Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossils

White fibers from a Late Cretaceous dinosaur Shuvuuia deserti stained positive for β-keratin antibodies in a 1999 paper, followed by many similar immunological claims for Mesozoic protein in bones and integument. Antibodies recognize protein epitopes derived from its tertiary and quaternary structure, so such results would suggest long polypeptide preservation allowing for sequencing with palaeobiological implications. However, proteins are relatively unstable biomacromolecules that readily hydrolyze and amino acids exhibit predictable instability under diagenetic heat and pressure. Furthermore, antibodies can yield false positives. We reanalyzed a Shuvuuia fiber using focused ion beam scanning electron microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, and laser-stimulated fluorescence imaging, finding it to be inorganic and composed mainly of calcium phosphate. Our findings are inconsistent with any protein or other original organic substance preservation in the Shuvuuia fiber, suggesting that immunohistochemistry may be inappropriate for analyzing fossils due to issues with false positives and a lack of controls.