Apatite in Hamipterus tianshanensis eggshell: advances in understanding the structure of pterosaur eggs by Raman spectroscopy
Pterosaur eggs can offer information about pterosaur reproductive strategies and are extremely precious because only a small number of specimens have been discovered. Previous studies have mainly focused on morphological descriptions of pterosaur eggs and their embryos while the chemical composition of pterosaur eggs has received little attention. The conventional view believed that the eggshell was composed of calcite. However, previous SEM–EDS results for Hamipterus tianshanensis showed that the eggshell contains phosphorus. Therefore, the object of this research is to determine the mineral composition of the eggshell of H. tianshanensis. Two eggs were analyzed by scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM–EDS) and Raman spectroscopy. The SEM–EDS results show that both surface and cross section are porous and characterized by small irregularly shaped particulates. Moreover, the distribution of Ca and P has a strict coincidence in the cross-section of eggshells. Furthermore, neither the intense peaks of calcite nor organic peaks can be observed by Raman spectroscopy in eggshells. Meanwhile, the Raman spectroscopy mapping analysis result shows a sharp and intense peak at approximately 966 cm？1 among the white eggshell, which can be hard evidence that H. tianshanensis eggs are mainly composed of calcium phosphate. Combined with the present of F in the eggshell, it can be inferred that fluorapatite Ca5(PO4)3F is the main mineral. The fluorapatite eggshell can be interpreted in two ways. One explanation is that H. tianshanensis laid apatite-shelled eggs, similar to living Salvator merianae, and the bioapatite transformed to fluorapatite over geological time. Another possible explanation is that the fluorapatite comes from the result of phosphatization of soft egg membrane tissues through taphonomic processes, indicating that H. tianshanensis might have laid soft eggs. Regardless, the results show that fluorapatite, rather than calcite is the main preserved mineral composition of H. tianshanensis eggshell, correcting the previous view. This study contributes to the present understanding of the mineral composition of pterosaur eggshells and may offer some insight into the pterosaur reproduction pattern.