If all living things died at this time, according to some estimates, only about 1 percent would become fossils. Even fewer would have preserved soft tissue. These rare tissue fossils offer crucial clues to biology and evolution, but their formation remains a mystery. Why do scientists find fossilized intestines, for example, but never a fossilized liver?
Fossils develop when minerals replace the body parts of organisms that die and become buried in sediments, such as the mixture of mud and seawater on the ocean floor. Paleontologists especially like calcium phosphate, a fossil-building mineral, because it can preserve soft organs in exquisite detail, sometimes down to the cell nucleus. This mineral forms only under specific acidity conditions, so scientists have hypothesized for decades that differences between the pH levels of decaying organs determine which ones are preserved.
To better understand how organs change after death, University of Birmingham paleontologist Thomas Clements took a trip to the fishmonger with a plan to ruin four delicious sea bass. His team stuck pH probes into the internal organs of the fish, then submerged the carcasses in artificial seawater and left them to rot.
For 70 days, the researchers watched the sea bass swell, shed its meat and disintegrate into piles of bones while probes monitored the changing chemistry of the body parts. recently published results in Paleontology, show that within 24 hours, the acidity of each organ reached the correct range for calcium phosphate to crystallize, and these conditions lasted up to five days. The team expected to find stark differences between the organs, but instead the entire carcass rotted evenly into a relatively homogeneous soup of decomposition byproducts, retained within by the skin for up to 20 days.
This surprising result led the researchers to consider other factors that could aid fossilization, such as phosphorous levels within an organ’s tissues. “Muscles are full of phosphate,” says Clements. “If you already have the phosphate in there, then there is already a high probability that [the organ] it will be replaced by calcium phosphate.”
“It would be interesting to do this in [nonfish organisms] too,” says paleontologist Victoria McCoy of the University of Wisconsin-Milwaukee, who was not involved in the study. She suggests that future work could monitor other aspects of the environments inside decaying organs, such as concentrations of various elements. The researchers could also investigate whether the physical structures of the tissues influence the formation of minerals. “In many ways, it raises more questions than there would have been if they had found organ-specific pH gradients,” says McCoy. “But that’s what makes it so cool.”