Crystal overgrowth on fossil remains is well-documented in the literature. Attention has specifically focused on bioapatite (i.e., an apatite of biochemical origin regardless of post-mortem changes) configurations, in order to decipher any possible relation to fossilization/diagenesis. This study investigates the Rare Earth Element (REE) and other High-Field-Strength Element (HFSE) composition of euhedral crystals formed on the surface of conodont elements compared with that of crystal-free surfaces. Euhedral crystals are by definition crystals characterized by sharp faces, developing solids that, for apatite, assume the form of hexagonal prisms, reflecting its crystal symmetry. Late Ordovician (Amorphognathus ordovicicus Zone) conodonts from two localities in Sardinia and the Carnic Alps (Italy) are herein investigated. Conodont elements reveal the occurrence of smooth surfaces and surfaces partially covered with euhedral crystals. Since euhedral crystals did not reasonably grow during the organism's lifetime, the REE and HFSE analysis can provide important insights into the crystal growth process. The experimental results indicated a substantial contribution of diagenetic imprinting for all the analyzed material, although more evident on euhedral crystals that are significantly enriched in middle and, subordinately, in heavy REE with respect to smooth surfaces. The positive correlations between La + Th vs log[ΣREE] and Ce + Th vs log[ΣREE] could support the hypothesis that the neoformed euhedral crystals grew also by depleting the pristine bioapatite of the conodont elements. Nevertheless, the occurrence of two types of apatite cannot be ruled out: euhedral crystals as neoformed products of diagenetic processes and smooth surfaces as remains of the pristine conodont bioapatite after diagenesis.

化石残骸上晶体的过度生长在文献中有详细记载。人们的注意力特别集中在生物磷灰石（即，无论死后变化如何，生化来源的磷灰石）构造，以便破译与化石化/成岩作用的任何可能的关系。本研究研究了牙形刺元素表面形成的自形晶体的稀土元素 (REE) 和其他高场强元素 (HFSE) 成分，与无晶体表面的成分进行比较。根据定义，自形晶体是一种具有尖锐表面的晶体，形成固体，对于磷灰石来说，呈现六棱柱的形式，反映了其晶体对称性。本文研究了来自撒丁岛和卡尼克阿尔卑斯山（意大利）两个地点的晚奥陶世（Amorphognathus ordovicicus带）牙形刺。牙形刺元素揭示了光滑表面和部分被自形晶体覆盖的表面的出现。由于自形晶体在生物体的一生中不会合理生长，因此 REE 和 HFSE 分析可以为晶体生长过程提供重要的见解。实验结果表明，成岩印记对所有分析的物质都有重大贡献，尽管在光滑表面的中稀土元素显着富集的自形晶体上更为明显，其次是重稀土元素。La + Th与log[ΣREE] 和 Ce + Th与log[ΣREE] 之间的正相关性可以支持新形成的自形晶体也是通过消耗牙形刺元素的原始生物磷灰石而生长的假设。然而，不能排除两种类型磷灰石的出现：作为成岩过程新形成产物的自形晶体和作为成岩作用后原始牙形刺生物磷灰石残留物的光滑表面。