Abstract Yunnanozoans are a key group of Cambrian fossils, and many specimens of these blade-shaped, soft-bodied animals show exceptional preservation. Debate continues over whether they were stem vertebrates, non-vertebrate chordates, stem deuterostomes or even protostome invertebrates. Previous studies of the yunnanozoan circulatory system were limited to optical observations and remain controversial. Here, we re-study the relevant soft-tissue structures using a suite of high-resolution three-