page_title: Ediacaran biota
section_title: Avalon-type assemblage
hierarchical_section_title: Ediacaran biota / Assemblages / Avalon-type assemblage
caption_attribution_description: English: Reconstruction of fossil soils and their biota in the Mistaken Point Formation of Newfoundland
context_page_description: The Ediacaran biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.
  The Ediacaran biota may have undergone evolutionary radiation in a proposed event called the Avalon explosion, 575 million years ago. This was after the Earth had thawed from the Cryogenian period's extensive glaciation. This biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still a matter of debate.
context_section_description: The Avalon-type assemblage is defined at Mistaken Point in Canada, the oldest locality with a large quantity of Ediacaran fossils.
  The assemblage is easily dated because it contains many fine ash-beds, which are a good source of zircons used in the uranium-lead method of radiometric dating. These fine-grained ash beds also preserve exquisite detail. Constituents of this biota appear to survive through until the extinction of all Ediacarans at the base of the Cambrian.
  One interpretation of the biota is as deep-sea-dwelling rangeomorphs such as Charnia, all of which share a fractal growth pattern. They were probably preserved in situ (without post-mortem transportation), although this point is not universally accepted. The assemblage, while less diverse than the Ediacara- or Nama-types, resembles Carboniferous suspension-feeding communities, which may suggest filter feeding – by most interpretations, the assemblage is found in water too deep for photosynthesis. The low diversity may reflect the depth of water – which would restrict speciation opportunities – or it may just be too young for a rich biota to have evolved. Opinion is currently divided between these conflicting hypotheses.
  An alternative explanation for the distinct composition of the Avalon-type assemblage is that it was a terrestrial assemblage of volcaniclastic coastal soils near a continental volcanic arc. This view is based on geochemical studies of the substrates of Mistaken Point fossils and associated matrix supported tuffs and volcanic bombs that could only form on land. Some of these fossils such as Fractofusus and Charniodiscus were found in strata that Retallack interprets to be red well drained paleosols of coastal plains, but others such as Aspidella were found in strata that Retallack interprets to be intertidal paleosols.
caption: Reconstruction of fossil soils and their biota in the Mistaken Point Formation of Newfoundland [129]
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