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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".

Morphology

The organisms range from 4 to 1400 millimetres 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. They are thicker at one end than the other, but there is no agreement as to which end is the front, and the organism does not appear to have a "head".

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.

Five species are distinguished; D. costata, D. lissa, D. tenuis, D. rex and D. menneri. They are distinguished mainly on morphometric grounds; when their dimensions are plotted, four distinct arrays are found: for example, D. costata has a narrow midrib and wide segments, and is oval in shape, where D. tenuis has narrow segments, a wide midrib and a long body. D. rex is the largest, and has much thinner/narrower segments than D. costata. D. lissa is extremely elongated, almost ribbon-like in shape. D. menneri (originally Vendomia menneri) differs from all other members of the genus in that the isomers of one end fused together to form a head-like unit. Aside from this, most individuals otherwise resemble very small or juvenile forms of D. costata.

The innards of Dickinsonia are thought to contain spindle-like fibres 0.5-1 mm in diameter.

Fossil record

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 Australiamarker, 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 Rangesmarker of South Australiamarker, as well as Rajastanmarker, Podolia, and the White Seamarker area and Central Uralsmarker of Russiamarker, 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.

Trackway fossils

Arcing trackways of Dickinsonia fossils 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 may also be "tumble tracks" created by an organism rolling along the sea floor, perhaps as it was buffeted by currents. This would require the organism to be symmetrical on its top and bottom, and it's not yet known whether that is the case. The imprints are almost identical, suggesting they were made by one organism—but this is not necessarily the case: they could be the bases of lichens or "mushrooms arranged in fairy rings". However, in some cases these trackway imprints overlap.

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.

Internal anatomy



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. However, this probably represents an artefact of the preservational process; the reported fossils may have been more decayed than usual for Ediacarans, producing a different appearance.

Taphonomy

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, agglutination (although this hypothesis is not mainstream!)

Where Dickinsonia is found to be folded or bent, it is not deformed in a brittle manner, as a "death mask" would be; indeed, it is not very flexible at all.

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.

Ecology

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.

The organisms are preserved in such a way that their resistant parts must have been a sturdy biopolymer (such as keratin) rather than a brittle mineral (such as calcite or a pyritised death mask).

Affinity

Dickinsonia is generally regarded as a member of the Vendazoa — a group of organisms that thrived just before most of the modern multicellular animal phyla appeared in the fossil record. Other vendazoa 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.

Retallack has attempted to use the mode of decay of the fossil as a clue to its affinity. He originally proposed that all of the Ediacara biota were lichens based on their post-burial compaction, but faced strong criticism.His revised opinion reiterates the fact that the decay mode of the organisms is most similar to that of leaves, fungi or lichens, and not at all like soft-bodied animals, which clot and distort as they wilt and decay.

A host of considerations reduces the possible affinities of Dickinsonia to fungi and lichens, if the organism is to be shoe-horned into an extant category of organisms.

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.

References

  1. Seilacher 1989
  2. Dzik 2000??
  3. Retallack, Gregory J. (2004) "Death, Decay and Destruction of Dickinsonia". Geological Society of America Abstracts with Programs, Vol. 36, No. 5, p. 521 [1]
  4. Dzik, Jerzy. (2000) "The Origin of the Mineral Skeleton in Chordates." in Max Knobler Hecht, Ross J. MacIntyre and Michael T. Clegg, eds. Evolutionary Biology Vol. 31. Pp. 105-46. Springer. ISBN 0306461781 [2] – URL retrieved February 10, 2007



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