Proarticulata is a proposedphylum of extinct, near-bilaterally symmetricalanimals known from fossils found in theEdiacaran (Vendian) marine deposits, and dates to approximately567 to 550 million years ago.[1][2] The name comes from the Greekπρο (pro-) = "before" andArticulata, i.e. prior to animals with true segmentation such asannelids andarthropods. This phylum was established byMikhail A. Fedonkin in 1985 for such animals asDickinsonia,Vendia,Cephalonega,Praecambridium[3] and currently many other Proarticulata are described (see list).[4][5]
Due to their simplistic morphology, their affinities and mode of life are subject to debate. They are almost universally considered to bemetazoans, 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) intoisomers as distinct from the transverse articulationsegments 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.[5] 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 ofgliding reflection.[6][7] Some recent research suggests that some proarticulatans likeDickinsonia have genuine segments, and the isomerism is superficial and due totaphonomic distortion.[8] However, other researchers dispute this.[9][10] Displacement of left-right axis is known in bilaterians, notablylancelets.[11][12]
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 andKarakhtia).[6][13][14][15]
These proarticulatans are incompletely segmented, as the anterior zone is free of isomers, often making a "hairband" like appearance (example cephalozoans includeYorgia,Praecambridium,Andiva,Archaeaspinus,Ivovicia,Podolimirus,Tamga,Spriggina,Marywadea andCyanorus).[6][13][15][16] Some cephalozoans from thefamilyYorgiidae 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.[6][7][15]
Artist's reconstruction ofCephalonega stepanovi.[17]Artist's reconstruction ofLossinia feeding on surface algae.
InCephalonega stepanovi andTamga hamulifera the zone containing the isomers is encircled by a peripheral, undivided zone.[16] TheCephalonega's isomers are connected to each other, forming a body resembling a rubber raft; theTamga's isomers are separated from each other, and do not touch.
InLossinia, 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."[16]
The dipleurozoan body is subradial, divided by isomers entirely (e.g.,Dickinsonia andPhyllozoon).Dickinsonia juveniles show undivided anterior areas but these regions were reduced in the course of ontogeny, and in the adult stagesDickinsonia-like proarticulates changed so radically that they became almost indistinguishable from isomers.[13][16][18]
^Fedonkin MA (1985). "Systematic Description of Vendian Metazoa". In Sokolov BS, Iwanowski AB (eds.).Vendian System: Historical–Geological and Paleontological Foundation. Vol. 1: Paleontology. Moscow: Nauka. pp. 70–106.
^Ivantsov Y, Fedonkin MA, Nagovitsyn AL, Zakrevskaya MA (September 2019). "Cephalonega, A New Generic Name, and the System of Vendian Proarticulata".Paleontological Journal.53 (5):447–454.doi:10.1134/s0031030119050046.S2CID203853224.
^Ivantsov AY (December 2010). "Paleontological evidence for the supposed precambrian occurrence of mollusks".Paleontological Journal.40 (12):1552–1559.doi:10.1134/S0031030110120105.S2CID86523806.
