Why some animals become fossils while others vanish without a trace

For centuries, we have marveled at the prints left by animals who existed thousands of years ago. But one enduring mystery remains: why do some animals leave behind beautifully preserved remains, while others vanish without a trace?

Scientists at the University of Lausanne (UNIL) have taken on this question head-on. Their groundbreaking research offers new insights into why certain creatures become fossils, and others simply disappear.

Unlocking the secrets of fossilization
At the outset, the scientists explain that the fossils are not limited to bones. In some extraordinary cases, they can even preserve muscles, skin, and even traces of internal organs. But this process is not guaranteed, and the event is rather quite selective.

Farid Saleh, a Swiss National Science Foundation Ambizione Fellow at UNIL, and his team discovered that the internal chemistry of a carcass plays a pivotal role in whether it fossilizes. During decomposition , the remains of some animals can cause sharp drops in oxygen levels in the surrounding sediment.
“This means that, in nature, two animals buried side by side could have vastly different fates as fossils, simply because of differences in size or body chemistry,” explained lead researcher Nora Corthésy.



Chemistry of decay
The research team observed that proteins tend to break down faster than lipids, which in turn accelerates oxygen consumption near the decaying body. This local depletion of oxygen can limit further breakdown and encourage the growth of bacteria that don’t rely on oxygen.

This shift sets up the perfect conditions for fossilization. As these bacteria work, the chemical environment around the body changes, allowing durable minerals, like pyrite or calcium phosphate, to replace soft tissues in fine detail.

“A large, protein-rich animal can slip into this state more easily, leading to a better chance of fossilization,” the scientists noted.

In contrast, smaller animals or those with less protein may not trigger such changes in the surrounding environment and are more likely to decay completely.

The bias of the fossil record
This helps explain why many fossil beds feature certain animals, like arthropods, while others, such as soft-bodied worms, are absent. The absence may not reflect extinction or rarity, but simply a failure to fossilize.

Implications for paleontology
“Understanding how size and body chemistry influence fossilization changes how paleontologists interpret ancient ecosystems,” said Corthésy. “It helps separate the absence of fossils due to true extinction from those missing due to poor preservation.”


The creatures we'll never know
The sobering reality is that many animals may have lived and died without leaving any physical trace. Their bodies decomposed so quickly, or failed to create the chemical conditions needed for preservation, that they were wiped clean from the geological record.

In this sense, the fossil record is less of a comprehensive archive and more of a highly selective snapshot. “Animals with soft bodies and low protein content, such as some early worms, may have existed in abundance, but we’ll likely never find their remains,” the researchers explained.