Diplomystus dentatus, Green River Formation, Wyoming, USA. Credit: Didier Descouens
Australian researchers have discovered why a fossilised fish (Diplomystus dentatus) was unearthed from the ‘Fossil Basin’ region of the US state of Wyoming with remarkably preserved skin and scales.
Complex decomposition and degradation processes usually destroy a organism’s organic matter after it dies. But, in some exceptional cases, conditions allow them to fossilise and persist over geological timeframes.
“Sediments of the Green River Formation (GRF) within Fossil Basin (Wyoming, USA) host some of the most extraordinary fossils and examples of soft tissue preservation in the geologic record,” the researchers write in new study published in Environmental Microbiology.
In life, the fish would have lived in the warmer, fresher upper layer of a highly stratified lake. After its death, the fish would have sank to the lake floor – a microenvironment rich in oxygen.
“We usually think of low-oxygen, or ‘anoxic’, conditions as essential for preserving soft tissues because oxygen promotes decay,” says Dr Amy Elson, lead author of the study from Australia’s Curtin University.
“But this case shows even in oxygen-rich settings, unique chemical conditions can protect delicate tissues for tens of millions of years.
Diplomystus dentatus fish fossil from the Fossil Basin of the Eocene Green River Formation, USA. Credit: Elson et al 2025, Environmental Microbiology
“Our work provides new insights into why some fossils preserve incredible detail while others do not.”
The found that when the fish skin broke down, it released short-chain saturated fatty acids and hydrogen ions which changed the chemistry of the surrounding microenvironment and led to phosphate mineralisation of the scales.
“The dermis layer of the skin anchors the scales, which in modern fish are composed of hydroxyapatite mineral and type I collagen (a protein),” the authors explain.
This hydroxyapatite mineral was changed into fluorapatite (Ca5(PO4)3F) – a phosphate mineral – rather than the usual carbonate precipitation, which would have caused the tissues to decay.
Senior author and University of Western Australia Professor, Kliti Grice, adds: “This discovery broadens our understanding of fossilisation and the chemical conditions that allow biological materials to persist.
“Beyond reconstructing Earth’s evolutionary history, understanding these processes could inspire new ways to preserve biological materials in medicine, guide exploration for energy and mineral resources and improve methods for locking away carbon in sediments to help tackle climate change.
“It shows how looking back deep into Earth’s past can help address challenges we face today and in the future.”