Specifically, three-dimensional reconstructions and illustrations of the anatomy of fossilized bones generated by clinical CT scanners and micro-CT have contributed significantly to this field. In this regard, CT imaging has become valuable in the field of vertebrate paleontology given its ability to measure even very dense objects in a nondestructive manner 13, 14. However, at present, there is no physical method that enables noninvasive chemical characterization of fossil objects. Given the higher chemical stability of fluorapatite as compared to hydroxyapatite, hydroxyl ions in the bone mineral are replaced by fluoride over time 7. Therefore, fluorine must have been delivered by intergranular fluid within the solidifying sediment and was incorporated into bone by a dissolution-crystallisation process. Uptake of fluorine by diffusion into solid media is excluded at the low temperatures of diagenesis based on grain boundary diffusion data 11 and specifically also crystal diffusion data for apatite 12. The uptake of fluorine into bone apatite within the geological environment is interpreted as an exchange mechanism by coupled dissolution and precipitation, thus preserving the orginal shape of the object 9, 10. This process relies on the absorption of fluoride ions by the bone mineral when exposed to groundwater or soil 7, 8. Those variations can be explained, inter alia, by the concept of diagenesis as hydroxyapatite in bones and teeth is replaced by thermodynamically more stable fluorapatite during the process of fossilization 4, 5, 6. Multiple studies have investigated fragmented bone specimens reporting varying elementary compositions of bony tissue of fossil objects 1, 2, 3. These findings highlight the relevance of radiological imaging techniques in the natural sciences by introducing quantitative DECT imaging as a nondestructive approach for material decomposition in fossilized objects, thereby potentially adding to the toolbox of paleontological studies. Moreover, the jaw bone mass of Tyrannosaurus rex showed areas of particularly high fluorine concentrations on DECT, while conventional CT imaging features supported the diagnosis of chronic osteomyelitis. The analysis shows that DECT material maps can differentiate bone from surrounding sediment and reveals fluorine as an imaging marker for fossilized bone and a reliable indicator of the age of terrestrial fossils. Specifically, DECT was developed and validated for imaging-based calcium and fluorine quantification in bones of five fossil vertebrates from different geological time periods and of one extant vertebrate. This study investigates quantitative DECT for the nondestructive density- and element-based material decomposition of fossilized bones. However, its potential to assist investigations in paleontology has not yet been explored. Dual-energy computed tomography (DECT) is an imaging technique that combines nondestructive morphological cross-sectional imaging of objects and the quantification of their chemical composition.
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