Dickinsonia is an extinct genus of fossils of the Ediacaran biota. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth is consistent with a bilaterian affinity, though some have suggested that it belongs to the fungi, or even an "extinct kingdom". The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal.
|Cast of Dickinsonia costata from Australia|
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 metres (4 ft 7 in) 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 segments are separated by a thin ridge or groove along the axis of symmetry into right and left halves. The segments are organized in an alternating pattern according to glide reflection symmetry rather than bilateral symmetry; thus, these "segments" are isomers. This glide reflection is also found in Spriggina, another mysterious organism from approximately the same era.
The segments of Dickinsonia have been described by Adolf Seilacher as self-organising "pneu structures", chambers filled with a liquid at higher than ambient pressure, analogous to a quilted air mattress.
Some spectacular fossils attributed to Dickinsonia appear to preserve internal anatomy, believed to represent a tract that both digested food and distributed it throughout the organism.
Since 1947, a total of nine species have been described:
- D. costata Sprigg, 1947 (type species)
- D. minima Sprigg, 1949
- D. spriggi Harrington et Moore, 1955
- D. elongata Glaessner et Wade, 1966
- D. tenuis Glaessner et Wade, 1966
- D. lissa Wade, 1972
- D. brachina Wade, 1972
- D. menneri Keller, 1976 =Vendomia menneri Keller, 1976, redefined as Dickinsonia by Ivantsov, 2007
- D. rex Jenkins, 1992
From these, it is possible to consider only four or five of these nine as valid species:
- D. costata. (junior synonyms D. minima, D. spriggi, and D. elongata; holotype). Unlike other species, D. costata has comparatively fewer, wider segments/isomers.
- D. tenuis (junior synonym D. brachina). Strongly resembles D. costata, but differs from it by more narrow and numerous segments, sparingly lengthened oval form of the body.
- D. lissa is extremely elongated (up to 15 cm), almost ribbon-like in shape, with numerous thin isomers. The isomers of the head area are short in comparison with those of the rest of the body. The fossil bears a distinct axial ledge consisting of two parallel bands extending from the head region to the posterior end of the body.
- D. menneri (junior synonym Vendomia menneri) is a small organism up to 8 mm in length, and strongly resembles juvenile specimens of D. costata with its small number of isomers and well-marked head. D. menneri differs from juvenile D. costata by its slightly more elongated form.
- D. rex was erected for selected paratypes of D. elongata. This species is represented by only several very big specimens (up to more than 1 m in length), and does not have a distinct determination. A large size is the major reason for D. rex's species status. Individuals identified as D. rex may simply be large specimens of D. costata and or D. tenuis.
Discovery and namingEdit
The first species and specimens of this fossil organism was first discovered in the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges in South Australia. Additional specimens of Dickinsonia are also known from the Mogilev Formation in the Dniester River Basin of Podolia, Ukraine and from the Lyamtsa, Verkhovka, Zimnegory and Yorga Formations in the White Sea area of the Arkhangelsk Region, and Chernokamen Fm. of the Central Urals, Russia. These deposits have been dated to 567–550 Myr.
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.
As a rule, 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. 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. 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 redefined as Dickinsonia.
Several trace fossils, including Epibaion and Phyllozoon, have been interpreted as feeding impressions of Dickinsonia and its relatives. Such fossils consist of large, rounded impressions with less relief that the usual upper side of these animals.
The affinities of Dickinsonia are uncertain. It has been variously interpreted as a jellyfish, coral, polychaete worm, turbellarian, mushroom, xenophyophoran protist, sea anemone, lichen, and even a close ancestor of the chordates. Genera such as Yorgia and Marywadea somewhat resemble Dickinsonia, and may be related. However, it is possible that Dickinsonia falls into a group of organisms that became extinct before the Cambrian. Its construction is loosely similar to other Ediacaran organisms, and the similarity of their architecture suggests that dickinsoniamorphs may belong in a clade with Charnia and other rangeomorphs. Paleontologist Adolph Seilacher even went so far as to suggest that most of the Ediacaran fauna represents a separate Kingdom termed "Vendozoa" (now: "Vendobionta") that thrived just before most of the modern multicellular animal phyla appeared in the fossil record.
There is an argument that Dickinsonia is more derived than a sponge, but less so than a eumetazoan. The idea that these organisms could move depends on whether Epibaion is its trace fossil or just a different preservation state. It lacks any convincing evidence for a mouth, anus or gut, and appears to have fed by absorption on its bottom surface. The placozoans are simple animals which feed with their soles and are phylogenetically between sponges and eumetazoa; this suggests that Dickinsonia may have been a stem-group placozoan, or somewhere more crownwards than sponges on the eumetazoan stem.  A study of inferred growth patterns determined that Dickinsonia is a Eumetazoan but a more accurate affiliation was not established. In a subsequent study Hoyal Cuthill & Han (2018) assigned Dickinsonia to the extinct animal group Petalonamae (placed as sister group to the Eumetazoa), which also included the genera Stromatoveris, Arborea and Pambikalbae, as well as rangeomorphs and erniettomorphs.
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