The Ediacaran Fossil Dickinsonia
Dickinsonia is a genus of extinct fossil organisms found in the latter portion of the Ediacaran Period (635-543 Ma). Species in this genus belong to what is known as the 'Ediacaran Biota' - a diverse group of macroscopic, multicellular marine organisms that appear, and then disappear, in the fossil record right before the Cambrian radiation of animals. While their taxonomic affinity is uncertain, Dickinsonia and fossils of its kind represent a major transition in the history of life - from seas populated mainly by microscopic organisms to those populated by macroscopic ones as well.
Dickinsonia is an iconic fossil of the Ediacaran biota. It (roughly) resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".
Since 1947, a total of nine species have been described:
- D. costata Sprigg, 1947
- 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, redescribed by Ivantsov, 2007 (=Vendomia menneri Keller 1976)
- D. rex Jenkins, 1992
From these, it is possible to consider only 4 or 5 of these 9 as valid species:
D. lissa is very elongated (up to 15 cm), almost ribbon-like in shape, with the numerous thin isomers, the head and adjoining to it isomers are short. 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 is a small organism up to 8 mm in long, resembles juvenile D. costata by the small number of isomers and well-marked head formed through fused or undivided isomers. D. menneri differs from D. costata by its somewhat elongated form.
D. rex. This form was selected from the paratypes of the D. elongata. This Dickinsonia represented by only several very big specimens (up to more than 1 m in length), and has not a distinct determination. Big size is a major reason for select it into independent species and actually can be large specimens of the D. costata and/or D. tenuis.
The organisms range from first millimetres to 1.4 metres in length, and are ovoid in outline. They consist of a number of rib-like segments emerging from a central groove or ridge; these ribs interdigitate, producing a glide symmetry.
The segments of Dickinsonia have been described as "pneus", chambers filled with a liquid at higher than ambient pressure, analogous to a quilted air mattress. Features in a few specimens have been interpreted as evidence of longitudinal muscle fibers, and a medial gut, but this interpretation has not reached acceptance.
Dickinsonia was first described by Reg Sprigg, the original discoverer of the Ediacaran biota in Australia, who named it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg.
Dickinsonia is known from unskeletonised impressions in late Ediacaran quartz sandstones in Ediacara and elsewhere in the Flinders Ranges of South Australia, as well as, Podolia of Ukraine, and the White Sea area and Central Urals of Russia, and has an estimated time range of 560-555 Myr.
Dickinsonia is a "resistant" fossil – that is to say, it is preserved as a (usually concave) cast on the underside of overlying bedding planes—unlike most Ediacaran fronds. Where part and counterparts of the same impression are known, they are separated by as much as 3 mm, with the ribbing most prominent on the top surface; this suggests that the ornament was displayed on the top surface only, and that underlying sand supported the impression.
Arcing trackways of Dickinsonia fossils, termed Epibaion, have been found, but their interpretation too is insecure. They may be impressions the organism made while it rested on the sediment surface – perhaps by secreting slime in order to form a platform on the underlying microbial mat, or by sitting and dissolving the underlying microbes in order to devour them. They have also been interpreted as "tumble tracks" created by an organism rolling along the sea floor, perhaps as it was buffeted by currents, and as the bases of lichens or "mushrooms arranged in fairy rings". However, in some cases these trackway imprints overlap. Ridges apparently produced by the channelling of sediment in digestive tubes seem to indicate that the trackways do indeed represent feeding traces; the sedimentary disturbance expected of tumbling-induced impressions is not observed.
Body fossil interactions
Halo-like "reaction rims" surround specimens. Adjacent specimens deform as if to avoid entering their neighbour's halo, suggesting they competed with one another. No body fossils have been found to overlap.
Some spectacular fossils which can be attributed to Dickinsonia appear to preserve internal anatomy, believed to represent a tract that both digested food and distributed it throughout the organism.
The organisms are preserved in positive or negative relief, usually in coarse sandstone, and are usually preserved by virtue of imprinting on microbial mats, though their preservation may also reflect the abundance of aerobic environments or microbial pyritisation in the Ediacaran era—or, if they are protists, possibly agglutination (although this hypothesis is not mainstream).
The height of the specimens preserved bears little relation to their length or width, suggesting that the mode of decay resembled that of a lichen, leaf or mushroom. Assuming their pneus were originally cylindrical, they were more rigid than worms, jellyfish or logs.
Organisms of all sizes are found on bedding plane assemblages; this shows that they were commonly preserved in life position, as currents would preferentially remove smaller specimens. Further, their preservation on the top of certain sedimentary structures shows that they must have been firmly attached to the substrate at their time of burial.
Dickinsonia is found in sedimentary beds 8 mm thick; allowing for compaction, this allows these specimens a maximum height of 1 cm.
The organisms displayed isometric, indeterminate growth – that is to say, they kept on expanding until they were covered with sediment or otherwise killed. They spent most, if not all, of their lives with most of their bodies firmly anchored to the sediment, although they may have moved from resting-place to resting-place. Their mode of anchorage may have been oyster-like concretion, lichen-like rooting with rhizines, or fungus-like attachment to an underground network of hyphæ.
Dickinsonia is generally regarded as a member of the Vendobionta — a group of organisms that thrived just before most of the modern multicellular animal phyla appeared in the fossil record. Other Vendobionta such as Yorgia and Marywadea somewhat resemble Dickinsonia, and may be related.
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.
However, it is possible that Dickinsonia falls into a group of organisms that went 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.
There is a strong argument that the organism is more derived than a sponge, but less so than a eumetazoan. The organism could clearly move, evidenced by its association with trackways which could only have been produced by feeding. However, 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. 
Gregory Retallack originally proposed that some Ediacaran fossils were lichens based on their unusual resistance to post-burial compaction, but faced heavy criticism. He proposes that the decay mode of the organisms is most similar to that of leaves, fungi or lichens, unlike soft-bodied animals which clot and distort as they wilt and decay. A detailed study of paleosols with Dickinsonia preserved in life position suggests, according to Retallack, that Dickinsonia could have lived on dry land.
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