Tribrachidium
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| Tribrachidium heraldicum Fossil range: Ediacaran - 558-542Ma[1] |
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 Fossil of Tribrachidium heraldicum
 Colourful reconstruction
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Tribrachidium heraldicum ("Heraldic Three Arms") was an early Ediacaran organism famous for its unusual tri-radial symmetry. It was named and first described from South Australia by Martin Glaessner and Brian Daily in 1959.[3]
Tribrachidium fossils are found in numerous locations throughout the world, including Newfoundland, the Northwest Territories, the cliffs along the shores of the White Sea in Russia, and Ediacara Hills, Australia, where the first specimens were found. They show a disc-shaped creature about 5cm in diameter on average, with three curved "arms" extending from the centre to nearly the edge. These arms were probably hollow and could be inflated or deflated. This threefold symmetry is almost unique in animals, which are usually either bilaterally or radially symmetric. Fleshy ridges, or possibly loose filaments, extended from the edges of the arms and formed the rest of the body.
Like many Precambrian fossils, the relationship of Tribrachidium to other animals is poorly known. To some extent it has become a poster child for the problem in general, often being shown as an example of that era's peculiar lifeforms. Among others, Tribrachidium has been described variously as a cnidarian, lophophore, echinoderm, ecdysozoan or even as an odd, outlying member of the dipleurozoa — a proposed stem group to the chordates. Some have even speculated that it is not a complete animal, but rather the holdfast of a larger creature. Still others suggest that it was not an animal at all, but, either a protist, or the member of some now-extinct kingdom of multicellular organisms.
Recently, A.Y. Ivantsov and M.A. Fedonkin have interpreted T. heraldicum, and its relatives, as being cnidarians.[4] This is because of the discovery and analysis of a Precambrian conulate, Vendoconularia, that they regard as related to the trilobozoans because of its sixfold symmetry.
Together with two other tri-radially symmetrical organisms, Anfesta stankovskii, and Albumares brunsae, both from the shores of the White Sea, Tribrachidium belongs to the family Tribrachididae.
[edit] Links
- Palaeos dendrogram [1]
[edit] References
- ^ Grazhdankin, Dima (2004). "Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution". Palæobiology 30 (2): 203–221.
- ^ Palaeos Proterozoic : Neoproterozoic: Ediacaran : Ediacaran Period - 4
- ^ Glaessner, M.F.; Daily, B. (1959). "The geology and Late Precambrian fauna of the Ediacara fossil reserve". Records of the South Australian Museum 13 (3): 369–401.
- ^ Ivantsov, A.Y.; Fedonkin, M.A. (2002). "Conulariid-like Fossil From The Vendian Of Russia: A Metazoan Clade Across The Proterozoic/palaeozoic Boundary". Palaeontology 45: 1219–1229. doi:.
- Anatomical Information Content in the Ediacaran Fossils and Their Possible Zoological Affinities, Jerry Dzik, Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland
- The Ediacaran Biotas in Space and Time, Ben Waggoner, Department of Biology, University of Central Arkansas, Conway, Arkansas 72035-5003
- McMenamin, Mark A. S. The Garden of Ediacara New York: Columbia University Press, 1998. ISBN 0-231-10559-2
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