Avalon explosion

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Dickinsonia, an enigmatic quilted organism with glide symmetry which may have been an early animal DickinsoniaCostata.jpg
Dickinsonia , an enigmatic quilted organism with glide symmetry which may have been an early animal
Cloudina may have been one of the first mineralized animals to appear, although its life appearance and evolutionary affinities remain unknown. Cloudina fossils (Neoproterozoic; Corumba, Brazil) (45453979394).jpg
Cloudina may have been one of the first mineralized animals to appear, although its life appearance and evolutionary affinities remain unknown.
Kimberella was originally interpreted as a cubozoan cnidarian, although it is now believed it was an early mollusc. Kimberella quadrata 4.jpg
Kimberella was originally interpreted as a cubozoan cnidarian, although it is now believed it was an early mollusc.
The Ediacaran trace fossils are a sign of animal movement as well as sediment disturbance, they show possible signs of the earliest true animals. Ediacaran trace fossil.jpg
The Ediacaran trace fossils are a sign of animal movement as well as sediment disturbance, they show possible signs of the earliest true animals.

The Avalon explosion, named from the Precambrian faunal trace fossils discovered on the Avalon Peninsula in Newfoundland, eastern Canada, is a proposed evolutionary radiation of prehistoric animals about 575 million years ago in the Ediacaran period, with the Avalon explosion being one of three eras grouped in this time period. [3] This evolutionary event is believed to have occurred some 33 million years earlier than the Cambrian explosion, which had been long thought to be when complex life started on Earth.

Contents

Scientists are still unsure of the full extent behind the development of the Avalon explosion, [3] which resulted in a rapid increase in metazoan biodiversity, including the first appearance of some extant infrakingdoms/superphyla such as cnidarians and bilaterians. Many of the Avalon explosion animals are sessile soft-bodied organisms living in deep marine environments, [4] and the first stages of the Avalon explosion were observed through comparatively minimal species. [3]

History

Charles Darwin predicted a time of ecological growth before the Cambrian Period, but there was no evidence to support it until the Avalon explosion was proposed in 2008 by Virginia Tech paleontologists after analysis of the morphological space change in several Ediacaran assemblages. [3] [4] The discovery suggests that the early evolution of animals may have involved more than one explosive event. [5] The original analysis has been the subject of dispute in the literature. [6] [7] [8]

Evidence

Trace fossils of these Avalon organisms have been found worldwide, with many found in Newfoundland, in Canada and the Charnwood Forest in England, [3] representing the earliest known complex multicellular organisms. [lower-alpha 1] The Avalon explosion theoretically produced the Ediacaran biota. [4] [3] The biota largely disappeared contemporaneously with the rapid increase in biodiversity known as the Cambrian explosion. At this time, all living animal groups were present in the Cambrian oceans. [3]

The Avalon explosion appears similar to the Cambrian explosion in the rapid increase in diversity of morphologies in a relatively small-time frame, followed by diversification within the established body plans, [3] [9] a pattern similar to that observed in other evolutionary events. [9]

Plants and animals

The Avalon explosion was a time of early evolution and low diversity in species. There were over 270 species defined, [10] with 50 different morphological characteristics categories, many of which the anatomical structure had to be inferred with fossils and casts. [4] [10] These species were placed into 20 different genera. [10]  

During this time, animals became bilateral and along with increasing complexity. [11] Many animals during this time fit into the annelid, arthropod, echinoderm, and cnidarian phyla. [11] Animals at this time developed bilateral symmetry with a clear anterior and posterior side, which included species like Spriggina , Charniodiscus and Yorgia . [10]

Many of the plants fit into a now-extinct phylum of Vendobionta. [11] The Vendobionta were radically arranged, with many tubular elements and a central stalk. Frondlets were a prominent aquatic plant during this time, with many different shapes, including Fractofusus which is a spindle shape, Fradgatia , a lettuce shape; and Rangea which was a leaf shape. [11]

See also

Notes

  1. Simple multicellular organisms such as red algae evolved at least 1,200  million years ago.

Related Research Articles

<span class="mw-page-title-main">Ediacaran</span> Third and last period of the Neoproterozoic Era

The Ediacaran is a geological period of the Neoproterozoic Era that spans 96 million years from the end of the Cryogenian Period at 635 Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last period of the Proterozoic Eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian Period marks the start of the Phanerozoic Eon, where recognizable fossil evidence of life becomes common.

<i>Pteridinium</i> Ediacaran fossil

Pteridinium is an erniettomorph found in a number of Precambrian deposits worldwide. It is a member of the Ediacaran biota.

<i>Dickinsonia</i> Extinct genus of early animals

Dickinsonia is a genus of extinct organism, most likely an animal, that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It is one of the best known members of the Ediacaran biota. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth has been considered consistent with a stem-group bilaterian affinity, though various other affinities have been proposed. It lived during the late Ediacaran. The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal, though these results have been questioned.

<span class="mw-page-title-main">Doushantuo Formation</span> Fossil formation in south-central China

The Doushantuo Formation is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran, is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion, as well as due to its microfossils' potential utility as biostratigraphical markers. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

<i>Kimberella</i> Primitive Mollusc-like organism

Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

<span class="mw-page-title-main">Rangeomorph</span> Form taxon of frondose Ediacaran fossils

The rangeomorphs are a form taxon of frondose Ediacaran fossils that are united by a similarity to Rangea. Some researchers, such as Pflug and Narbonne, suggest that a natural taxon Rangeomorpha may include all similar-looking fossils. Rangeomorphs appear to have had an effective reproductive strategy, based on analysis of the distribution pattern of Fractofusus, which consisted of sending out a waterborne asexual propagule to a distant area, and then spreading rapidly from there, just as plants today spread by stolons or runners.

<i>Spriggina</i> Extinct genus of animals

Spriggina is a genus of early animals whose relationship to living animals is unclear. Fossils of Spriggina are known from the late Ediacaran period in what is now South Australia. Spriggina floundersi is the official fossil emblem of South Australia; it has been found nowhere else.

<span class="mw-page-title-main">Vendobionta</span> Group of extinct creatures that were part of the Ediacaran biota

Vendobionts or Vendozoans (Vendobionta) are a proposed very high-level, extinct clade of benthic organisms that made up of the majority of the organisms that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran period. They became extinct shortly after the so-called Cambrian explosion, with the introduction of fauna forming groups more recognizably related to modern animals. It is very likely that the whole Ediacaran biota is not a monophyletic clade and not every genus placed in its subtaxa is an animal.

<span class="mw-page-title-main">Ediacaran biota</span> Life of the Ediacaran period

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were 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 term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

The end-Ediacaran extinction is a mass extinction believed to have occurred near the end of the Ediacaran period, the final period of the Proterozoic eon. Evidence suggesting that such a mass extinction occurred includes a massive reduction in diversity of acritarchs, the sudden disappearance of the Ediacara biota and calcifying organisms, and the time gap before Cambrian organisms "replaced" them. Some lines of evidence suggests that there may have been two distinct pulses of the extinction event, one occurring 550 million years ago and the other 539 million years ago.

The Cambrian explosion is an interval of time approximately 538.8 million years ago in the Cambrian period of the early Paleozoic when a sudden radiation of complex life occurred, and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

Ediacaran type preservation relates to the dominant preservational mode in the Ediacaran period, where Ediacaran organisms were preserved as casts on the surface of microbial mats.

<span class="mw-page-title-main">Erniettomorph</span> Extinct clade of fossils

The Erniettomorphs are a form of Ediacaran fossil consisting of rows of airbed-like tubes arranged along a midline with a glide symmetry. Representative genera include Ernietta, Phyllozoon, Pteridinium, Swartpuntia. Undisputed Erniettomorphs were Ediacaran, but the species Erytholus, Rutgersella, and Protonympha, who have by some been included in this group but are by no means clear members, are found through to the Late Devonian. Their affinity is uncertain; they probably form a clade and are most likely a sister group to the rangeomorphs, which bear a similar construction. Placements within the metazoan crown-group have been rebutted, and it is most likely that these peculiar organisms lie in the stem group to the animals. There is no evidence that they possessed a mouth or gut. Because they may have been found in water which was too deep to permit photosynthesis – and in some cases, lived half-buried in sediment, it is speculated that they fed by osmosis from the sea water. Such a lifestyle requires a very high surface area to volume ratio – higher than is observed in fossils. However, this paradox can be resolved if much of the volume of the organisms was not metabolically active. Many Pteridinium fossils are found completely filled with sand; if this sand were present within the organism while it was alive, this would reduce its metabolically active volume enough to make osmotic feeding viable.

<span class="mw-page-title-main">Mistaken Point Formation</span>

The Mistaken Point Formation is a geologic formation in Newfoundland and Labrador. It is recognized as a Lagerstätte preserving fossils dating back to the Ediacaran period. It contains a stratum dated to 565 ± 3 million years ago.

<span class="mw-page-title-main">Petalonamae</span> Proposed extinct group of animals

The petalonamids (Petalonamae) are an extinct group of archaic animals typical of the Ediacaran biota, also called frondomorphs, dating from approximately 635 million years ago to 516 million years ago. They are benthic and motionless animals, that have the shape of leaves, fronds (frondomorphic), feathers or spindles and were initially considered algae, octocorals or sea pens. It is now believed that there are no living descendants of the group, which shares a probable relation to the Ediacaran animals known as Vendozoans.

Shuhai Xiao is a Chinese-American paleontologist and professor of geobiology at Virginia Tech, Blacksburg, Virginia, U.S.A.

<span class="mw-page-title-main">Nama assemblage</span> Ediacaran biotic assemblage

The Nama assemblage was the last of the Ediacaran biotic assemblages. Following the Avalon and White Sea assemblages, it spanned from c. 550 Ma to c. 539 Ma, coinciding with the Terminal Ediacaran biozone. The assemblage was characterized by a faunal turnover, with the decline of the preexisting White Sea biota. The drop of diversity has been compared to the mass extinctions of the Phanerozoic. A second drop of diversity occurred at the Ediacaran–Cambrian boundary, concluding the Nama assemblages with the end-Ediacaran extinction.

<span class="mw-page-title-main">White Sea assemblage</span> Ediacaran biotic assemblage

The White Sea assemblage was the second of the three Late Ediacaran biotic assemblages, following the Avalon assemblage and preceding the Nama assemblage. It spanned from c. 560 Ma to c. 550 Ma. Showing an increase in genus diversity from the Avalon assemblage, it concluded with a faunal turnover often characterized as the first pulse of the end-Ediacaran extinction, with only 20% of White Sea taxa found in the later Nama assemblage despite similar taphonomic processes.

<span class="mw-page-title-main">Avalon assemblage</span> Ediacaran biotic assemblage

The Avalon assemblage was the first of the three Late Ediacaran biotic assemblages, spanning from c. 575 Ma to c. 560 Ma. It was followed by the White Sea assemblage, although temporal overlaps have been noted between the biotic assemblages. While earlier macroscopic fossils, mostly of algal origin, are known from the Lantian Formation, Avalon-type localities provide the first evidence of putative metazoan ancestors, as part of the Ediacaran biota.

References

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