First Steroid Biomarkers in a Pterosaur Fossil Overturn a Century of Fossilization Theory

For more than a century, paleontologists assumed that oxygen and the microbes that use it are the sworn enemies of soft-tissue preservation. The textbook logic was simple: microbes consume organic matter, and exceptional fossils require anoxic, microbe-suppressing conditions. A groundbreaking study published in June 2026 in iScience shatters that paradigm by revealing that microbial oxidation was not just compatible with molecular preservation in a 113-million-year-old pterosaur wing—it was the reason the fossil survived at all .

A Wing Bone That Refuses to Decay

The fossil, a hollow wing phalanx (finger bone) from the Romualdo Formation in the Araripe Basin of northeastern Brazil, was preserved inside a carbonate concretion . Lead author Kliti Grice of Curtin University and an international team deployed geochemical and microscopic analysis to unlock its secrets .

Diet: What the Pterosaur Ate, Read Directly from Its Molecules

For the first time, researchers detected sterane biomarkers—cholesterol derivatives—in any pterosaur fossil . Carbon isotope analysis of these compounds points to a diet of fish or squid-like marine animals, consistent with the animal's tooth and skull morphology . The molecular evidence offers a direct chemical window into the pterosaur's trophic ecology that bone shape alone cannot provide.

The Geological Vault: How Multi-Staged Mineralization Preserved the Bone in 3D

The team documented a sequence of mineral barriers that acted as a natural "geological vault" . First, fluorapatite (a calcium phosphate) formed rapidly within and around the bone, stabilizing fine structural features. Next, successive layers of calcite gradually filled the bone cavity. Critically, the calcite is depleted in carbon-13, indicating it originated from the decay of the pterosaur's own fatty tissues and lipids . The multi-layered mineral coat shielded organic compounds—including steroid biomarkers and microscopic structures resembling collagen fibers—from chemical degradation over 113 million years .

Microbes as Both Destroyers and Preservers

The study documents a complex, multi-staged mineralization process driven by local redox (oxidation-reduction) shifts during early diagenesis . Sulfur-oxidizing bacteria (SOB)—identified by the minerals barite and celestite they left behind—were key players . These microbes broke down soft tissues and fats, releasing carbon that fed calcite precipitation. At the same time, their activity created the chemical conditions that sealed the bone in protective minerals before delicate structures could be lost .

Why This Changes the Rules of Fossilization

Conventional thinking held that oxygen and microbial oxidation are destructive—that decay microbes consume and erase soft tissues and biomolecules, and that exceptional preservation requires anoxic conditions to suppress microbial activity. This study overturns that assumption in two ways :

• Microbial oxidation was essential, not detrimental. Sulfur-oxidizing and other decay microbes did not simply destroy the tissue; they actively created the geochemical gradients that drove fluorapatite and calcite precipitation, which physically preserved the bone in three dimensions .
• Oxygen-microbe interactions enabled biomolecule survival. The same oxidative processes that broke down fatty tissue released carbon that fed calcite formation, while the rapidly forming mineral rind trapped and shielded steroid biomarkers and collagen-like structures from further degradation .

As Grice stated, "Rather than being destroyed by oxygen, some fossils are preserved because of it, through oxidative processes carried out by ancient microbiomes" . The team proposes this as a new global Lagerstätten mechanism—a common pathway for exceptional fossil preservation that is now being identified at other fossil sites .

The Bigger Picture

The Curtin-led study is the first to recover steroid biomarkers from a pterosaur, revealing a fish/squid diet. It demonstrates that multi-staged mineralization, driven by sulfur-metabolizing microbes and local redox shifts, was the key to the wing's 3D preservation. And it fundamentally reframes the role of microbial oxidation from a purely destructive force to a necessary, constructive step in certain types of exceptional fossil preservation .