Dickinsonia

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 (final part of Precambrian). 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.

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 1.4 m in length, and from a fraction of a millimetre to a few millimetres thick. 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. The body is divided into two by a midline ridge or groove, except for a single unpaired segment at one end, dubbed the "anterior most unit" suggested to represent the front of the organism. It is disputed whether the segments are offset from each other following glide reflection, and are thus isomers, 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. The number of segments/isomer pairs varies from 12 in smaller individuals to 74 in the largest Australian specimens.

The body of Dickinsonia is suggested to have been sack-like, with the outer layer being made of a resistant but unmineralised material. Some specimens from Russia show the presence of branched internal structures. Some authors have suggested that the underside of the body bore cilia, as well as infolded pockets.

Dickinsonia is suggested to have grown by adding a new pair of segments/isomers at the end opposite the unpaired "anterior most unit". Dickinsonia probably exhibited indeterminate growth (having no maximum size), though it is suggested that the addition of new segments slowed down later in growth. Deformed specimens from Russia indicate that individuals of Dickinsonia could regenerate after being damaged.

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. These trace fossils have been assigned to the genus Epibaion. 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.

Discovery
The first species and specimens of this fossil organism were first discovered in the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges in South Australia. Reg Sprigg, the original discoverer of the Ediacaran biota in Australia, described Dickinsonia, naming it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg. Additional specimens of Dickinsonia are also known from the Mogilev Formation in the Dniester River Basin of Podolia, Ukraine, the Lyamtsa, Verkhovka, Zimnegory and Yorga Formations in the White Sea area of the Arkhangelsk Region, Chernokamen Formation of the Central Urals, Russia, (these deposits have been dated to 567–550 Myr.  ), the Dengying Formation in the Yangtze Gorges area, South China. (ca. 551–543 Ma).

Taphonomy
As a rule, Dickinsonia fossils are preserved as negative impressions ("death masks") 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. 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" on the decaying body, or perhaps it was due to the carbonate cementation of the sand. 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.

Rarely, Dickinsonia have been preserved as a cast in massive sandstone lenses, where it occurs together with Pteridinium, Rangea and some others. 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. These specimens are products of events where organisms were first stripped from the sea-floor, transported and deposited within sand flow. In such cases, stretched and ripped Dickinsonia occur. The first such specimen was described as a separate genus and species, Chondroplon bilobatum and later re-identified as Dickinsonia.

Species
Since 1947, a total of nine species have been described, of which three are currently considered valid:

A claimed specimen of Dickinsonia from India was later determined to be the remains of a beehive.

External relationships
Dickinsonia is classified as part of the group Proarticulata or Dickinsoniomorpha. Proarticulata includes a number of morphologically similar organisms, such Spriggina, Yorgia, Andiva and Cephalonega, which share the same segmented articulation. The affinities of Proarticulata to other organisms, including to other members of the Ediacaran biota, like rangeomorphs, have long been contentious. It has been historically proposed that most Ediacaran organisms were closely related to each other, as part of the grouping "Vendobionta", though recent authors argue that this grouping as a whole is likely to be polyphyletic. Gregory Retallack has proposed that the fossils of Dickinsonia and other Ediacaran biota represent lichens that grew in a terrestrial environment, but this has been broadly rejected by other authors, who argue that a marine environment of deposition better fits available evidence. Other proposal have included giant protists, as proposed by Adolf Seilacher. Most modern research suggest that Dickinsonia and other proarticulatans are likely to be animals, possibly belonging to Eumetazoa. A chemical study of Russian specimens found that they were enriched with cholesterol, which is only produced by animals, supporting an animal affinity, though these results have been questioned by other authors, who consider the association between the cholesterol molecules and the Dickinsonia fossils to not be definitive. Within Animalia, a number of affinities have been proposed, including as stem-eumetazoans forming a clade with rangeomorphs, to Placozoa, and to Cnidaria. A number of researchers have proposed close affinities to Bilateria, based on the bilateral or nearly bilateral organisation of proarticulatans, though proarticulatans are not likely to be a member of the bilaterian crown group.