Yorgia

Taxobox
name = "Yorgia"
fossil_range = Ediacaran

---

regnum = Animalia
phylum = Proarticulata
classis = Cephalozoa
familia = Yorgiidae
genus = "Yorgia"
species = "Y. waggoneri"
binomial = "Yorgia waggoneri"
binomial_authority = Ivantsov 1999

"Yorgia waggoneri" Ivantsov, 1999cite journal
author = Ivantsov, A.Y.
year = 1999
title = A New Dickinsonid from the Upper Vendian of the White Sea Winter Coast (Russia, Arkhangelsk Region)
journal = Paleonological Journal
volume = 33
issue = 3
pages = 233-241] is a member of the Ediacara biota, and resembles the organisms "Dickinsonia" and "Spriggina". It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of 25 cm. It is classified into an extinct animal phylum, the Protarticulata, a phylum roughly as unrelated to extant (non-sponge) animals as the sponges.cite journal
author = Buss, L.W. and Seilacher, A.
year = 1994
title = The Phylum Vendobionta: A Sister Group of the Eumetazoa?
journal = Paleobiology
volume = 20
issue = 1
pages = 1–4
issn = 0094-8373
url = http://links.jstor.org/sici?sici=0094-8373%28199424%2920%3A1%3C1%3ATPVASG%3E2.0.CO%3B2-F
accessdate = 2007-06-21 ]

Morphology

The body plan of the "Yorgia" and other proarticulates is unusual for solitary (non-colonial) metazoans. These bilateral organisms have a segmented metameric body, but left and right segments (isomers) are organized in an alternating pattern relatively to the axis of the body – they are not direct mirror images. This phenomenon is described as the symmetry of gliding reflection. Some proarticulates demonstrate obvious asymmetry of left and right parts of the body. "Yorgia’s" initial right isomer is the only one which spreads far towards the left side of the body.cite journal
author = Ivantsov, A.Y.
year = 2001
title = "Vendia" and Other Precambrian "Arthropods"
journal = Paleonological Journal
volume = 35
issue = 4
pages = 335-343
url = http://www.vend.paleo.ru/pub/Ivantsov_2001.pdf ] Ivantsov, A.Y. (2004) [http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/ivantsov.pdf "Vendian Animals in the Phylum Proarticulata"] . The Rise and Fall of the Vendian Biota. IGSP Project 493. Abstracts. Prato, Italy, p. 52.] This lack of true bilateral symmetry, along with other considerations, has led some scientists to suspect that the organism falls in a sister group to the eumetazoa (i.e. all animals except the sponges and a few other oddballs).

Fossil record

Imprints of the "Yorgia waggoneri" has been found in the rocks of Vendian period (Ediacaran) White sea region of Russia, dated around 555.5 Ma. and "Yorgia" sp. has been found in the Central Uralscite journal
author = Grazhdankin D. V.
coauthors = Maslov A. V., Mustill T. M. R.& Krupenin M. T.
year = 2005
title = The Ediacaran White Sea Biota in the Central Urals
journal = Doklady Earth Sciences
volume = 401
issue = 6
pages = 784–788] of Russia and Flinders Ranges, Australia.cite journal
author = Droser, M.
coauthors = Gehling, J. & Jensen, S.
year = 2006
title = Assemblage palaeoecology of the Ediacara biota: The unabridged edition?
journal = Palaeogeography, Palaeoclimatology, Palaeoecology
volume = 232
pages = 131-147
url = http://www.earthscience.ucr.edu/people/droser/Droser_et_2006.pdf ] Most body imprints of "Yorgia" have in the past been primarily preserved on the sole of sandstone beds in negative relief. Other "Yorgia" fossils show internal structure in the original organism, showing two symmetrical rows of nodules, a central tube, rib like tubes, and a semicircular shape with a hole in the circle centre positioned towards the head end. This structure has been interpreted as the impression of gonads, intestine and mouth.Verify source|date=July 2008

Trace fossils

Some fossils appear as chains of positive imprints, terminated by the negative imprint of the animal. Such positive imprints are confined to the "elephant skin" surface texture that is interpreted as the remains of a microbial film. They have been interpreted as the feeding tracks produced as "Yorgia" fed on the surface of the microbial mat that lined the sea floor. Grazing of that bacterial film could have been accomplished by the work of numerous hair-like organs, cilia, located on the ventral side of the body, which directed micro-organisms to the central groove of "Yorgia". A chevron-like fabric on the positive imprints has been taken as evidence of this activity.cite journal
author = Ivantsov, A.Y.
coauthors = Malakhovskaya, Y.E.
year = 2002
title = Giant Traces of Vendian Animals
journal = Doklady Earth Sciences
volume = 385
issue = 6
pages = 618–622
url = http://vend.paleo.ru/pub/Ivantsov_et_Malakhovskaya_2002-e.pdf ] Ivantsov, A.Y. (2004) [http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/ivantsov.pdf "Vendian Animals in the Phillum Proarticulata"] . The Rise and Fall of the Vendian Biota. IGSP Project 493. Abstracts. Prato, Italy, p. 52.] This feeding habit is unknown in post-Cambrian deposits, but the late Cambrian ichnogenus "Climactichnites] " appears to reflect similar behaviour.

Taphonomic details revealed in "Yorgia" due to its large size allow interpretation of the chains of positive imprints of other proarticulates as grazing traces, as opposed to trails created as organisms were swept along the sea floor by currents. In addition to "Yorgia", two fossil taxa, "Epibaion" and "Phyllozoon", seem to have produced similar grazing traces. Small groups of positive body imprints are documented for "Dickinsonia costata" as well and "Dickinsonia cf. tenuis".cite journal
author = Ivantsov, A.Y.
coauthors = Malakhovskaya, Y.E.
year = 2002
title = Giant Traces of Vendian Animals
journal = Doklady Earth Sciences
volume = 385
issue = 6
pages = 618–622
url = http://vend.paleo.ru/pub/Ivantsov_et_Malakhovskaya_2002-e.pdf ] cite journal
author = Retallack, G.J.
year = 2007
title = Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil
journal = Alcheringa: An Australasian Journal of Palaeontology
volume = 31
issue = 3
pages = 215–240
url = http://www.informaworld.com/index/781217204.pdf ]

References

* cite journal
author = Dzik, Jerzy
year = 2003
title = Anatomical Information Content in the Ediacaran Fossils and Their Possible Zoological Affinities
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 114–126
url = http://icb.oxfordjournals.org/cgi/reprint/43/1/114.pdf
doi = 10.1093/icb/43.1.114
* Fedonkin, M. A. (2003) " [http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf The origin of the Metazoa in the light of the Proterozoic fossil record] ". Paleontological Research, vol. 7, no. 1, pp. 9-41.,