Dickinsonia
Template:Short description Template:Good article Template:DistinguishTemplate:Use dmy dates Template:Automatic taxobox Dickinsonia is a genus of extinct organism that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It had a round, approximately bilaterally symmetric body with multiple segments running along it. It could range from a few millimeters to over a meter in length, and likely lived in shallow waters, feeding on the microbial mats that dominated the seascape at the time.
As a member of the Ediacaran biota, its relationships to other organisms has been heavily debated. It was initially proposed to be a jellyfish, and over the years has been claimed to be a land-dwelling lichen, placozoan, or even a giant protist. Currently, the most popular interpretation is that it was a seafloor dwelling animal, perhaps a primitive stem group bilaterian, although this is still contentious. Among other Ediacaran organisms, it shares a close resemblance to other segmented forms like Vendia, Yorgia and Spriggina and has been proposed to be a member of the phylum Proarticulata or alternatively the morphogroup Dickinsoniomorpha. It is disputed whether the segments of Dickinsonia are bilaterally symmetric across the midline, or are offset from each other via glide reflection, or possibly both.
Since the description of Dickinsonia costata in 1947 by Reginald Sprigg, eight other species have been proposed, although only two others—Dickinsonia tenuis and Dickinsonia menneri—are widely considered valid.
Description
Dickinsonia fossils are known only in the form of imprints and casts in sandstone beds. The specimens found range from a few millimetres to about Template:Convert in length, and from a fraction of a millimetre to a few millimetres thick.<ref name=Rise2>Template:Cite book</ref> They are nearly bilaterally symmetric, segmented, round or oval in outline, slightly expanded to one end (i.e. egg-shaped outline). The rib-like segments are radially inclined towards the wide and narrow ends, and the width and length of the segments increases towards the wide end of the fossil.<ref name=Ivantsov2007>Template:Cite journal</ref><ref name=Ivantsov2012>Template:Cite journal</ref> The body is divided into two by a midline ridge or groove,<ref name=Ivantsov2007/><ref name=Ivantsov2012/><ref name=":1">Template:Cite journal</ref> except for a single unpaired segment at one end, dubbed the "anterior most unit" suggested to represent the front of the organism.<ref name=":1" /> It is disputed whether the segments are offset from each other following glide reflection, and are thus isomers,<ref name=Ivantsov2007/><ref name=Ivantsov2012/><ref name=Ivantsov2011>Template:Cite journal</ref><ref name=":3"/> or whether the segments are symmetric across the midline, and thus follow true bilateral symmetry, as the specimens displaying the offset may be the result of taphonomic distortion. Dickinsonia could perhaps have had both at the same time, with one side of the organism being glide reflected and the other having true symmetry.<ref name=":1" /><ref name=":2"/>
The body of Dickinsonia is suggested to have been sack-like, with the outer layer being made of a resistant but unmineralised material.<ref name=":3">Template:Cite journal</ref> Some specimens from Russia show the presence of branched internal structures.<ref name=Ivantsov2004>Template:Cite journal</ref><ref name=":3"/> Some authors have suggested that the underside of the body bore cilia, as well as infolded pockets.<ref name=":3"/>
Dickinsonia is suggested to have grown by adding a new pair of segments/isomers at the end opposite the unpaired "anterior most unit".<ref name=":1"/><ref name=":5">Template:Cite journal</ref> Dickinsonia probably exhibited indeterminate growth (having no maximum size), though it is suggested that the addition of new segments slowed down later in growth.<ref name=":4">Template:Cite journal</ref> Deformed specimens from Russia indicate that individuals of Dickinsonia could regenerate after being damaged.<ref name=":5"/>
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Ecology
Dickinsonia is suggested to have been a mobile marine organism that lived on the seafloor and fed by consuming microbial mats growing on the seabed using structures present on its underside. Dickinsonia-shaped trace fossils, presumed to represent feeding impressions, sometimes found in chains demonstrating this behaviour have been observed.<ref name=":3"></ref> These trace fossils have been assigned to the genus Epibaion.<ref name="Ivantsov2011" /><ref name="Ivantsov2013" /><ref name="Ivantsov_Malakhovskaya2002">Template:Cite journal</ref> A 2022 study suggested that Dickinsonia temporarily adhered itself to the seafloor by the use of mucus, which may have been an adaptation to living in very shallow water environments.<ref>Template:Cite journal</ref>
Taphonomy
Dickinsonia fossils are preserved as negative impressions on the bases of sandstone beds. Such fossils are imprints of the upper sides of the benthic organisms that have been buried under the sand.<ref name="Ivantsov2009">Template:Cite journal</ref><ref name="Gehling1999">Template:Cite journal</ref> The imprints formed as a result of cementation of the sand before complete decomposition of the body. The mechanism of cementation is not quite clear; among many possibilities, the process could have arisen from conditions which gave rise to pyrite "death masks"<ref name="Gehling1999" /> on the decaying body, or perhaps it was due to the carbonate cementation of the sand.<ref name="Serezhnikova2011">Template:Cite book</ref> The imprints of the bodies of organisms are often strongly compressed, distorted, and sometimes partly extend into the overlying rock. These deformations appear to show attempts by the organisms to escape from the falling sediment.<ref name="Ivantsov2011"></ref><ref name="Ivantsov2013">Template:Cite journal</ref><ref name="Runnegar1982">Template:Cite journal</ref>
Rarely, Dickinsonia have been preserved as a cast in massive sandstone lenses, where it occurs together with Pteridinium, Rangea and some others.<ref name="Grazhdankin20042">Template:Cite journal</ref><ref name="Keller_Fedonkin_1976">Template:Cite journal</ref><ref name="Keller_Fedonkin_1977">Template:Cite journal</ref><ref name="Gehling_Droser2013">Template:Cite journal</ref> Large beds containing many hundreds of Dickinsonia (along with many other species) are preserved in situ within Nilpena Ediacara National Park, with park rangers providing on-site guided tours in the cooler months of the year.<ref>Template:Cite web</ref> These specimens are products of events where organisms were first stripped from the sea-floor, transported and deposited within sand flow.<ref name="Grazhdankin20042" /><ref name="Gehling_Droser2013" /> In such cases, stretched and ripped Dickinsonia occur. The first such specimen was described as a separate genus and species, Chondroplon bilobatum<ref name="Wade1971">Template:Cite journal</ref> and later re-identified as Dickinsonia.
Taxonomy
History
Dickinsonia was first discovered in 1946 at the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges in South Australia. Reg Sprigg described Dickinsonia the following year<ref name="Sprigg19472">Template:Cite journal</ref> and named it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg.<ref>Template:Cite web</ref> Additional specimens of Dickinsonia have also been found in the Mogilev Formation in the Dniester River Basin of Ukraine,<ref name="Fedonkin1983Ukr">Template:Cite book</ref> the White Sea in Russia,<ref>Template:Cite journal</ref> and the Dengying Formation in the Yangtze Gorges area, South China. (ca. 551–543 Ma).<ref>Template:Cite journal</ref>
Sprigg's initial interpretation was that Dickinsonia was a jellyfish-like organism from the early Cambrian. He suspected that the imprint left behind was a cast of the flattened bell, and that the grooves radiating from the center were possibly some sort of canal system or rigid structure.<ref name="Sprigg19472" /> Further analysis in 1949 theorized that the bilateral nature of Dickinsonia could have been a sign of higher complexity, but was unwilling to firmly classify it into any taxon.<ref name="Sprigg1949">Template:Cite journal</ref> In 1955, Harrington and Moore published their own classification of Dickinsonia, assigning it to class Dipleurozoa, order Dickinsoniida, and family Dickinsoniidae in the now defunct group Coelenterata.<ref name="Harrington_Moore1955" />
After the discovery of the undisputibly Precambrian Charnia in 1958,<ref>Template:Cite journal</ref> the existence of Proterozoic life became more widely accepted among paleontologists. This discovery lead Dickinsonia and other South Australian organisms to be properly recognized as Precambrian in age. The segmentation of the recently discovered Spriggina from the same locality lead it and the similarly segmented Dickinsonia to be classified as annelids, which remained the leading hypothesis for the next few decades,<ref name=":2">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> albeit with reservations.<ref name=":2" /><ref>Template:Cite book</ref><ref>Template:Cite journal</ref>
In 1985, following studies that concluded that Dickinsonia and related taxa had glide symmetry rather than bilteral symmetry, a new phylum, Proarticulata, was erected to include the Ediacaran organisms that were assumed to have glide reflection, which included Spriggina, Vendia, and several others.<ref>Template:Cite journal</ref> Their relationships to other organisims remain uncertain and numerous hypotheses have been offered since. Adolf Seilacher proposed that most Ediacaran organisms were closely related to each other, as part of the grouping "Vendobionta",<ref name="Seilacher1992">Template:Cite journal</ref> though recent authors argue that this grouping is likely polyphyletic.<ref name="Bobrovskiy">Template:Cite journal</ref> Some authors do not use Proarticulata and instead use the clade Dickinsoniomorpha.<ref name=":3" /> In 2013 Gregory Retallack proposed that Dickinsonia and other Ediacaran lifeforms were lichens, arguing that their preservation methods were similar.<ref>Template:Cite journal</ref> This has been broadly rejected by most authors, who argue that a marine environment better fits available evidence.<ref>Template:Cite journal</ref><ref name="Bobrovskiy" /><ref>Template:Cite journal</ref> Other proposals have included giant protists, placozoans, or cnidarians.<ref name="Sperling2008">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>J.W. Valentine Dickinsonia as a polypoid organism Paleobiology, 18 (1992), pp. 378-382</ref>
Modern classification
While Dickinsonia's relationships to other organisms are still highly contentious,<ref>Template:Cite journal</ref> most biologists consider an animal with stem-bilaterian affinity to be the most likely interpretation.<ref name=":1"></ref><ref name=":3" /><ref name=":0">Template:Cite journal</ref><ref>Template:Cite journal</ref> In 2018 it was found that many Russian specimens contained cholesterol, which is only produced by animals, supporting an animal affinity.<ref name="Bobrovskiy" /> These results have been questioned by other authors, however, who consider the association between the cholesterol and Dickinsonia fossils to not be definitive.<ref name="Love2021">Template:Cite book</ref> The predictable growth patterns, clear left and right sides, and a posterior-anterior axis all suggest that Dickinsonia was a bilaterian. However, most modern bilaterians have a mouth and anus connected by a gut, none of which has been found in Dickinsonia. This almost certainly rules out Dickinsonia to be a crown-bilaterian, but could mean it was a stem-bilaterian.<ref name=":1" />
Species
Since 1947, a total of nine species have been described, three of which are currently considered valid:<ref name="Evans2023">Template:Cite journal</ref>
| Species | Authority | Location | Status | Notes | Refs |
|---|---|---|---|---|---|
| Dickinsonia brachina | Template:Harvp | Australia | Template:No | synonym of D. tenuis | <ref name="Wade1972">Template:Cite journal</ref> |
| Dickinsonia costata | Template:Harvp | Australia, Russia, and Ukraine | Template:Yes | Template:Efn | <ref name="Sprigg19472" /> |
| Dickinsonia elongata | Template:Harvp | Australia | Template:No | synonym of D. costata | <ref name="Glaessner1966">Template:Cite journal</ref> |
| Dickinsonia lissa | Template:Harvp | Australia | Template:No | synonym of D. tenuis | <ref name="Wade1972" /> |
| Template:Nobr | Template:Harvp | Russia | Template:Yes | Template:EfnTemplate:Efn | <ref name="Keller_Fedonkin_1976" /> |
| Dickinsonia minima | Template:Harvp | Australia | Template:No | synonym of D. costata | <ref name="Sprigg1949" /> |
| Dickinsonia rex | Template:Harvp | Australia | Template:No | synonym of D. tenuis | <ref name="Jenkins1992">Template:Cite book</ref> |
| Dickinsonia spriggi | Template:Harvp | Australia | Template:No | synonym of D. costata | <ref name="Harrington_Moore1955">Template:Cite journal</ref> |
| Dickinsonia tenuis | Template:Harvp | Australia and Russia | Template:Yes | Template:Efn | <ref name="Glaessner1966" /> |