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-dimensional imaging and chemical analytical techniques. These preserved structures exhibit tubular shapes, three-dimensional nature and iron-rich and proteinaceous chemical signatures, collectively consistent with the dorsal and ventral aortae of vertebrates. Similar, smaller vessels are revealed within four pairs of ‘circular structures’ behind the pharynx. Their vessel-rich nature, anatomical position and uniform spherical morphology favour an interpretation of these circular structures as kidney glomeruli rather than previously suggested gonadal or glandular tissues, although the absence of evidence for a heart in yunnanozoans complicates the physiological inference. This work provides the first micrometre-scale characterization of putative blood vessels in Cambrian animals and offers new insights into the early evolution of vertebrate-type circulatory and urogenital systems.
Tian et al. (Wed,) studied this question.