Proarticulata is a proposed phylum of extinct, bilaterally symmetricalanimals known from fossils found in the Ediacaran marine deposits, and dates to approximately. 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. Due to their simplistic morphology, their affinities and mode of life are subject to debate. They are almost universially considered to be metazoans, and due to possessing a clear central axis have been suggested to be stem-bilaterians. In the traditional interpretation, the Proarticulartan body is divided into transverse articulation into 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. 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..
Cephalozoa
These proarticulatans are incompletely segmented, as the anterior zone is free of isomers, often making a "hairband" like appearance. Some cephalozoans from the familyYorgiidae 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.
Dipleurozoa
The dipleurozoan body is subradial, divided by isomers entirely. 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.
Proarticulata ''Incertae sedis''
In Cephalonega stepanovi and Tamga hamulifera all isomers are encircled by a peripheral undivided zone. In the former, the isomers remain in contact with each other, forming a raft-like region, while in the latter, the isomers are isolated 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."
