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Proarticulata is a phylum of extinct, near-bilaterally symmetrical animals known from fossils found in the Ediacaran (Vendian) 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 (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 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 Glide reflection. Some recent research suggests that some proarticulatans like Dickinsonia have genuine segments, and the isomerism is superficial and due to Taphonomy. However, other researchers dispute this. Displacement of left-right axis is known in bilaterians, notably lancelets.
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."
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, Praecambridium, Andiva, Archaeaspinus, Ivovicia, Podolimirus, Tamga, Spriggina, Marywadea and Cyanorus).
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 Proarticulates
Body fossils
Trace fossils
See also
External links
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