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Yunnanozoon

Not to be confused with Yuyuanozoon.

Yunnanozoon
Temporal range: Terreneuvian, 525 Ma
↓
Fossil of Yunnanozoon lividum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Family: †Yunnanozoonidae
Dzik, 1995
Genus: †Yunnanozoon
Hou, Ramskold & Bergstrom, 1991
Species:
†Y. lividum
Binomial name
†Yunnanozoon lividum
Hou, Ramskold & Bergstrom, 1991
Synonyms
  • Haikouella Chen, Huang & Li, 1999

Yunnanozoon lividum (Yunnan + Greek ζῷον zôion (animal), with species name Latin lividum; (lead-coloured), referring to preserved colour of specimens[1]) 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

Well preserved specimen and diagram of body anatomy

The body of Yunnanozoon was fusiform, with specimens ranging from 2 to 6 centimetres (0.79 to 2.36 in) 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.[2]

Taxonomy

Yunnanozoon was described by Hou, Ramskold & Bergstrom in 1991, who considered its placement to be incertae sedis within Metazoa.[1] 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.[2]

In 1995, Jerzy Dzik placed Yunannozoon into its own class, Yunnanozoa.[3] In 1999, another genus of yunnanozoan, Haikouella described from the same deposits as Yannonzoon, with the species Haikouella lanceolata.[4] A second species of the genus Haikouella jianshanensis was described in 2003.[5] 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.[2]

The same comprehensive 2015 study considered Yunnanozoon to be a member of Bilateria of uncertain placement.[2] 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.[6]

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:[7][8]

Chordata
(total group)
"Vetulicolia"

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.[2]

See also

References

  1. ^ a b Xianguang, Hou; RamsköLd, Lars; BergströM, Jan (October 1991). "Composition and preservation of the Chengjiang fauna –a Lower Cambrian soft‐bodied biota". Zoologica Scripta. 20 (4): 395–411. doi:10.1111/j.1463-6409.1991.tb00303.x. ISSN 0300-3256.
  2. ^ a b c d e Cong, Pei-Yun; Hou, Xian-Guang; Aldridge, Richard J.; Purnell, Mark A.; Li, Yi-Zhen (2015). Smith, Andrew (ed.). "New data on the palaeobiology of the enigmatic yunnanozoans from the Chengjiang Biota, Lower Cambrian, China". Palaeontology. 58 (1): 45–70. doi:10.1111/pala.12117. S2CID 84567733.
  3. ^ "Yunnantozoon and the ancestry of chordates - Acta Palaeontologica Polonica". www.app.pan.pl. Retrieved 11 February 2024.
  4. ^ Chen, Jun-Yuan; Huang, Di-Ying; Li, Chia-Wei (December 1999). "An early Cambrian craniate-like chordate". Nature. 402 (6761): 518–522. doi:10.1038/990080. ISSN 0028-0836.
  5. ^ Shu, Degan; Morris, Simon Conway; Zhang, Z. F.; Liu, J. N.; Han, Jian; Chen, Ling; Zhang, X. L.; Yasui, K.; Li, Yong (28 February 2003). "A New Species of Yunnanozoan with Implications for Deuterostome Evolution". Science. 299 (5611): 1380–1384. doi:10.1126/science.1079846. ISSN 0036-8075.
  6. ^ Tian, Qingyi; Zhao, Fangchen; Zeng, Han; Zhu, Maoyan; Jiang, Baoyu (8 July 2022). "Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in yunnanozoans". Science. 377 (6602): 218–222. Bibcode:2022Sci...377..218T. doi:10.1126/science.abm2708. ISSN 0036-8075. PMID 35857544. S2CID 250380981.
  7. ^ Mussini, G.; Smith, M. P.; Vinther, J.; Rahman, I. A.; Murdock, D. J. E.; Harper, D. A. T.; Dunn, F. S. (2024). "A new interpretation of Pikaia reveals the origins of the chordate body plan". Current Biology. 34 (13): 2980–2989.e2. doi:10.1016/j.cub.2024.05.026. PMID 38866005. (Note: The cladogram is simplified using the definition of Vetulicolidae from Li et al. 2018, cited separately on this page)
  8. ^ Li, Yujing; Williams, Mark; Gabbott, Sarah E.; Chen, Ailen; Cong, Peiyun; Hou, Xianguang (2018). "The enigmatic metazoan Yuyuanozoon magnificissimi from the early Cambrian Chengjiang Biota, Yunnan Province, South China". Journal of Paleontology. 92 (6): 1081–1091. doi:10.1017/jpa.2018.18.
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