Kimberella
| Kimberella | ||||||||||||
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Kimberella fossil |
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| Temporal occurrence | ||||||||||||
| Ediacarium | ||||||||||||
| 558 to 555 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Kimberella | ||||||||||||
| Wade , 1972 | ||||||||||||
Kimberella is an extinct animal genus uncertain systematic position, before about 558 to 555 million years ago in the Ediacaran lived and the Ediacaran biota belongs. So far only the species K. quadrata has been included in the genus.
The first fossils of Kimberella were found in 1959 in the Ediacara Hills of Australia , the eponymous locality of the Ediacarium (635 to 542 mya ). A more precise radiometric dating of the stratum typicum for an absolute temporal classification of Kimberella was not possible. In 1997, however, newer and much better preserved fossils were found in the White Sea region of Russia and in 2007 the strata were radiometric dating from 558 to 555 mya .
Kimberella was originally classified as a medusa stage (jellyfish) of a hollow animal. When the bilateral symmetry and traces of a possible rasp tongue (radula) were discovered in 1997 , it was classified as a basal (phylogenetically original) representative of the molluscs . However, the interpretation of the remains as a rasp tongue and thus the classification at the base of the molluscs has remained controversial. However, the paleontologists still agree on the classification of Kimberella as a member of the Bilateria (two-sided animals).
distribution
Kimberella is only known from two sites: from the Australian Ediacara Hills and from the Ust 'Pinega Formation on the White Sea near the mouth of the Onega .
The Kimberella fossils from the White Sea are often found together with algae and the Ediacara fossils Tribrachidium and Dickinsonia - meandering trace fossils that may have been produced by Kimberella itself. A layer of volcanic ash that lies between layers of Kimberella fossils could be dated radiometrically from the decay of uranium in zircons to an age of 558 to 555.3 million years. There are Kimberella -Fossilien known from other layers, the younger and older than them using the ash layer very precisely dated fossils are. In contrast to the finds from Russia, the fossils from the Ediacara Hills in Australia could not yet be precisely dated.
description
So far, over 1000 specimens of Kimberella from all ages and sizes have been found in the fine-grained sandstone layers of the Ust'-Pinega Formation on the White Sea. The large number of fossils and the good state of preservation in the fine-grained sandstone allowed well-founded conclusions to be drawn about the external morphology, internal anatomy, the musculoskeletal system and the way of life of this genus.
All sizes of Kimberella have an oval to pear-shaped basic shape, with larger individuals being more elongated-oval or elongated-pear-shaped. The smallest fossils found are only two to three millimeters long. The largest individual is incomplete and, when reconstructed, should have reached a length of 13 to 15 cm. In the statistical size distribution, no maxima can be observed that could indicate a gender dimorphism or a genetically fixed adult size. Also, no ontogenetic changes can be observed in the population; the smallest copies are simply small copies of the largest copies. Only the value for the length / width ratio is slightly higher for larger specimens.
The specimens were axially symmetrical (bilateral) and had a single shell plate on the back (dorsal), which extended over the entire length of the body. With larger specimens, this back shell plate could be up to 15 centimeters long, five to seven centimeters wide and three to four centimeters thick. The shell plate was firm, but still flexible and does not appear to have been made of any mineralized material. Since it probably got thicker with age, it also got stiffer and stiffer. At one end of the shell plate this had a hump-shaped structure, which probably marked the front end of the animal. Some specimens show fine, strip-shaped depressions over the entire length of the shell plate, which are vertical to the body axis. They are interpreted as attachment points for the muscles. Similar striped depressions have also been discovered on the ends of some dish plates. This is where the retraction muscles (retractor muscles) for pulling in the foot could have started.
The organism had a raised "ridge" along the body axis and laterally not a smooth, but a slightly wavy surface. Kimberella does not show any visible segmentation, but still a kind of modularization. Each module contained a well-developed muscle ligament that was vertical to the body axis from the highest point (shell plate) to the muscular foot. Likewise, smaller muscles in the foot, which were also vertical to the body axis from the front to the back of the body. The combination of the movement of the two types of muscles enabled Kimberella to crawl by undulating the foot.
The body had a ruffled edge. It is believed that this could have been part of the respiratory system and functioned similarly to today's gills . The fact that this seam was very stretched suggests that it was either a not very efficient respiratory system or that it had some other function.
Way of life
The facies of the Ust 'Pinega Formation and the embedding of the Kimberella fossils from the White Sea area suggest that the animals lived on the seabed . Kimberella lived in shallow to about ten meters deep water, especially in shallow gullies on muddy ground and shared the habitat with photosynthetically active organisms. Kimberella was found along with other representatives of the Ediacaran fauna such as Yorgia , Dickinsonia , Tribrachidium and Charniodiscus . This suggests that these forms lived in the same habitat. Kimberella probably grazed microbacterial mats without specializing in any particular food source. While eating, it probably crawled backwards and grazed the microbe mats flat, as the pasture traces show. How Kimberella reproduced is not known.
The sediment surface on which Kimberella lived has occasionally been spilled by fine-grained sand, e.g. B. by storm events or changes in the current. In response to this stress, the animals withdrew under their shell plate, similar to the reaction of many molluscs . It is unlikely that they were quick enough to escape such an event. Some specimens survived the burial and were able to dig themselves out of the sand like they show signs of escape. But especially younger animals remained trapped in the sediment despite attempts to escape and were fossilized at the end of a tunnel a few centimeters long.
conservation
The preservation of the Kimberella fossils was made possible on the one hand by the rapid sedimentation, which quickly cut off the dead bodies from contact with the sea water and also scavengers, and on the other hand also a rapid solidification (lithification) of the sediments below and above. The lithification of the sediments may have been accelerated by the decomposition products of other organisms. Some scientists even suggest that the animal's slime may also have influenced its own fossilization. Against this is the fact that the slime probably evaporated very quickly and thus could not play a role in the fossilization. The rapid solidification of the sediments meant that the three-dimensional shape of the animal was preserved during fossilization.
Systematics
The first Kimberella fossil was discovered in Australia in 1959 and was first scientifically described and named as Kimberia in 1966 by Martin Glaessner and Mary Wade . The name honors John Kimber, a teacher and collector who was killed on an expedition to central Australia in 1964. Originally the fossil was classified as the medusa stage of a coelenterate . In 1972 Mary Wade renamed the genus Kimberella , as the name Kimberia was already taken. Kimberella has now been interpreted as a box jellyfish (Cubozoa). This systematic classification persisted until the fossils were found in the White Sea region. Mikhail A. Fedonkin and Benjamin M. Wagoner rewrote Kimberella in 1997 on the basis of this extensive material. They found beyond doubt that Kimberella was bilaterally symmetrical. The systematisation as a medusa stage (jellyfish) of a cnidarians or rib jellyfish was thus refuted, since the basic plan of these groups includes a radially symmetrical construction plan. Fedonkin and Wagoner observed a dorsal, non-mineralized, flexible shell plate under which the animal could possibly retreat. The trail that a specimen of Kimberella left on the surface of the sediment shortly before it was buried shows scratches. These scratch marks were interpreted by these authors as grazing marks, which arose when the specimen grazed the microbial lawn on the sediment surface with a rasp tongue (radula). Kimberella is therefore regarded by these authors as a single-shell mollusk or a very closely related animal. However , the typical rasping tongue of a modern mollusc has not yet been detected in the fossils of Kimberella . However, the rasp tongue (radula) of molluscs has a very low fossilization potential due to its exclusively organic structure, and so far only very few fossilized radulae have been found (e.g. ammonites , but so far no snail radulae !). Therefore, this negative finding is not an argument that Kimberella did not have a rasp tongue.
The interpretation of Kimberella as a mollusc has not gone unchallenged. Critics, however, argue that the few available facts about Kimberella do not allow them to be systematically related to molluscs or related animals. The scratch marks could also have been caused by other body structures. One even has to question the classification as a bilateral animal (cf. discussion in Butterfield).
Today it is almost certain that Kimberella , possibly after Vernanimalcula , is the oldest triploblastic bilateral animal ever documented . The interpretation as a mollusc, however, is still fraught with doubts.
Significance for the tribal history of animals
The systematic position of Kimberella is important for understanding Cambrian Radiation . The theory of the Cambrian Explosion assumes that the basic blueprints of today's species were created between 543 million years and 518 million years. The interpretation of kimberella as a bilateral animal would shift the diversification of the animal overgroups to the Precambrian . Should it be at Kimberella actually a mollusk and thus a Urmundtier act, then would have not only the animal over trunks of Protostome (Protostomia) and the new mouths (Deuterostomia) already some time years ago 555 million and thus the beginning of the Cambrian (border Ediacaran / Cambrian: 542 million years) evolved apart, but also the animal phyla of the molluscs and the annelids / arthropods , which are currently interpreted as sister groups. The existence of one animal tribe (molluscs) thus also proves the existence of the other sister group (annelids / arthropods), even if no reliable fossils of this other animal tribe have yet been found ( terminus post quem non ).
Web links
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s t u M. A. Fedonkin, A. Simonetta, AY Ivantsov: New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia) : palaeoecological and evolutionary implications. In: Patricia Vickers-Rich, Patricia Komarower: The Rise and Fall of the Ediacaran Biota. (= Special publications. 286). Geological Society, London 2007, ISBN 978-1-86239-233-5 , pp. 157-179. doi: 10.1144 / SP286.12
- ↑ M. Glaessner, B. Daily: The Geology and Late Precambrian Fauna of the Ediacara Fossil Reserve . (PDF; 69.4 MB). In: Records of the South Australian Museum . 13, 1959, pp. 369-401.
- ↑ MW Martin, DV Grazhdankin, SA Bowring, DAD Evans, MA Fedonkin, JL Kirschvink: Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution. In: Science. Volume 288, No. 5467, May 5, 2000, p. 841. doi: 10.1126 / science.288.5467.841 . PMID 10797002 . Retrieved May 10, 2007.
- ↑ A. Yu. Ivantsov: New reconstruction of Kimberella, problematic Vendian metazoan. In: Paleontological Journal. Volume 43, No. 6, 2009, pp. 601-611. doi: 10.1134 / S003103010906001X .
- ↑ a b c d M. A. Fedonkin, BM Wagoner: The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. In: Nature. Volume 388, No. 6645, 1997, p. 388. doi: 10.1038 / 42242
- ^ PR Getty: Producing And Preserving Climactichnites. Philadelphia Annual Meeting, 2006. (online) , accessed June 2, 2008.
- ^ MF Glaessner, M. Wade: The late Precambrian fossils from Ediacara, South Australia . In: Palaeontology . tape 9 , no. 4 , 1966, pp. 599 ( palass.org [PDF]).
- ^ M. Wade: Hydrozoa and Scyphozoa and other medusoids from the Precambrian Ediacara fauna, South Australia. In: Palaeontology. Issue 15, 1972, pp. 197-225.
- ^ NJ Butterfield: Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale. In: Bioessays. Volume 28, No. 12, 2006, pp. 1161-1166. doi: 10.1002 / bies.20507
- ↑ JY Chen, DJ Bottjer, P. Oliveri, SQ Dornbos, F. Gao, S. Ruffins, H. Chi, CW Li, EH Davidson: Small bilaterian fossils from 40 to 55 million years before the Cambrian. In: Science. 305, 2004, pp. 218-222.
- ^ Douglas H. Erwin, Eric H. Davidson: The last common bilaterian ancestor. In: Development. Edition 129, 2002, pp. 3021-3032. (on-line)
- ^ R. Cowen: History of Life . 3. Edition. Blackwell Science, 2000, ISBN 0-632-04444-6 , pp. 63 .