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Dunkleosteus BW

Dunkleosteus was a gigantic, 10 meter (33 ft) long prehistoric fish.[1]

Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

Origin and fossil recordEdit

Vernanimalcula

Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria.

Animals are generally considered to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.

The first fossils that might represent animals appear towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible. However some paleontologists and geologists would suggest that animals appeared much earlier than previously thought, possibly even as early as 1 billion years ago. Trace fossils such as tracks and burrows found in Tonian era indicate the presence of triploblastic worm like metazoans roughly as large (about 5 mm wide) and complex as earthworms.[2] In addition during the beginning of the Tonian period around 1 billion years ago (roughly the same time that the trace fossils previously discussed in this article date back to) there was a decrease in Stromatolite diversity which may indicate the appearance of grazing animals during this time as Stromatolites also increased in diversity shortly after the end-Ordovician and end-Permian rendered large amounts of grazing marine animals extinct and decreased shortly after their populations recovered. The discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts further doubt on their interpretation as evidence of early animal evolution.[3][4]

See alsoEdit

ReferencesEdit

  1. ^ Monster fish crushed opposition with strongest bite ever, smh.com.au
  2. ^ Seilacher, A., Bose, P.K. and Pflüger, F. (1998). "Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India". Science 282 (5386): 80–83. doi:10.1126/science.282.5386.80. PMID 9756480. http://www.sciencemag.org/cgi/content/abstract/282/5386/80. Retrieved on 2007-08-20. 
  3. ^ Matz, Mikhail V.; Tamara M. Frank, N. Justin Marshall, Edith A. Widder and Sonke Johnsen (2008-12-09). "Giant Deep-Sea Protist Produces Bilaterian-like Traces". Current Biology (Elsevier Ltd) 18 (18): 1–6. doi:10.1016/j.cub.2008.10.028. http://www.biology.duke.edu/johnsenlab/pdfs/pubs/sea%20grapes%202008.pdf. Retrieved on 2008-12-05. 
  4. ^ Reilly, Michael (2008-11-20). "Single-celled giant upends early evolution". MSNBC. http://www.msnbc.msn.com/id/27827279/. Retrieved on 2008-12-05. 


External linksEdit

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