Dickinsonia

Dickinsonia Temporal range: Ediacaran, 560–555 Ma PreЄ Є O S D C P T J K Pg N ↓
A fossil and reconstruction of D. costata
Scientific classification
Kingdom:	Animalia
Phylum:	Proarticulata
Class:	Dipleurozoa[1]
Family:	Dickinsoniidae
Genus:	Dickinsonia Sprigg, 1947
Species
D. costata Sprigg, 1947 D. lissa Wade, 1972 D. tenuis Glaessner & Wade, 1966 D. menneri Keller 1976,[2] [reinterpretation of Vendomia menneri Keller 1976][3] D. rex Glaessner & Wade, 1966 See text
Synonyms
Vendomia[2]

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".

Species variety

Schematic reconstructions of Dickinsonia costata, D. lissa, D. tenuis, D. menneri, D. sp. and Ivovicia rugulosa

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^[2] (=Vendomia menneri Keller 1976^[3])
• D. rex Jenkins, 1992

From these, it is possible to consider as reliable 4 or 5 only:

D. costata. (D. minima, D. spriggi, and D. elongata (holotype) are its junior synonyms). Unlike other species, D. costata has fewer, wider segments/isomers.

D. tenuis (D. brachina is it junior synonyms). Strongly resembles D. costata, differs from it by more narrow and numerous segments, sparingly lengthened oval form of the body.

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 long), 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.

Morphology

Ontogeny of the Dickinsonia costata

The organisms range from 4 millimetres to 1.4 metres in length,^[4] 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.^[5] Features in a few specimens have been interpreted as evidence of longitudinal muscle fibers,^{[citation needed][6]} and a medial gut,^{[citation needed]} but this interpretation has not reached acceptance.

The innards of Dickinsonia are thought to contain spindle-like fibres 0.5–1 mm in diameter.^[4]

Fossil record

Dickinsonia was first described by Reg Sprigg, the original discoverer of the Ediacaran biota in Australia,^[7] 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^[4] 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.^[8]

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, termed Epibaion, have been found,^[9] 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,^[4] or by sitting and dissolving the underlying microbes in order to devour them.^[4][10] They have also been interpreted as "tumble tracks" created by an organism rolling along the sea floor, perhaps as it was buffeted by currents,^[4] and as the bases of lichens or "mushrooms arranged in fairy rings".^[4] 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.^[11]

Body fossil interactions

Halo-like "reaction rims" surround specimens.^[4] Adjacent specimens deform as if to avoid entering their neighbour's halo, suggesting they competed with one another.^[4] No body fossils have been found to overlap.^[4]

Internal anatomy

The structure of some Dickinsonia specimens has been interpreted as a putative "digestive–distributive" system. Image after Ivantsov 2004.

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.^[12]

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, possibly agglutination (although this hypothesis is not mainstream).^[4]

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.^[4]

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.^[4] Assuming their pneus were originally cylindrical, they were more rigid than worms, jellyfish or logs.^[13]

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.^[4] 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.^[4]

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.^[4] 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.^{[citation needed]} 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.^[4]

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,^[4][14] and even a close ancestor of the chordates.^[15]

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,^[13] but faced strong criticism.^[16] 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.^[4]

A host of considerations^{[clarification needed]} reduces the possible affinities of Dickinsonia to fungi and lichens, if the organism is to be shoe-horned into an extant category of organisms.^{[citation needed]}

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.^[17]

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.^[18] 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. ^[18][19]

See also

• List of Ediacaran genera

References

1. ^ Harrington, N. J. and Moore. R. C. (1955). "Kansas Pennsylvanian and other jellyfishes". Bull. Kansas geol. Surv. 114 (5): 153–163. http://www.kgs.ku.edu/Publications/Bulletins/114_5/index.html. 
2. ^ ^{a b c} Ivantsov, A. Yu (2007). "Small Vendian transversely Articulated fossils". Paleontological Journal 41 (2): 113–122. doi:10.1134/S0031030107020013. 
3. ^ ^{a b} B. M. Keller and M. A. Fedonkin (1976). "New Records of Fossils in the Valdaian Group of the Precambrian on the Syuz’ma River" (in Russian) (PDF). Izv. Akad. Nauk SSSR, Ser. Geol. 3: 38–44. http://vend.paleo.ru/pub/Keller_Fedonkin_1976.pdf. 
4. ^ ^{a b c d e f g h i j k l m n o p q r s} Retallack, G.J. (2007). "Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil" (PDF). Alcheringa: an Australasian Journal of Palaeontology 31 (3): 215–240. doi:10.1080/03115510701484705. http://www.informaworld.com/index/781217204.pdf. Retrieved 2007-11-24. 
5. ^ Seilacher 1989
6. ^ Dzik 2000??
7. ^ Sprigg, Reg C. (1947). "Early Cambrian (?) Jellyfishes from the Flinders Ranges, South Australia" (PDF). Trans. Roy. Soc. S. Aust. 71: 212–24. http://www.samuseum.sa.gov.au/Journals/TRSSA/TRSSA_v071/trssa_v071_p212p224.pdf. 
8. ^ Grazhdankin, Dima (2004). "Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution" (PDF). Palæobiology 30 (2): 203–221. doi:10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2. http://paleobiol.geoscienceworld.org/cgi/reprint/30/2/203.pdf. Retrieved 2007-03-08. 
9. ^ Ivantsov, A.Y.; Malakhovskaya, Y.E. (2002). "Giant Traces of Vendian Animals" (PDF). Doklady Earth Sciences 385 (6): 618–622. http://vend.paleo.ru/pub/Ivantsov_et_Malakhovskaya_2002-e.pdf. Retrieved 2008-02-24. 
10. ^ Jensen, Sören; Droser, Mary L.; Gehling, James G. (2005). "Trace fossil preservation and the early evolution of animals". Palaeogeography, Palaeoclimatology, Palaeoecology 220 (1–2): 19–29. doi:10.1016/j.palaeo.2003.09.035. 
11. ^ Ivantsov, A. Yu. (2011). "Feeding traces of Proarticulata — the Vendian metazoa". Paleontological Journal 45 (3): 237–248. doi:10.1134/S0031030111030063.  edit
12. ^ Ivantsov, A. Yu (2004). "New Proarticulata from the Vendian of the Arkhangel'sk Region" (PDF). Paleontological Journal 38 (3): 247–253. http://vend.paleo.ru/pub/Ivantsov_2004_eng.pdf. 
13. ^ ^{a b} Retallack, G.J. (1994). "Were the Ediacaran fossils lichens?". Paleobiology 17: 523–544. ISSN 0094-8373. 
14. ^ Retallack, Gregory J. (2004) "Death, Decay and Destruction of Dickinsonia". Geological Society of America Abstracts with Programs, Vol. 36, No. 5, p. 521 [1]
15. ^ 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
16. ^ Waggoner, B.; Collins, A.G. (2004). "Reductio Ad Absurdum: Testing The Evolutionary Relationships Of Ediacaran And Paleozoic Problematic Fossils Using Molecular Divergence Dates". Journal of Paleontology 78 (1): 51–61. doi:10.1666/0022-3360(2004)078<0051:RAATTE>2.0.CO;2. ISSN 0022-3360. 
17. ^ Brasier, M.; Antcliffe, J. (2004). "Decoding the Ediacaran Enigma". Science 305 (5687): 1115–1117. doi:10.1126/science.1102673. PMID 15326344. 
18. ^ ^{a b} Sperling, Erik; Vinther, Jakob; Pisani, Davide; Peterson, Kevin (2008). "A placozoan affinity for Dickinsonia and the evolution of Late Precambrian metazoan feeding modes". In Cusack, M; Owen, A; Clark, N. Programme with Abstracts. 52. Palaeontological Association Annual Meeting. Glasgow, UK. p. 81. http://downloads.palass.org/annual_meeting/2008/Glasgow2008abstracts.pdf. 
19. ^ Sperling, Erik; et al. (2008). "A Placozoan Affinity for Dickinsonia and the Evolution of Late Precambrian Metazoan Feeding Modes". Geological Society of America Abstracts with Programs 40 (6): 508. http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_151743.htm. 

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