Yunnanozoon

Yunnanozoon lividum (Yunnan + Greek ζῷον zôion (animal), with species name Latin lividum; (lead-coloured), referring to preserved colour of specimens ) is an extinct species of bilaterian animal from the Lower Cambrian Chengjiang biota of Yunnan province, China. Its affinities have been long the subject of controversy.

Description
The body of Yunnanozoon was fusiform, with specimens ranging from 2 to 6 cm in length. The body was strongly laterally compressed, meaning that it was taller than it was wide. A segmented dorsal unit was present on the top of the body. The first segment of which was triangular, while the other segments were approximately rectangular. Axial stripes also ran down the body in the region immediately below the dorsal unit. Towards the front of the animal were 7 pairs of filamentous arches. These arches were covered by sac-like structures which had openings between them. Towards the back of the body a tube-like structure was present, possibly the gut, with four pairs of enigmatic circular structures (possibly gonads) were present at the mid-point of the body. The front half of the body had rod-like structures running along the top and bottom, which were probably sclerotized.

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 protosome of uncertain position, or a relative of Ecdysozoa. A 2015 study which comprehensively described Yunnanozoon considered it a member of Bilateria of uncertain placement. A 2022 study of the filamentous arches using advanced imaging techniques suggested that they were "composed of cartilage within an extracellular matrix of micofibrils", suggesting that yunannozoans were stem-vertebrates.

In 1995, Jerzy Dzik placed Yunannozoon into its own class, Yunnanozoa. In 1999, another genus of yunnanozoan, Haikouella described from the same deposits as Yannonzoon, 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.

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.