Reassessing yunnanozoans from the Chengjiang fauna: Insights from ultrastructure and thermal analysis

Yunnanozoans (Yunnanozoon and Haikouella) from the early Cambrian Chengjiang fauna have been classified as stem vertebrates, cephalochordates, hemichordates, stem deuterostomes or even problematic bilaterians. The claimed ultrastructure of their branchial arches comprising cellular cartilage and fibrillin microfibers has led to speculations linking yunnanozoans to vertebrates. Raman spectrum analysis reveals that these yunnanozoans fossils experienced a maximum temperature of approximately 300°C during their geological history. Scanning electron microscopy (SEM) and transmission electron Microscopy (TEM) of branchial arches of yunnanozoans fossils, revealed that the organic matter was preserved as homogeneous membranous organic carbonaceous films, ruling out of potential preservation of cellular cartilage and fibrillin microfibers. The integrated research incorporating ultrastructure and thermal analysis have led us to removes yunnanozoans from vertebrates.