Tribrachidium
| Tribrachidium | ||||||||
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Petrification of Tribrachidium heraldicum |
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| Temporal occurrence | ||||||||
| Ediacarium | ||||||||
| 558 to 555 million years | ||||||||
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| Systematics | ||||||||
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| Scientific name | ||||||||
| Tribrachidium | ||||||||
| Glaessner , 1959 | ||||||||
| species | ||||||||
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Tribrachidum heraldicum is a three-ray (triradial) symmetrical fossil of the late Ediacariums . It is the best-known representatives of the extinct tribe of trilobozoa .
etymology
The term Tribrachidium ("little three-armed") is a word created from the ancient Greek τρία (tria) = "three", βραχίων (brachion) = "arm, forearm" and the Latin diminutive suffix –idium . The species name heraldicum refers to the similarity of the fossil with the well-known heraldic symbol of the triskele , as it is used for example in the coat of arms of the Isle of Man .
Discovery and first description
Tribrachidium was discovered in the Ediacara Member des Rawnsley Quartzite in the Flinders Range in South Australia and was first described in 1966 by the Australian geologist and paleontologist Martin Glaessner .
Occurrence
Other occurrences of Tribrachidium are the Mogilev Formation on the Dniester in Podolia ( Ukraine ) as well as the Verkhovka Formation , the Zimnegory Formation and the Yorga Formation on the White Sea in the Russian Oblast of Arkhangelsk .
description
Tribrachidium heraldicum is usually obtained as a negative impression (cast) on the underside of sandstone banks, but is occasionally also found as a positive (model). In addition, reverse embedding took place by overturning the organism, which is very rarely the case with other fossils of the Ediacara biota . Sideways displacements are also known to produce fossils with characteristic, concentric rings.
The circular or nearly circular fossil, which can vary in diameter from 3 to 50 millimeters (usually 20 to 30 millimeters), has a trilobal shape with straight or clover-like edges. The central part has three backs or interlocked, spiral-shaped "arms" turned back up to 90 ° clockwise. The flat discs are rarely more than 2 millimeters thick. The surface of the fossil is traversed by numerous radial furrows, which in larger individuals cover the entire surface, but in smaller individuals only occur from the bend back of the arms to the edge. Larger individuals (diameter> 15 millimeters) have a border reinforced with tentacles, which does not occur in smaller, linearly limited forms. Each arm bend has a convex elevation (bulla), which Jenkins (1992) interpreted as a gonad or reproductive organ.
Reconstruction and family status
Tribrachidium was originally described by Martin Glaessner as a problematic taxon that differed from all known animal groups due to its triradial symmetry. A superficial resemblance to edrioasteroid echinoderms was nevertheless considered by those who worked on it at the time. Glaessner later decidedly rejected any affinity of Tribrachidium with any recent phyla of the animal kingdom, leaving its taxonomic position completely unclear. Nevertheless, a relationship with echinoderms, sponges, and cnidarians continued to be considered.
The recognizable structures on the poorly preserved Australian fossils were interpreted either as tentacles, as peculiar arms or as a Y-shaped mouth opening, but the latter hypothesis was dropped again. The mechanism of locomotion of Tribrachidium is also unknown.
The discovery of the closely related albumares and anfesta stankovskii and other, much better preserved, Russian fossils, led Mikhail Fedonkin the new taxon trilobozoa - an extinct group of three streams-symmetrical, the hollow animals like organisms - to set up. The Trilobozoa initially formed only one class within the tribe of the Coelenterata. However, in the course of the division of the Coelenterata into the two tribes of cnidarians (Cnidaria) and rib jellyfish (Ctenophora), the Trilobozoa were also raised to one tribe.
Fedonkin (1985) was able to show that Tribrachidium is the imprint of the upper surface of an animal body, which details certain elements of its external and internal anatomy. The radial furrows of the fossil originate from radial incisions on the surface of the living animal, whereas the three interlocked, spirally twisted backs of the arms in the central part reproduce impressions of cavities in its body.
Habitat
Tribrachidium was a benthically living soft-body organism that was not permanently but only temporarily connected to its substrate of microbe mats . The fossil was not evenly distributed across the ocean floor, but occurred in cohorts. Since it is found in very different facies areas, it can be classified as an adaptable generalist.
Individual evidence
- ^ A b c Glaessner, MF and Daily, B .: The geology and Late Precambrian fauna of the Ediacara fossil reserve . In: Records of the South Australian Museum . tape 13 (3) , 1959, pp. 369-401 .
- ^ Glaessner, MF and Wade, M .: The late Precambrian fossils from Ediacara, South Australia . In: Palaeontology . tape 9 (4) , 1966, pp. 599 .
- ↑ Fedonkin, MA: The Vendian of the Ukraine (in Russian) . Ed .: Velikanov, VA et al. Naukova Dumka, Kiew 1983, p. 128-139 .
- ^ Fedonkin, MA: New locality of non-skeletal Metazoa in the Vendian of Winter Coast (in Russian) . In: Doklady Akademii Nauk SSSR . tape 239 (6) , 1978, pp. 1423-1426 .
- ↑ Fedonkin MA et al: The Rise of Animals. Evolution and Diversification of the Kingdom Animalia . Johns Hopkins University Press, 2007, ISBN 978-0-8018-8679-9 , pp. 326 .
- ^ A b Hall, CMS: Paleoecology of Tribrachidium: New Data from the Ediacaran of South Australia (diploma thesis) . University of California, Riverside 2015.
- ↑ a b Ivantsov, A., Yu. and Leonov MV: The imprints of Vendian animals - unique paleontological objects of the Arkhangelsk region (in Russian) . Ackhangelsk 2009, ISBN 978-5-903625-04-8 , pp. 91 .
- ^ Jenkins, RJF: Functional and ecological aspects of Ediacaran assemblages . Ed .: Lipps, JH and Signor, PW, Origin and Early Evolution of Metazoa. Plenum Press, New York 1992, pp. 131-171 .
- ^ Glaessner, MF: Precambrian . Ed .: Robison, RA and Teichen, C. Treatise on invertebrate paleontology, Part A. Boulder, Colorado, Geological Society of America. University of Kansas Press, 1979, pp. 79-118 .
- ^ A b Fedonkin, MA: Systematic Description of Vendian Metazoa . In: Sokolov, BS and Iwanowski, AB Vendian System (eds.): Historical-Geological and Paleontological Foundation . 1: Paleontology (in Russian). Nauka, Moscow 1985, p. 70-106 .
- ↑ Runnegar, BN and Fedonkin, M. A: Proterozoic Metazoan Body Fossils . Ed .: Schopf, JW and Klein, C. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, 1992, pp. 373 .
