Dickinsonia
Encyclopedia
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
".
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.
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.
The innards of Dickinsonia are thought to contain spindle-like fibres 0.5–1 mm in diameter.
, 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.
, 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.
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.
in the Ediacaran era—or, if they are protist
s, possibly 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.
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).
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 eumetazoa
n. 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.
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
".
Species variety
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 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
The organisms range from 4 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
Air mattress
An air mattress is an inflatable mattress/sleeping pad. Due to its buoyancy, it is also often used as a water toy/flotation device, and in UK is termed as a lilo .-For sleeping:...
. Features in a few specimens have been interpreted as evidence of longitudinal muscle fibers, and a medial gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
, but this interpretation has not reached acceptance.
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 SpriggReg Sprigg
Reginald Claude Sprigg, AO, HonDSc ANU, HonDSc Flinders, MSc Adelaide, FTSE was an Australian geologist and conservationist. At age 17 he became the youngest Fellow of the Royal Society of South Australia. In 1946, in the Ediacara Hills, South Australia he discovered the Ediacara biota, an...
, the original discoverer of the Ediacaran biota in Australia, who named it after Ben Dickinson, then Director of Mines for South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, and head of the government department that employed Sprigg.
Dickinsonia is known from unskeletonised impressions in late Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
quartz sandstones in Ediacara
Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around 650 km north of Adelaide. The area has many old copper and silver mines from mining activity in the late 19th century...
and elsewhere in the Flinders Ranges
Flinders Ranges
Flinders Ranges is the largest mountain range in South Australia, which starts approximately north west of Adelaide. The discontinuous ranges stretch for over from Port Pirie to Lake Callabonna...
of South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, as well as, Podolia
Podolia
The region of Podolia is an historical region in the west-central and south-west portions of present-day Ukraine, corresponding to Khmelnytskyi Oblast and Vinnytsia Oblast. Northern Transnistria, in Moldova, is also a part of Podolia...
of Ukraine
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
, and the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
area and Central Urals of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, 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, termed EpibaionEpibaion
Epibaion is an Ediacaran Proarticulatan imprint, often occurring in chains, that is interpreted as a feeding trace; some chains terminate in a body fossil, allowing their maker to be identified. Several specimens are known; E. waggoneris was produced by Yorgia waggoneri; E. costatus by Dickinsonia...
, 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.Internal anatomy
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 pyritisationPyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
in the Ediacaran era—or, if they are protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s, possibly agglutination
Agglutination (biology)
Agglutination is the clumping of particles. The word agglutination comes from the Latin agglutinare, meaning "to glue."This occurs in biology in three main examples:...
(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
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
) rather than a brittle mineral (such as calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
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 YorgiaYorgia
Yorgia waggoneri is a member of the Ediacara biota, and resembles a cross between the organisms Dickinsonia and Spriggina. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of...
and Marywadea
Marywadea
Marywadea is a genus of Ediacaran biota shaped like an oval with a central ridge. It is a bilaterian organism as evidenced by its symmetry. The fossil has an asymmetrical first chamber of the quilt. It has transverse ridges away from the central axis that may be gonads. The head is shaped as a...
somewhat resemble Dickinsonia, and may be related.
The affinities of Dickinsonia are uncertain. It has been variously interpreted as a jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
, polychaete worm, turbellarian, mushroom, xenophyophoran
Xenophyophore
Xenophyophores are marine protozoa, giant single-celled organisms found throughout the world's oceans, at depths of up to 10,641 meters . Xenophyophores are found in the greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges in 1889, then as testate...
protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
, sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
, lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
, 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
Charnia
Charnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...
and other rangeomorphs.
There is a strong argument that the organism is more derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
than a sponge, but less so than a eumetazoa
Eumetazoa
Eumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage...
n. 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.