Proarticulata is a proposed

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

of extinct, near-bilaterally symmetrical

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...

known from fossils found in the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

(Vendian) marine deposits, and dates to approximately . The name comes from the Greek () = "before" and Articulata, i.e. prior to animals with true segmentation such as

annelids The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

and

arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

. This phylum was established by Mikhail A. Fedonkin in 1985 for such animals as ''

Dickinsonia ''Dickinsonia'' is a genus of extinct organism that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It had a round, bilaterally symmetric body with multiple segments running along it. It could range f ...

'', '' Vendia'', '' Cephalonega'', '' Praecambridium'' and currently many other Proarticulata are described (see list). Due to their simplistic morphology, their affinities and mode of life are subject to debate. They are almost universally considered to be metazoans, and due to possessing a clear central axis have been suggested to be stem-

bilaterians Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...

. In the traditional interpretation, the Proarticulatan body is divided into transverse articulation (division) into

isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the existence or possibili ...

as distinct from the transverse articulation segments in annelids and arthropods, as their individual isomers occupy only half the width of their bodies, and are organized in an alternating pattern along the longitudinal axis of their bodies. In other words, one side is not the direct mirror image of its opposite (''

chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...

''). Opposite isomers of left and right side are located with displacement of half of their width. This phenomenon is described as the symmetry of gliding reflection. Some recent research suggests that some proarticulatans like ''Dickinsonia'' have genuine segments, and the isomerism is superficial and due to taphonomic distortion. However, other researchers dispute this. Displacement of left-right axis is known in bilaterians, notably lancelets.

Morphology

Vendiamorpha

The body is completely segmented, with all isomers curved towards the posterior, and the first isomer is normally much larger than the rest. The first two isomers at the anterior dorsal end are partly fused. (e.g., '' Vendia'', '' Paravendia'' and '' Karakhtia'').

Cephalozoa

These proarticulatans are incompletely segmented, as the anterior zone is free of isomers, often making a "hairband" like appearance (example cephalozoans include ''

Yorgia ''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 . It is classified within the extinct animal phylum Proart ...

'', '' Praecambridium'', ''

Andiva ''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 ...

'', ''

Archaeaspinus ''Archaeaspinus fedonkini'' is an extinct proarticulatan organism from the Late Ediacaran period. Background ''Archaeaspinus'' was discovered in Zimnii Bereg, the Winter Coast of the White Sea in Russia, by A. Yu. Ivantsov in 2001. Since the ...

'', '' Ivovicia'', '' Podolimirus'', '' Tamga'', ''

Spriggina ''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 o ...

'', '' Marywadea'' and '' Cyanorus''). Some cephalozoans from the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Yorgiidae demonstrate pronounced asymmetry of the left and right parts of the body. For instance, ''Yorgia''’s initial right isomer is the only one which spreads far towards the left side of the body. ''Archaeaspinus'' has an unpaired anterior lobe confined by the furrow to the left side only. left, Artist's reconstruction of '' Cephalonega stepanovi''. Artist's reconstruction of '' Lossinia'' feeding on surface algae. In '' Cephalonega stepanovi'' and '' Tamga hamulifera'' the zone containing the isomers is encircled by a peripheral, undivided zone. The '' Cephalonega''s isomers are connected to each other, forming a body resembling a rubber raft; the '' Tamga''s isomers are separated from each other, and do not touch. In '' Lossinia'', the center undivided region has no visible isomers, instead having the lobe-like isomers emanate from the periphery of the undivided region as "transverse articulations."

Dipleurozoa

The dipleurozoan body is subradial, divided by isomers entirely (e.g., ''

'' and '' Phyllozoon''). ''Dickinsonia'' juveniles show undivided anterior areas but these regions were reduced in the course of ontogeny, and in the adult stages ''Dickinsonia''-like proarticulates changed so radically that they became almost indistinguishable from isomers.

List of Proarticulates

Body fossils

*'' Armillifera'' Fedonkin, 1980 :''A. parva'' Fedonkin, 1980 *''

'' Fedonkin, 2002 :''A. ivantsovi'' Fedonkin, 2002 *''

'' Ivantsov, 2007 (=''Archaeaspis'' Ivantsov, 2001) :''A. fedonkini'' Ivantsov, 2001 *'' Cephalonega'' Ivantsov ''et al.'', 2019 :''C. stepanovi'' (Fedonkin, 1976) *'' Chondroplon'' Wade, 1971 (possible =''Dickinsonia'') :''C. bilobatum'' Wade, 1971 *'' Cyanorus'' Ivantsov, 2004 :''C. singularis'' Ivantsov, 2004 *''

'' Sprigg, 1947 :''D. costata'' Sprigg, 1947 :''D. menneri'' Keller 1976 (=''Vendomia menneri'' Keller 1976) :''D. tenuis'' Glaessner & Wade, 1966 *'' Ivovicia'' Ivantsov, 2007 :''I. rugulosa'' Ivantsov, 2007 *'' Karakhtia'' Ivantsov, 2004 :''K. nessovi'' Ivantsov, 2004 *'' Lossinia'' Ivantsov, 2007 :''L. lissetskii'' Ivantsov, 2007 *'' Marywadea'' Glaessner, 1976 :''M. ovata'' Glaessner & Wade, 1966 *'' Ovatoscutum'' Glaessner & Wade, 1966 :''O. concentricum'' Glaessner & Wade, 1966 *'' Paravendia'' Ivantsov, 2004 :''P. janae'' Ivantsov, 2001 (=''Vendia janae'' Ivantsov, 2001) *'' Podolimirus'' Fedonkin, 1983 (=''Valdainia'' Fedonkin, 1983) :''P. mirus'' Fedonkin, 1983 (''Valdainia plumosa'' Fedonkin, 1983) *'' Praecambridium'' Glaessner & Wade, 1966 :''P. siggilum'' Glaessner & Wade, 1966 *''

'' Glaessner, 1958 :''S. floundersi'' Glaessner, 1958 *'' Tamga'' Ivantsov, 2007 :''T. hamulifera'' Ivantsov, 2007 *'' Vendia'' Keller, 1969 :''V. sokolovi'' Keller, 1969 :''V. rachiata'' Ivantsov, 2004 *? '' Windermeria'' Narbonne, 1994 :''W. aitkeni'' Narbonne, 1994 *''

'' Ivantsov, 1999 :''Y. waggoneri'' Ivantsov, 1999

Trace fossils

*''

Epibaion ''Epibaion'' is a trace fossil imprint of the Ediacaran animals of the phylum Proarticulata, which became extinct in the Precambrian. Imprints often occurring in chains, that is interpreted as a feeding trace; some chains terminate in a body fos ...

'' Ivantsov, 2002 :''E. axiferus'' Ivantsov, 2002. :''E. waggoneris'' Ivantsov, 2011. This is a trace of ''Yorgia waggoneri'' :''E. costatus'' Ivantsov, 2011. This is a trace of ''Dickinsonia costata'' *'' Phyllozoon'' Jenkins & Gehling, 1978 :''P. hanseni'' Jenkins & Gehling, 1978

References

External links

Database of Ediacaran Biota
Advent of Complex Life {{Taxonbar, from=Q2566978 Bilaterian phyla Vendobionta