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Was Dickinsonia One of the Earliest Multicellular Animals?
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[QUOTE="BillTre, post: 6061458, member: 581757"] Multicellular animals ([URL='https://en.wikipedia.org/wiki/Animal']metazoans[/URL]) are thought (based upon molecular divergence data between metazoanas and chanoflagellates) to have initially evolved about 720 MYA, at beginning of the [URL='https://en.wikipedia.org/wiki/Cryogenian']Cryogenian[/URL] (720-635 MYA). These first pre-metazoans are assumed to have started out very small with no hard parts (including no cell walls) and therefore did not preserve well as fossils. During this period, the first components of the metazoan system of development (working from a single fertilized egg to a clone of cells that reproducibly shape themselves into a functional adult form, in order to reproduce) that generates the adult body plans were established. Metazoans are thought to have separated from [URL='https://en.wikipedia.org/wiki/Choanoflagellate']Chanoflagelates[/URL] (eucayrotic single celled flagellated metazoan precursors) during the [URL='https://en.wikipedia.org/wiki/Cryogenian']Cryogenian[/URL] (720-635 MYA). Near microscopic fossils, often interpreted as [URL='https://en.wikipedia.org/wiki/Embryo_fossil'][B]embryos[/B][/URL], have been found at the beginning or the Ediacaran. The [URL='https://en.wikipedia.org/wiki/Ediacaran']Ediacaran[/URL] Period (635-541 MYA) is characterized by the first macro scale fossils that were not thought to be bacterial mats. These fossils are of unusual morphology (strange looking), casting uncertainty on what exactly they are: plant, animal, fungus, colonial bacteria. The Ediacaran was followed by the Cambrian period where the number and variety of obviously animal body plans increased rapidly. At this time, the body plans of many of the animals found today were established. The identity and relationships of the [URL='https://en.wikipedia.org/wiki/Ediacaran_biota']Ediacaran fossils[/URL] are still a matter of some discussion. Now, additional strong evidence identifying one of the Ediacaran animals, [I]Dickinsonia[/I], as an animal, based on "[URL='https://www.sciencemag.org/news/2018/09/fossil-one-world-s-earliest-animals-according-fat-molecules-preserved-half-billion?utm_campaign=news_weekly_2018-09-21&et_rid=295682744&et_cid=2384417']fossil" cholesterol associated with an organic layer of the fossil animal[/URL]. Cholesterol is only produced by animals, not plants, fungi, bacteria, or algae. [URL='https://en.wikipedia.org/wiki/Dickinsonia'][I]Dickinsonia[/I][/URL] is one of the most likely of the Ediacaran fossils to be an animal, based on apparent trace fossils (like footprints), showing its movement and feeding behavior (see areas in a bacterial mat that were fed upon below). The cholesterol findings provide additional strong support for this this being an animal. [I]Dickinsonia[/I] may be a fossil member of the obscure [URL='https://en.wikipedia.org/wiki/Placozoa']Placozoan phylum[/URL]. Placozoa are among the most primitive of multicellular animals. They have been variously located either above or below Sponges, Cnidarians (hydra, coral, sea anemone), and [URL='https://en.wikipedia.org/wiki/Ctenophora']Ctenophores[/URL] (comb jellies). [I]Considering all that, Dickinsonia[/I] seems to be something like this to: [LIST] [*]two or maybe three tissue layers ([URL='https://en.wikipedia.org/wiki/Endoderm']Endoderm[/URL], [URL='https://en.wikipedia.org/wiki/Ectoderm']ectoderm[/URL], [URL='https://en.wikipedia.org/wiki/Mesoderm'](maybe mesoderm)[/URL]). (Placozoa only have two tissue layers) [*]has a front to back body axis (therefore expect something like a Hox code for ID positions along this axis) [*]looks bilaterally symmetrical (goes with having a front and back axis combined with a top and bottom axis) [*]may have slithered along on cilia (like Placozoa). [*]Eat things (bacterial or algae cells?) it settles upon, then moves on. Probably no predators, for a while. [/LIST] https://www.physicsforums.com/attachments/231018 [ATTACH=full]231075[/ATTACH] [/QUOTE]
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