Yunnanozoon
| Yunnanozoon | ||||||||||
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Yunnanozoon lividum |
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| Temporal occurrence | ||||||||||
| Cambrian , 2nd series, 3rd stage | ||||||||||
| 521 to 514 million years | ||||||||||
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| Scientific name | ||||||||||
| Yunnanozoon | ||||||||||
| Hou , Ramsköld & Bergström , 1991 | ||||||||||
| Art | ||||||||||
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Yunnanozoon (“animal from Yunnan” from Greek ζῷον [zóon], “animal”, “living being”) is a genus of extinct worm-like tissue animals (Eumetazoa) from the early Cambrian . The genus belongs to the Chengjiang Fauna Community , a fossil deposit in Yunnan Province, China, and is at least 514 million years old. The fossil has attracted a great deal of scientific attention, as many scientistshave interpreted itin connection with the origin of the new mouths (deuterostomia), the group to which vertebrates, including humans, belong. Although numerous interpretations of the systematic position of Yunnanozoon have been proposed and discussed, its exact affiliation within the Bilateria is currently (as of 2016) still unclear. The only scientifically described species is Y. lividum .
description
Yunnanozoon fossils reach about 25 to 40 millimeters in length. They are preserved as a thin, carbon- rich and dark-colored impression on light limestone slabs . Unlike many other Chengjiang fossils, Yunnanozoon fossils are not mineralized by iron minerals ( pyrite or iron oxides). The fossil conservation was described as a similar to the Burgess schist , it is a form of the conservation deposit that was essentially restricted to the early Cambrian . Some Yunnanozoon fossils are in excellent condition and show the finest anatomical details. Almost all fossils are preserved in the lateral position, this is taken as a strong indication that the animal was laterally flattened in life, i.e. much higher than it was wide. This points to a freely swimming (pelagic) than to a crawling (benthalen) organism .
The body of Yunnanozoon consists of two units lying one above the other. Above (dorsal) a body unit that consists of repetitive units that are uniformly arranged one behind the other, including an elongated body that appears disjointed, but in many individuals has stripes along the longitudinal axis of the body that roughly corresponds to the dorsal units. Both units are often embedded separately from one another. This suggests that either they were connected to one another in a relatively flexible manner in the living animal, so that they could be separated from one another during partial decomposition before final embedding, or that the embedding conditions were more favorable for the preservation of one or the other structure.
The articulated dorsal units appear predominantly rectangular in the fossil, with the exception of the first, which shows an elongated triangular outline, and the last, which is conical and ends in a short, pointed tail. Their exact number is only clearly recognizable from a few fossils, so that their possible variability in the living animals cannot be specified, about 22 to 24 of the rectangular units are given. The serial units have already been interpreted very differently, including as myomers of pronounced muscles. Today it seems most likely that these are mutually movable plates of a protective, sclerotized dorsal shield, possibly due to a reinforced cuticle .
The most striking feature of the rest of the body near the front end is a series of darker (ie probably more stable or more sclerotized) paired arch-like structures that appear to be covered by finer processes (filaments). Fossils with a different number of such arches have been described, after which u. a. two genera were also characterized. According to more recent results, this is probably due to conservation, the number in the living animal could always have been seven pairs. The structures have aroused the scientific imagination of numerous paleontologists , as they have a striking resemblance to the gill arches of vertebrates, which would make an interpretation of the fossil as the common ancestor of all vertebrates seem possible. The homology (related similarity) of the structures with gills is neither proven nor refuted. Functionally, the arches are interpreted either as gills or, more likely, as filtering devices of an animal with a filtering diet. In the case of a few particularly well-preserved fossils, they are surrounded by a fine, veil-like structure, which indicates a bag-like covering on the living animal; accordingly they would not have been exposed to the living animal, but would have been enclosed in a hose-like bag. At the level of each arch it probably had a round opening to the outside. The front end of the animal hardly shows any special structures. Structures that were previously interpreted as eyes or head attachments are now considered to be artifacts due to preservation .
In the center of the body, directly below the articulated dorsal units, the fossils have a band-shaped zone that often appears articulated by fine transverse stripes similar to the dorsal units. Some editors saw a chorda dorsalis in this structure , which would make Yunnanozoon an original representative of the chordates. Today it seems more likely that it could be the imprint of a fluid-filled body cavity (a coelom or pseudocoelom), i.e. an element of a hydroskeleton . Underneath (ventrad) behind the arched structures in many fossils is an elongated, sometimes somewhat tortuous structure, presumably an intestinal tube . Four paired dark circular imprints can often be seen directly behind the arches, which are mostly interpreted as gonads (gonads).
Systematics
Traditionally, a distinction was made between two genera, Yunnanozoon with the species Yunnanozoon lividum Hou, Ramsköld & Bergström, 1981, and Haikouella with the two species Haikouella lanceolata and Haikouella jianshanensis . According to recent studies, these are synonyms of Yunnanozoon lividum , i.e. H. there is only one species. Yunnanozoon is morphologically completely isolated and has no close relatives. Although an assignment to a separate ( monotypical ) family, order and class has been formally proposed, these ranks, since they only include one species, are currently of no cognitive value.
The relationship of Yunnanozoon is still a mystery today. Various authors have suggested the fossil to vertebrates or acrania to you, because myomeres and a notochord were present. Others see them in the trunk group of the chordates , the gill loach (Hemichordata), the trunk group of the ambulacraria , among other things due to external gills, or that of the deuterostomia as a whole. They have also been compared with the equally enigmatic Vetulicolia . Others even see them completely outside the new mouths and assign them (due to the sclerotized body shell) to the Ecdysozoa or the trunk group of the entire Bilateria. Relationships have also been established with other Chenjiang fossils, particularly of the genus Myllokunmingia . The combination of features of the fossil partially supports each of these assignments, while other features are unknown to other representatives of the respective group. Although most authors emphasize the importance of the fossil and a position as the original deuterostomian does not seem unlikely, a reliable assignment is not possible to this day.
Finds
Yunnanozoon is known only from the Chengjiang Fauna Community and has not been found anywhere else. Well-known sites are Maotianshan, Xiaolantian and Ma'anshan near Chenjiang and Ercaicun and the area around Haikou near Kunming. Within this it is considered common. Many hundreds of individuals are documented in the collections , often gregarious with assemblies of up to a few dozen on the same slab of stone.
swell
- Pei-Yun Cong, Xian-Guang Hou, Richard J. Aldridge, Mark A. Purnell, Yi-Zhen Li (2014): 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 .
- Genus Yunnanozoon. In: Xian-guag Hou, Richard Aldridge, Jan Bergström, David J. Siveter, Derek Siveter, Xiang-Hong Feng: The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life. Wiley, 2008. ISBN 978-04-7099994-3 (on pages 212-213).
Individual evidence
- ↑ Yunnanozoon. In: Lexicon of Biology . Spectrum Academic Publishing House. Heidelberg, 1999, accessed October 6, 2016 .
- ↑ Pei-Yun Cong, Xian-Guang Hou, Richard J. Aldridge, Mark A. Purnell, Yi-Zhen Li (2014): 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
- ^ Robert R. Gaines (2015): Burgess Shale-type Preservation and its Distribution in Space and Time. In: Marc Laflamme, James D. Schiffbauer, and Simon AF Darroch (editors): Reading and Writing of the Fossil Record: Preservational Pathways to Exceptional Fossilization. Paleontological Society Papers 20: 124-146.