ediacaran biota


  • [114] It has since been found to be a siphonophore, possibly even sections of a more complex species,[115] though this in turn has raised suspicions for a similar status for
    at least some Ediacaran organisms.

  • They found that, while the White Sea assemblage had the most species, there was no significant difference in disparity between the three groups, and concluded that before
    the beginning of the Avalon timespan these organisms must have gone through their own evolutionary “explosion”, which may have been similar to the famous Cambrian explosion .

  • [74] A suite of fossils known as the small shelly fossils are represented in the Ediacaran, most famously by Cloudina[75] a shelly tube-like fossil that often shows evidence
    of predatory boring, suggesting that, while predation may not have been common in the Ediacaran Period, it was at least present.

  • [21] However, since they lay below the “Primordial Strata” of the Cambrian that was then thought to contain the very first signs of animal life, a proposal four years after
    their discovery by Elkanah Billings that these simple forms represented fauna was dismissed by his peers.

  • Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape;[88] these fossils are thought to be associated with ciliary feeding
    but the precise method of formation of these disconnected and overlapping fossils largely remains a mystery.

  • [93][94] The link between certain frond-like Ediacarans and sea pens has been thrown into doubt by multiple lines of evidence; chiefly the derived nature of the most frond-like
    pennatulacean octocorals, their absence from the fossil record before the Tertiary, and the apparent cohesion between segments in Ediacaran frond-like organisms.

  • [92] However, more recent discoveries have established that many of the circular forms formerly considered “cnidarian medusa” are actually holdfasts – sand-filled vesicles
    occurring at the base of the stem of upright frond-like Ediacarans.

  • Their more resistant nature is reflected in the fact that, in rare occasions, quilted fossils are found within storm beds as the high-energy sedimentation did not destroy
    them as it would have the less-resistant discs.

  • [49] Other “embryos” have been interpreted as the remains of the giant sulfur-reducing bacteria akin to Thiomargarita,[50] a view that, while it had enjoyed a notable gain
    of supporters[51][52] as of 2007, has since suffered following further research comparing the potential Doushantuo embryos’ morphologies with those of Thiomargarita specimens, both living and in various stages of decay.

  • He points out the chitinous walls of lichen colonies would provide a similar resistance to compaction, and claims the large size of the organisms — up to 1.5 metres (4 ft
    11 in) long, far larger than any of the preserved burrows — also hints against classification with animals.

  • For macroorganisms, the Cambrian biota appears to have almost completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still
    a matter of debate.

  • [129] There are at present no widely accepted reports of Ediacara-type organisms in the Cambrian period, though there are a few disputed reports, as well as unpublished observations
    of ‘vendobiont’ fossils from 535 Ma Orsten-type deposits in China.

  • Alternate explanations include holdfasts and protists;[58] the patterns displayed where two meet have led to many ‘individuals’ being identified as microbial colonies,[59][60]
    and yet others may represent scratch marks formed as stalked organisms spun around their holdfasts.

  • [84] Putative “burrows” dating as far back as 1,100 million years may have been made by animals that fed on the undersides of microbial mats, which would have shielded them
    from a chemically unpleasant ocean;[85] however their uneven width and tapering ends make a biological origin so difficult to defend[86] that even the original proponent no longer believes they are authentic.

  • [71] There is however little evidence for any trace fossils in the Ediacaran Period, which may speak against the active grazing theory.

  • There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought the fossils were preserved by virtue
    of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived.

  • The earliest known embryos, from China’s Doushantuo Formation, appear just a million years after the Earth emerged from a global glaciation, suggesting that ice cover and
    cold oceans may have prevented the emergence of multicellular life.

  • [15] A 2018 study confirmed that one of the period’s most-prominent and iconic fossils, Dickinsonia, included cholesterol,[16] suggesting affinities to animals, fungi, or
    red algae.

  • Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian rather than the Ediacaran.

  • Indeed, there does seem to be a slow increase in the maximum level of complexity seen over this time, with more and more complex forms of life evolving as time progresses,
    with traces of earlier semi-complex life such as Nimbia, found in the 610 million year old Twitya formation,[120] and older rocks dating to 770 million years ago in Kazakhstan,[121] possibly displaying the most complex morphology of the time.

  • [16] Cnidarians[edit] A sea pen, a modern cnidarian bearing a passing resemblance to Charnia Since the most primitive eumetazoans—multi-cellular animals with tissues—are cnidarians,
    and the first recognized Ediacaran fossil Charnia looks very much like a sea pen, the first attempt to categorise these fossils designated them as jellyfish and sea pens.

  • As soft-bodied organisms, they would normally not fossilize and, unlike later soft-bodied fossil biota such as the Burgess Shale or Solnhofen Limestone, the Ediacaran biota
    is not found in a restricted environment subject to unusual local conditions: they were a global phenomenon.

  • So while this shows that Cnidarians were already well established in the Ediacaran period, the question remains if there were only sessile polyps or that a medusal stage had
    also developed.

  • A notable example is the form known as Charniodiscus, a circular impression later found to be attached to the long ‘stem’ of a frond-like organism that now bears the name.

  • [58] In 2018 analysis of ancient sterols was taken as evidence that one of the period’s most-prominent and iconic fossils, Dickinsonia, was an early animal.

  • [87] The burrows observed imply simple behaviour, and the complex efficient feeding traces common from the start of the Cambrian are absent.

  • [53] A recent discovery of comparable Ediacaran fossil embryos from the Portfjeld Formation in Greenland has significantly expanded the paleogeograpical occurrence of Doushantuo-type
    fossil “embryos” with similar biotic forms now reported from differing paleolatitudes.

  • [127] Preservation bias[edit] The paucity of Ediacaran fossils after the Cambrian could simply be due to conditions that no longer favoured the fossilisation of Ediacaran
    organisms, which may have continued to thrive unpreserved.

  • [5][6][7] While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago), the earlier fossil communities disappear
    from the record at the end of the Ediacaran leaving only curious fragments of once-thriving ecosystems.

  • These disparate morphologies can be broadly grouped into form taxa: “Embryos” Recent discoveries of Precambrian multicellular life have been dominated by reports of embryos,
    particularly from the Doushantuo Formation in China.

  • [65] Including such fossils as the iconic Charnia and Swartpuntia, the group is both the most iconic of the Ediacaran biota and the most difficult to place within the existing
    tree of life.

  • [41] Soft-bodied organisms today rarely fossilize during such events, but the presence of widespread microbial mats probably aided preservation by stabilising their impressions
    in the sediment below.

  • This could place them in the bilateral clade of animals[83] but they could also have been made by simpler organisms feeding as they slowly rolled along the sea floor.

  • Martin Glaessner proposed in The Dawn of Animal Life (1984) that the Ediacaran biota were recognizable crown group members of modern phyla, but were unfamiliar because they
    had yet to evolve the characteristic features we use in modern classification.

  • Further, in some cases, the bacterial precipitation of minerals formed a “death mask”, ultimately leaving a positive, cast-like impression of the organism.

  • A sampling, reported in 2018, of late Ediacaran strata across Baltica (
    <560 mya) suggests the flourishing of the organisms coincided with conditions of low overall productivity with a very high percentage produced by bacteria, which may have led to high concentrations of dissolved organic material in the oceans.

  • [1][2] Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.

  • [64] These organisms appear to form two groups: the fractal rangeomorphs and the simpler erniettomorphs.

  • [68] Non-Vendobionts Some Ediacaran organisms have more complex details preserved, which has allowed them to be interpreted as possible early forms of living phyla excluding
    them from some definitions of the Ediacaran biota.

  • Examples of such scenarios today include plankton, whose small size allows them to reproduce rapidly to take advantage of ephemerally abundant nutrients in algal blooms.

  • Competition[edit] Cambrian animals such as Waptia may have competed with, or fed upon, Ediacaran life-forms.

  • While putative fossils are reported from 3,460 million years ago,[116][117] the first uncontroversial evidence for life is found 2,700 million years ago,[118] and cells with
    nuclei certainly existed by 1,200 million years ago:[119] The reason why it took so long for forms with an Ediacaran grade of organisation to appear is uncertain.

  • Misra’s discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the
    description of features that were previously undiscernible.

  • If these enigmatic organisms left no descendants, their strange forms might be seen as a “failed experiment” in multicellular life, with later multicellular life evolving
    independently from unrelated single-celled organisms.

  • [11][12][13] Kimberella may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, although this is controversial.

  • It is suggested that by the Early Cambrian, organisms higher in the food chain caused the microbial mats to largely disappear.

  • [132] Change in environmental conditions[edit] While it is difficult to infer the effect of changing planetary conditions on organisms, communities and ecosystems, great changes
    were happening at the end of the Precambrian and the start of the Early Cambrian.

  • Some finds[47] generated intense media excitement[48] though some have claimed they are instead inorganic structures formed by the precipitation of minerals on the inside
    of a hole.

  • On the early Earth, reactive elements, such as iron and uranium, existed in a reduced form that would react with any free oxygen produced by photosynthesising organisms.

  • [125] Periods of intense cold have also been suggested as a barrier to the evolution of multicellular life.

  • But for large size never to be favourable, the environment would have to be very different indeed.

  • [80] Fungus-like filaments found in the Doushantuo Formation have been interpreted as eukaryotes and possibly fungi, providing possible evidence for the evolution and terrestrialization
    of fungi ~635 Ma.

  • [126] Potentially, complex life may have evolved before these glaciations, and been wiped out.

  • [8] Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators and competition from
    other life-forms.

  • In early 2008 a team analysed the range of basic body structures (“disparity”) of Ediacaran organisms from three different fossil beds: Avalon in Canada, 575 to 565 million
    years ago; White Sea in Russia, 560 to 550 million years ago; and Nama in Namibia, 550 to 542 million years ago, immediately before the start of the Cambrian.

  • It is possible that increased competition due to the evolution of key innovations among other groups, perhaps as a response to predation,[131] drove the Ediacaran biota from
    their niches.

  • [123] Oxygen seems to have accumulated in two pulses; the rise of small, sessile (stationary) organisms seems to correlate with an early oxygenation event, with larger and
    mobile organisms appearing around the second pulse of oxygenation.

  • [54] Microfossils dating from 632.5 million years ago – just 3 million years after the end of the Cryogenian glaciations – may represent embryonic ‘resting stages’ in the
    life cycle of the earliest known animals.

  • [43] However, it is more common to find Ediacaran fossils under sandy beds deposited by storms or high-energy bottom-scraping ocean currents known as turbidites.

  • [21] In 1933, Georg Gürich discovered specimens in Namibia[22] but the firm belief that complex life originated in the Cambrian led to them being assigned to the Cambrian
    Period and no link to Aspidella was made.

  • Due to the difficulty of deducing evolutionary relationships among these organisms, some palaeontologists have suggested that these represent completely extinct lineages that
    do not resemble any living organism.

  • Palaeontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds[29][30] and with a combination of improved dating of existing specimens
    and an injection of vigour into the search many more instances were recognised.

  • Lacking any mouth, gut, reproductive organs, or indeed any evidence of internal anatomy, their lifestyle was somewhat peculiar by modern standards; the most widely accepted
    hypothesis holds that they sucked nutrients out of the surrounding seawater by osmotrophy[66] or osmosis.

  • [55] An alternative proposal is that these structures represent adult stages of the multicellular organisms of this period.

  • Donald Canfield detected records of the first significant quantities of atmospheric oxygen just before the first Ediacaran fossils appeared[122] – and the presence of atmospheric
    oxygen was soon heralded as a possible trigger for the Ediacaran radiation.


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