PHYTOPHILASPIS: A BONY ARTHROPOD FROM LOWER CAMBRIAN OF SIBERIA

Biomineralization is one of the important events during Cambrian radiation of animals that belong to modern phyla. Although there are many deposits of exceptional preservation known as Burgess Shale-type deposits worldwide in the Cambrian, most of them are preserved in fine-grained siliciclastics that are not the ideal environments for preserving biomineralized elements. One important occurrence of Cambrian non-trilobite arthropods, the Sinsk biota (lower Sinsk Formation, Botomian) from the Siberian Platform, has been discovered in carbonates. In addition to trilobites with calcified exoskeleton, this deposit also contains one enigmatic arthropod with phosphatic cuticle. One complete specimen was prepared with acid treatment. The results show that the phosphatic portion of the cuticle is fragile but continuous throughout the entire exoskeleton and there is sharp boundary of phosphorus concentration between the specimen and the matrix, whereas the biomineralized exoskeleton of coeval trilobite consists of relatively pure calcium carbonate. Two fragments of this arthropod were cut and the cross-sections were polished to reveal internal structure. One sample had undergone relatively more extensive of weathering as indicated by the frequent occurrences of dolomites and voids in the matrix. The outline of fibrous cuticular structures looks bright under backscattering imaging of scanning electron microscope; however, the fine details inside the phosphatic microfibers are not preserved. In the other sample, there are much less dolomites and voids in the matrix, the cuticle contains really fine zonation of concentric bands within the phosphatic microfibers. These features are considered as genuine cuticular features. At present, there is no exact modern analogue of phosphatic microfibers within arthropod cuticle known for comparison.