Relics of internal organs identified as kidney, liver and heart, and also eyes of acanthodian and actinopterygian fishes, have been found in specimens from the Middle Devonian nodule bed localities of Tynet Burn, Gamrie (Den of Findon), Lethen Bar and ?Clune in NE Scotland.
The organs are represented by dark stains positioned where the organs were situated in life. This phenomenon has been observed in the acanthodian genera Diplacanthus, Cheiracanthus, Mesacanthus and Rhadinacanthus from Tynet and Gamrie, and in the actinopterygian Cheirolepis from Lethen Bar and ?Clune. It is likely that the unique diagenetic conditions at these sites allow a visible expression of these structures to be preserved. The fossil bone of the fish in the Tynet nodules is stained by iron oxide deposited by the action of chemotrophic bacteria during early burial. Concentration of iron deposition at these haemoglobin-rich organ sites may provide the visible contrast that enables traces of these organs to be seen.
The mode of preservation of heart or liver traces in the Lethen Bar and ?Clune material appears to be very similar to that in the Tynet Burn material. The Den of Findon material has undergone considerable weathering in situ, rendering the bone and scales very pale brown, perhaps further enhancing the contrast of these dark organic traces. At most other nodule localities, the bones and scales are preserved black in a dark grey matrix and any remaining dark stains would be masked.