Fossils. This can tell scientists a lot about the biology, ecology and evolution of these ancient organisms that bones alone cannot convey. For example, earlier this year, researchers combined advanced imaging techniques to create a highly detailed 3D model of a 365-million-year-old ammonite fossil from the Jurassic period, revealing previously unseen internal muscles.

“One of the best ways for soft tissues to turn into rock is to replace them with a mineral called calcium phosphate (sometimes called apatite),” Co-author Thomas Clements said of the University of Birmingham. “Scientists have been studying calcium phosphate for decades to try to understand how this process happens — but one question we don’t understand is why certain internal organs seem to be more likely to be preserved than others.”

Specifically, muscles, stomach, and gut are more “phosphorylated” than other organs, such as kidneys and gonads. There are two common assumptions that explain this. The first is that different organs decay at different rates, and some organs will have a pH below the critical threshold of 6.4. As these organs decay, they develop a unique pH microenvironment that increases the likelihood that these organs will become fossils. Different areas of the same carcass may form different minerals.

The second hypothesis is that tissue biochemistry plays a major role. Specifically, a pervasive pH environment is created within the body cavity and persists until the body decomposes.

according to Clement et al ., no previous studies have focused on recording pH gradients associated with decay of specific anatomical features as carcass decays in real time; past experiments have focused on recording pH fluctuations outside the carcass. So the team decided to correct this gap and conduct experiments on decomposed fish, recording how the pH gradient changed over two and a half months.

First, they purchased several adult European bass from a local fishmonger as soon as possible after death (no more than 24-36 hours). The fish was kept on ice to slow decay, but not frozen to avoid any cellular damage. Next, they inserted pH probes into different locations on the six sea bass carcasses to target specific organs: the stomach, liver, intestine, and the external axis muscle. The fifth probe is used to monitor the pH of the surrounding environment 1 to 2 mm from the carcass.