Proarticulata is a proposed

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclatu ...

of extinct, bilaterally symmetrical

animals Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

known from fossils found in the

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...

(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 (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolo ...

and

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...

. This phylum was established by

Mikhail A. Fedonkin Academician Mikhail Aleksandrovich Fedonkin (russian: Михаи́л Алекса́ндрович Федо́нкин; born June 19, 1946) is a Russian paleontologist specializing in documentation of the earliest animals' body fossils, tracks, a ...

in 1985 for such animals as '' Dickinsonia'', ''

Vendia ''Vendia'' is a genus of oval-shaped, Ediacaran fossils ranging from 4.5 to 12.5 mm long. The body is completely segmented into isomers, which are arranged alternately in two rows longitudinal to the axis of the body. The larger isomers co ...

'', '' 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. 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 formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. ...

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''). 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., ''

'', ''

Paravendia ''Paravendia'' is an extinct genus of proarticulate vendiamorph that lived in the Ediacaran period, about 553 million years ago. It shares the Vendiidae family with ''Vendia'' and ''Karakhtia ''Karakhtia nessovi'' is a species of Proar ...

'' and ''

Karakhtia ''Karakhtia nessovi'' is a species of Proarticulate from the Ediacaran period, around 555 Million Years Ago. K. nessovi is the only species in the genus ''Karakhtia''. The genus Haootia has been compared minorly to ''Karakhtia'' in the way tha ...

'').

Cephalozoa

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

Tamga A tamga or tamgha (from otk, 𐱃𐰢𐰍𐰀, tamga, lit=stamp, seal; tr, damga; mn, tamga; ; ); an abstract seal or stamp used by Eurasian nomads and by cultures influenced by them. The tamga was normally the emblem of a particular tribe, ...

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

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Yorgiidae demonstrate pronounced asymmetry of 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 ''Tamga hamulifera'' is a disk-shaped fossil from Precambrian strata of the White Sea area, in Russia, the only member of the genus ''Tamga''. Description The small (3–5 mm in length), disk-shaped fossil has seven hook-like isomers in ...

'' 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 ''

''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., '' Dickinsonia'' 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 Proarticulata

Body fossils

*'' Armillifera'' Fedonkin, 1980 :''A. parva'' Fedonkin, 1980 *'' Andiva'' Fedonkin, 2002 :''A. ivantsovi'' Fedonkin, 2002 *'' Archaeaspinus'' Ivantsov, 2007 (=''Archaeaspis'' Ivantsov, 2001) :''A. fedonkini'' Ivantsov, 2001 *'' Cephalonega'' Ivantsov ''et al.'', 2019 :''C. stepanovi'' (Fedonkin, 1976) *''

Chondroplon ''Chondroplon bilobatum'' is a medusoid Ediacaran fossil. It has sand-filled tubes, although these may not have been sand-filled in life. It has a shield-like shape, with one end different from the other, and bilateral symmetry, and although it ...

'' Wade, 1971 (possible =''Dickinsonia'') :''C. bilobatum'' Wade, 1971 *'' Cyanorus'' Ivantsov, 2004 :''C. singularis'' Ivantsov, 2004 *'' Dickinsonia'' Sprigg, 1947 :''D. costata'' Sprigg, 1947 :''D. lissa'' Wade, 1972 :''D. menneri'' Keller 1976 (=''Vendomia menneri'' Keller 1976) :''D. tenuis'' Glaessner & Wade, 1966 *'' Ivovicia'' Ivantsov, 2007 :''I. rugulosa'' Ivantsov, 2007 *''

'' 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 *''

'' Ivantsov, 2004 :''P. janae'' Ivantsov, 2001 (=''Vendia janae'' Ivantsov, 2001) *'' Phyllozoon'' Jenkins & Gehling, 1978 :''P. hanseni'' Jenkins & Gehling, 1978 *'' Podolimirus'' Fedonkin, 1983 (=''Valdainia'' Fedonkin, 1983) :''P. mirus'' Fedonkin, 1983 (''Valdainia plumosa'' Fedonkin, 1983) *'' Praecambridium'' Glaessner & Wade, 1966 :''P. siggilum'' Glaessner & Wade, 1966 *'' Spriggina'' Glaessner, 1958 :''S. floundersi'' Glaessner, 1958 *''

'' Ivantsov, 2007 :''T. hamulifera'' Ivantsov, 2007 *''

'' Keller, 1969 :''V. sokolovi'' Keller, 1969 :''V. rachiata'' Ivantsov, 2004 *? '' Windermeria'' Narbonne, 1994 :''W. aitkeni'' Narbonne, 1994 *'' Yorgia'' Ivantsov, 1999 :''Y. waggoneri'' Ivantsov, 1999

Trace fossils

*'' Epibaion'' 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''

References

External links

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