American Tree Sparrow

by | Dec 20, 2017 | Sparrows

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Myllokunmingia is a genus of basal chordate from the Lower Cambrian Maotianshan shales of China 518 mya and is thought to be a vertebrate,[2] although this is not conclusively proven.[3] The type species M. fengjiaoa is 28 mm long and 6 mm high. It is among the oldest possible craniates, found in the lower Cambrian Chengjiang (518 million years ago). It appears to have a skull and skeletal structures made of cartilage. There is no sign of biomineralization of the skeletal elements. The holotype was found in the Yuanshan member of the Qiongzhusi Formation in the Eoredlichia Zone near Haikou at Ercaicun, Kunming City, Yunnan, China. Some researchers have considered the other primitive chordate Haikouichthys to be synonymous with this taxon,[4] but subsequent studies led by the British paleontologist Simon Conway Morris identified both genera to be distinct, separate taxa on the basis of different gill arrangement,[5] the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.[6]

The animal has a distinct head and trunk with a forward sail-like (1.5 mm) dorsal fin and a ventral finfold (probably paired) further back. The maximum height of M. fengjiaoa is at 6mm. The maximum height point is located around 11mm from the anterior. The head has five or six gill pouches with hemibranchs. In the trunk there are 25 segments (myomeres) with rearward-facing chevrons. There is a notochord, a pharynx and a digestive tract that may run all the way to the rear tip of the animal. The mouth cannot be clearly identified. There may be a pericardial cavity. There are no fin radials on M. fengjiaoa. There is only one specimen, which has the tip of the tail buried in sediment.[2]

See also

References

  1. ^ Yang, C.; Li, X.-H.; Zhu, M.; Condon, D. J.; Chen, J. (2018). "Geochronological constraint on the Cambrian Chengjiang biota, South China" (PDF). Journal of the Geological Society. 175 (4): 659–666. Bibcode:2018JGSoc.175..659Y. doi:10.1144/jgs2017-103. ISSN 0016-7649. S2CID 135091168.
  2. ^ a b Shu, D-G.; Luo, H-L.; Conway Morris, S.; Zhang, X-L.; Hu, S-X.; Chen, L.; Han, J.; Zhu, M.; Li, Y.; Chen, L-Z. (1999). "Lower Cambrian vertebrates from south China". Nature. 402 (6757): 42. Bibcode:1999Natur.402...42S. doi:10.1038/46965. S2CID 4402854.
  3. ^ Donoghue, P.C.J.; Purnell, M.A. (2005). "Genome duplication, extinction and vertebrate evolution" (PDF). Trends in Ecology & Evolution. 20 (6): 312–319. doi:10.1016/j.tree.2005.04.008. PMID 16701387. Archived from the original (PDF) on 2008-12-17. Retrieved 2008-11-06.
  4. ^ Hou, X.-G.; Aldridge, R. J.; Siveter, D. J.; Feng, X.-H. (2002). "New evidence on the anatomy and phylogeny of the earliest vertebrates". Proc Biol Sci. 269 (1503): 1865–1869. doi:10.1098/rspb.2002.2104. PMC 1691108. PMID 12350247.
  5. ^ Conway Morris, S. (29 June 2006). "Darwin's dilemma: the realities of the Cambrian 'explosion'". Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1470): 1069–1083. doi:10.1098/rstb.2006.1846. PMC 1578734. PMID 16754615.
  6. ^ Conway Morris, Simon; Caron, Jean-Bernard (2012). "Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". Biological Reviews. 87 (2): 480–512. doi:10.1111/j.1469-185X.2012.00220.x. PMID 22385518. S2CID 27671780.

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