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 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 protostome of uncertain position, or a relative of Ecdysozoa.

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.

The same comprehensive 2015 study considered ''Yunnanozoon'' to be 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.

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.

See also

*'' Metaspriggina''
*''Pikaia''

References

External links

*A picture can be found at https://web.archive.org/web/20030511135309/http://www.gs-rc.org:80/repo/repoe.htm

{{Taxonbar, from=Q690567

Maotianshan shales fossils
Enigmatic prehistoric animal genera
Cambrian chordates
Fossil taxa described in 1995
Prehistoric chordate genera
Chordates
Cambrian genus extinctions