Structural, chemical and isotope evidence for secondary phosphate mineralization of grasping spines of Early Palaeozoic chaetognaths

Microscope, cathodoluminescence, chemical and oxygen isotope studies have been conducted to determine the original mineralogy and diagenetic alteration of phosphatic grasping spines of Cambrian chaetognaths. The obtained data, along with a comparison to the composition of conodont apatite, show the presence of a few generations of diagenetic phosphate phases. A thin, outer layer of the spines is composed of very early diagenetic phosphate, which incrusted the original cuticle layer contributing to the preservation of an original shape and structural details of the fossils. This process can be linked to the ‘Orsten’‐type phosphatization mainly known from Lower Palaeozoic strata. Successive phosphate generations are observed within the middle and inner layers as well as internal cavities of the spines. Plastic deformations and diagenetic features of the spines show that they were originally composed of slightly flexible chitinous organic matter similar to that of modern chaetognaths. This conclusion is substantiated by rare findings of purely organic remnants of Cambrian chaetognath grasping spines.