<p>This study investigates the atypical formation of carbonaceous compressions in fossil echinoderms. The analyses focus on Devonian encrinasterids from the Ponta Grossa Lagerstätte and other fossil ophiuroids from laboratory-based diagenetic experiments. The results indicate that the kerogenization potential of organic remains is mainly limited by the skeletal structure and the type of burial sediment. Less protected viscera, such as those beneath the thin disk cover of ophiuroids, are more prone to compression and polymerization than other labile tissues and organs housed within robust calcareous modules. The antibacterial properties of purer kaolinitic clays may also contribute to this process, leading to thicker and more defined carbonaceous compressions compared to those formed in microbially complex mixed clays. Despite chemical and structural alterations during diagenesis, carbonaceous remains assist in mapping the anatomy of extinct groups, such as the disc-restricted viscera in the encrinasterids analyzed. Raman spectra of carbonaceous compressions in these fossils also proved excellent geothermometers, showing that the Ponta Grossa Lagerstätte reached a peak temperature of about 170&#xa0;°C.</p> Graphical abstract <p></p>