Secondary Phosphatization of the Earliest Cambrian Small Shelly Fossil Anabarites from Southern Shaanxi

Biomineralization may have an extremely long evolutionary history since the Paleoarchean, while the widespread biomineralization among metazoan lineages started at the earliest Cambrian. However, the primary mineralogy of Anabarites shell remains controversial. Optical microscopic observations combined with the Back-Scattered Electron (BSE) and Energy-Dispersive X-ray Spectroscopy (EDS) analyses are used to study the shell of the fossil Anabarites from the Kuanchuanpu fauna in southern Shaanxi Province in China, which is correlated to the Cambrian Fortunian Stage. The EDS analysis shows that the phosphorus-rich layer closely adjacent to the calcified layer exhibits a Ca: P: C ratio compositionally similar to the mineral fluorapatite (Ca₅(PO₄, CO₃)₃(F, CO₃). The result that the calcified layer and the phosphorus-rich layer have different chemical compositions is consistent with the optical observation that there is an obvious gap between these two layers and the phosphorus-rich layer can extend to the phosphatic material inside of the tube, suggesting the phosphorus-rich layer doesn't belong to the original shell. We suggest that the phosphorous-rich layer is diagenetic in origin, precipitated as a result of phosphorus release during the decay of organic matter by microbes. Considering the outermost shell layer (OMS, biologically controlled carbonate shell layer) should display different isotopic information from the carbonate matrix (i.e., OMS is ¹²C concentrated due to the biogenic organic matter template is readily rich in ¹²C), NanoSIMS was used to map ion distributions of C and N in the shell of Anabarites and matrix. However, ion images show that the concentration differences of ¹²C, ¹³C and ²⁶CN among the OMS and the matrix are unclear, while ¹²C and ²⁶CN are supposed to be enriched in the OMS. Therefore, the minor isotopic differences between the shell and the matrix is hard to be detected by NanoSIMS, at least in our sample, probably due to alteration of the ¹²C-rich characteristic of the Anabarites OMS during the late diagenesis.