Dickinsonia

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Dickinsonia
Temporal range: Late Ediacaran, 567–550  Ma
DickinsoniaCostata.jpg
Cast of Dickinsonia costata from Australia
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum:† Proarticulata
Class:† Dipleurozoa
Family:† Dickinsoniidae
Genus:† Dickinsonia
Sprigg, 1947
Type species
†Dickinsonia costata
Sprigg, 1947
Species
  • D. costata(Sprigg, 1947)
  • D. menneri(Keller, 1976)
  • D. tenuis(Glaessner & Wade, 1966)
Synonyms
Genus synonymy
  • Chondroplon? (Wade, 1971) [1]
  • PapilionataSprigg, 1947
  • VendomiaKeller, 1976 [2]
D. costata synonymy
  • Papilionata eyrei(Sprigg, 1947)
  • D. minima(Sprigg, 1949
  • D. elongata(Glaessner & Wade, 1966)
  • D. spriggi(Harrington & Moore, 1955)
D. tenuis synonymy
  • D. brachina(Wade, 1972)
  • D. lissa(Wade, 1972)
  • D. rex(Jenkins, 1992)

Dickinsonia is a genus of extinct organism, most likely an animal, that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It is one of the best known members of the Ediacaran biota. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth has been considered consistent with a stem-group bilaterian affinity, [3] though various other affinities have been proposed. [4] [5] [6] It lived during the late Ediacaran (final part of Precambrian). [7] The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal, [8] though these results have been questioned. [9]

Description

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. [10] 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. [2] [11] The body is divided into two by a midline ridge or groove, [2] [11] [12] except for a single unpaired segment at one end, dubbed the "anterior most unit" suggested to represent the front of the organism. [12] Is it disputed whether the segments are offset from each other following glide reflection, and are thus isomers, [2] [11] [13] [14] or whether the segments are symmetric across the midline, and thus follow true bilateral symmetry, as the specimens displaying the offset may be the result of taphonomic distortion. [12] [15] The number of segments/isomer pairs varies from 12 in smaller individuals to 74 in the largest Australian specimens. [15]

The body of Dickinsonia is suggested to have been sack-like, with the outer layer being made of a resistant but unmineralised material. [14] Some specimens from Russia show the presence of branched internal structures. [16] [14] Some authors have suggested that the underside of the body bore cilia, as well as infolded pockets. [14]

Dickinsonia is suggested to have grown by adding a new pair of segments/isomers at the end opposite the unpaired "anterior most unit". [12] [17] Dickinsonia probably exhibited indeterminate growth (having no maximum size), though it is suggested that the addition of new segments slowed down later in growth. [18] Deformed specimens from Russia indicate that individuals of Dickinsonia could regenerate after being damaged. [17]

Ecology

Dickinsonia is suggested to have been a mobile marine organism that lived on the seafloor and fed by consuming microbial mats growing on the seabed using structures present on its underside. Dickinsonia-shaped trace fossils, presumed to represent feeding impressions, sometimes found in chains demonstrating this behaviour have been observed. [14] These trace fossils have been assigned to the genus Epibaion . [13] [19] [20] A 2022 study suggested that Dickinsonia temporarily adhered itself to the seafloor by the use of mucus, which may have been an adaptation to living in very shallow water environments. [21]

Discovery

The first species and specimens of this fossil organism were first discovered in the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges in South Australia. Reg Sprigg, the original discoverer of the Ediacaran biota in Australia, [22] described Dickinsonia, naming it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg. [23] Additional specimens of Dickinsonia are also known from the Mogilev Formation in the Dniester River Basin of Podolia, Ukraine, [24] the Lyamtsa, Verkhovka, Zimnegory and Yorga Formations in the White Sea area of the Arkhangelsk Region, Chernokamen Formation of the Central Urals, Russia, [10] (these deposits have been dated to 567–550 Myr. [25] [26] [27] ), the Dengying Formation in the Yangtze Gorges area, South China. (ca. 551–543 Ma). [28]

Taphonomy

As a rule, Dickinsonia fossils are preserved as negative impressions ("death masks") 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. [29] [30] 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" [30] on the decaying body, or perhaps it was due to the carbonate cementation of the sand. [31] 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. [13] [19] [32]

Rarely, Dickinsonia have been preserved as a cast in massive sandstone lenses, where it occurs together with Pteridinium , Rangea and some others. [33] [34] [35] [36] These specimens are products of events where organisms were first stripped from the sea-floor, transported and deposited within sand flow. [33] [36] In such cases, stretched and ripped Dickinsonia occur. The first such specimen was described as a separate genus and species, Chondroplon bilobatum [37] and later re-identified as Dickinsonia.

Taxonomy

Species

Since 1947, a total of nine species have been described, of which three are currently considered valid: [38]

SpeciesAuthorityLocationStatusNotesRefs
Dickinsonia brachina Wade (1972) Australiainvalidsynonym of D. tenuis [39]
Dickinsonia costata Sprigg (1947a) Australia, Russia, and Ukrainevalid [upper-alpha 1] [40]
Dickinsonia elongata Glaessner & Wade (1966) Australiainvalidsynonym of D. costata [41]
Dickinsonia lissa Wade (1972) Australiainvalidsynonym of D. tenuis [39]
Dickinsonia menneri  Keller (1976) Russiavalid [upper-alpha 2] [upper-alpha 3] [42]
Dickinsonia minima Sprigg (1949) Australiainvalidsynonym of D. costata [43]
Dickinsonia rex Jenkins (1992) Australiainvalidsynonym of D. tenuis [44]
Dickinsonia spriggi Harrington & Moore (1955) Australiainvalidsynonym of D. costata [45]
Dickinsonia tenuis Glaessner & Wade (1966) Australia and Russiavalid [upper-alpha 4] [41]
  1. ↑ Unlike other species, D. costata has comparatively rounded body and fewer, wider segments / isomers.
  2. ↑ Dickinsonia menneri originally was identified as Vendomia but re-classified as Dickinsonia by Ivantsov (2007) [2]
  3. ↑ D. 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.
  4. ↑ Dickinsonia tenuis strongly resembles D. costata, but differs from it by more narrow and numerous segments, sparingly lengthened oval form of the body.

A claimed specimen of Dickinsonia from India was later determined to be the remains of a beehive. [46]

External relationships

Dickinsonia is classified as part of the group Proarticulata or Dickinsoniomorpha. [14] Proarticulata includes a number of morphologically similar organisms, such Spriggina , Yorgia , Andiva and Cephalonega , which share the same segmented articulation. [47] The affinities of Proarticulata to other organisms, including to other members of the Ediacaran biota, like rangeomorphs, have long been contentious. [8] It has been historically proposed that most Ediacaran organisms were closely related to each other, as part of the grouping "Vendobionta", [5] though recent authors argue that this grouping as a whole is likely to be polyphyletic. [8] Gregory Retallack has proposed that the fossils of Dickinsonia and other Ediacaran biota represent lichens that grew in a terrestrial environment, [48] but this has been broadly rejected by other authors, who argue that a marine environment of deposition better fits available evidence. [49] [8] [50] Other proposal have included giant protists, as proposed by Adolf Seilacher. [51] Most modern research suggest that Dickinsonia and other proarticulatans are likely to be animals, possibly belonging to Eumetazoa. [18] [12] [14] A chemical study of Russian specimens found that they were enriched with cholesterol, which is only produced by animals, supporting an animal affinity, [8] though these results have been questioned by other authors, who consider the association between the cholesterol molecules and the Dickinsonia fossils to not be definitive. [9] Within Animalia, a number of affinities have been proposed, including as stem-eumetazoans forming a clade with rangeomorphs, [52] to Placozoa, [53] and to Cnidaria. [54] A number of researchers have proposed close affinities to Bilateria, based on the bilateral or nearly bilateral organisation of proarticulatans, [14] [3] though proarticulatans are not likely to be a member of the bilaterian crown group. [12]

Related Research Articles

<i>Kimberella</i> Primitive Mollusc-like organism

Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

<i>Tribrachidium</i> Extinct genus of invertebrates

Tribrachidium heraldicum is a tri-radially symmetric fossil animal that lived in the late Ediacaran (Vendian) seas. In life, it was hemispherical in form. T. heraldicum is the best known member of the extinct group Trilobozoa.

<i>Spriggina</i> Extinct genus of annelid worms

Spriggina is a genus of early animals whose relationship to living animals is unclear. Fossils of Spriggina are known from the late Ediacaran period in what is now South Australia. Spriggina floundersi is the official fossil emblem of South Australia; it has been found nowhere else.

<i>Yorgia</i> Extinct proarticulate animal

Yorgia waggoneri is a discoid Ediacaran organism. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of 25 cm (9.8 in). It is classified within the extinct animal phylum Proarticulata.

<i>Cephalonega</i> Extinct genus of invertebrates

Cephalonega stepanovi is a fossil organism from Ediacaran deposits of the Arkhangelsk Region, Russia. It was described by Mikhail A. Fedonkin in 1976

<i>Parvancorina</i> Genus of fossil arnimal

Parvancorina is a genus of shield-shaped bilaterally symmetrical fossil animal that lived in the late Ediacaran seafloor. It has some superficial similarities with the Cambrian trilobite-like arthropods.

<span class="mw-page-title-main">Trilobozoa</span> Extinct phylum of triradially symmetrical animals

Trilobozoa is a phylum of extinct, sessile animals that were originally classified into the Cnidaria. The basic body plan of trilobozoans is often a tri-radial or radial sphere-shaped form with lobes radiating from its centre. Fossils of trilobozoans are restricted to marine strata of the Late Ediacaran period.

<i>Praecambridium</i> Extinct genus of marine animals

Praecambridium sigillum is an extinct organism that superficially resembles a segmented trilobite-like arthropod. It was originally described as being a trilobite-like arthropod, though the majority of experts now place it within the Proarticulata as a close relative of the much larger Yorgia. It is from the Late Ediacaran deposit of Ediacara Hills, Australia, about 555 million years ago. On average, P. sigillum had at least 5 pairs of segments, with each unit becoming progressively larger as they approach the cephalon-like head.

<i>Ovatoscutum</i> Extinct species of enigmatic organism

Ovatoscutum concentricum is one of many enigmatic organisms known from the Ediacaran deposits of the Flinders Ranges, Australia, and the White Sea area in Russia, dating around 555 Ma.

<span class="mw-page-title-main">Ediacaran biota</span> All organisms of the Ediacaran Period (c. 635–538.8 million years ago)

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

<i>Albumares</i> Extinct genus of soft-bodied Trilobozoan

Albumares brunsae is a tri-radially symmetrical fossil animal that lived in the late Ediacaran seafloor. It is a member of the extinct group Trilobozoa.

<span class="mw-page-title-main">Proarticulata</span> Extinct phylum of animals

Proarticulata is a proposed phylum of extinct, near-bilaterally symmetrical animals known from fossils found in the Ediacaran (Vendian) marine deposits, and dates to approximately 567 to 550 million years ago. The name comes from the Greek προ = "before" and Articulata, i.e. prior to animals with true segmentation such as annelids and arthropods. This phylum was established by Mikhail A. Fedonkin in 1985 for such animals as Dickinsonia, Vendia, Cephalonega, Praecambridium and currently many other Proarticulata are described.

<i>Rugoconites</i> Extinct genus of invertebrates

Rugoconites is a genus of Ediacaran biota found as fossils in the form of a circular or oval-like impression preserved in high relief, six or more centimeters in diameter. The fossils are surrounded by frills that have been interpreted as sets of tentacles. The bifurcating radial ribs, spreading from a central dome, serve to distinguish this genus from the sponge Palaeophragmodictya, and may represent the channels of the gastrovascular system. Fossils of Rugoconites have been interpreted as early sponges, although this is countered by Sepkoski et al. (2002), who interpreted the organism as a free-swimming jellyfish-like cnidarian; similar to Ovatoscutum. However, the fossil is consistently preserved as a neat circular form and its general morphology does not vary, therefore a benthic and perhaps slow-moving or sessile lifestyle is more likely. Ivantstov & Fedonkin (2002), suggest that Rugoconites may possess tri-radial symmetry and be a member of the Trilobozoa.

<span class="mw-page-title-main">Vendiamorpha</span> Extinct class of simple animals

Vendiamorpha is a class of extinct animals within the Ediacaran phylum Proarticulata.

<span class="mw-page-title-main">Isomer (Proarticulata)</span> Element in fossils

Isomer is an element of transverse body articulation of the bilateral fossil animals of the Phylum Proarticulata from the Ediacaran (Vendian) period. This term has been proposed by Andrey Yu. Ivantsov, a Russian paleontologist from the Laboratory of the Precambrian organisms, Paleontological Institute, Russian Academy of Sciences.

<i>Andiva</i> Vendian fossil

Andiva ivantsovi is a Vendian fossil, identified to be a bilaterian triploblastic animal in the Ediacaran phylum Proarticulata, known from the Winter Coast, White Sea, Russia. It was first discovered in 1977, and described as a new species in a new genus by Mikhail Fedonkin in 2002. It lived about 555 million years ago. Fossils of Andiva also occur in South Australia. All known fossils of Andiva are external molds.

Palaeoplatoda is a genus from the Ediacaran biota. It is a soft-bodied organism with a segmented body that resembles Dickinsonia, another Ediacaran organism.

<span class="mw-page-title-main">Petalonamae</span> Proposed extinct group of animals

The petalonamids (Petalonamae) are an extinct group of archaic animals typical of the Ediacaran biota, also called frondomorphs, dating from approximately 635 million years ago to 516 million years ago. They are benthic and motionless animals, that have the shape of leaves, fronds (frondomorphic), feathers or spindles and were initially considered algae, octocorals or sea pens. It is now believed that there are no living descendants of the group, which shares a probable relation to the Ediacaran animals known as Vendozoans.

<i>Archaeaspinus fedonkini</i>

Archaeaspinus fedonkini is an extinct proarticulatan organism from the Late Precambrian (Ediacaran) period.

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