Yunnanozoon lividum (from Yunnan, and Ancient Greek ζῷον (zôion), meaning "animal", with specific name coming from Latin lividum; (lead-coloured), referring to preserved colour of specimens
Taxonomy
Yunnanozoon was described by Hou, Ramskold & Bergstrom in 1991, who considered its placement to be incertae sedis within Metazoa. The placement of Yunnanozoon has been controversial, with various studies suggesting placements as a cephalochordate, a stem-chordate, a hemichordate, a stem-vertebrate, a stem-deuterostome, an ambulacrarian, a bilaterian of uncertain placement, as a protostome of uncertain position, or a relative of Ecdysozoa. In 1999, another genus of yunnanozoan, Haikouella described from the same deposits as Yunnanozoon, with the species Haikouella lanceolata. A second species of the genus Haikouella jianshanensis was described in 2003. The describers of Haikouella distinguished it from Yunnanozoon based on the number of filamentous arches and circular structures, and the placement of structure proposed to be pharyngeal teeth. However a comprehensive study of yunnanozoans in 2015 found that the number of filamentous arches and circular structures were the same in all yunnanozoans examined and that the supposed "pharyngeal teeth" were instead remnants of other structures, suggesting that there was only one species, Yunnanozoon lividum, with both Haikouella species being junior synonyms of it.
A 2024 study motivated by a reinterpretation of the fossil Pikaia placed Yunnanozoon as a transitional stem-chordate form between a paraphyletic Vetulicolia and the more crownward Chordata as shown on this simplified cladogram:
Ecology
Yunannozoon is suggested to have used its filamentous arches to feed with the openings on the side of the body being used to expel water.