Isomer (Proarticulata)
Encyclopedia
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Proarticulata
Proarticulata
Proarticulata is a proposed extinct phylum of very early bilateral animals known from fossils found in the Ediacaran fauna. Mikhail A. Fedonkin describes animals in Proarticulata as having a "shield-like" shape, an unmineralized, thin, flexible carapace and a thick, segmented body...
from the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
(Vendian) period. This term has been proposed by Andrey Yu. Ivantsov, a Russian paleontologist from the Laboratory of the Precambrian organisms, Paleontological Institute, Russian Academy of Sciences.
Morphology
Proarticulatan isomers are distinct from the segmentsSegmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
of the Annelida and Panarthropoda
Panarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
, as each of these elements occupies only half of width of a body and are organized in an alternating pattern relatively to the axis of the body. In other words, although proarticulatans are bilaterally symmetrical, 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 its width. This phenomenon is described as the symmetry of gliding reflection.
The first element in the row is a right isomer on the anterior end of the dorsal side. All successive isomers are similar to it, but gradually decrease in size and in angle of inclination from the anterior part (head) of the body to the posterior end.
New isomers were added at the growth point at the posterior end of the proarticulate body. With age, the addition of new isomers slowed down and probably stopped, the growth point shifted from the posterior end, and the relative length of the posterior isomers increased, sometimes significantly.
The overall number of body isomers amounts from several pairs (Vendia
Vendia
Vendia is an oval-shaped, Ediacaran fossil from 4.5 to 12.5 mm long. Its body is completely segmented into isomers, which are arranged alternately in two rows longitudinal to the axis of the body. The larger isomers cover the smaller ones externally but the posterior ends of all the isomers...
, Onega
Onega (fossil)
Onega stepanovi is one of the lesser known fossil taxa from the Ediacaran Period. It was described by Mikhail A. Fedonkin in 1976and was placed in the Phylum Proarticulata in 1985....
) to several hundreds (Dickinsonia
Dickinsonia
Dickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A...
).
Vendiamorpha
The body is completely segmented, with all isomers curved towards the posterior, and the first isomer is normally much larger than the rest. In some species the first two isomers at the anterior dorsal end are partly fused together to form a headshield-like structure. (e.g., VendiaVendia
Vendia is an oval-shaped, Ediacaran fossil from 4.5 to 12.5 mm long. Its body is completely segmented into isomers, which are arranged alternately in two rows longitudinal to the axis of the body. The larger isomers cover the smaller ones externally but the posterior ends of all the isomers...
, Paravendia, Podolimirus, and Karakhtia).
Cephalozoa
These proarticulatans demonstrate incomplete segmentation, as the anterior zone is free of isomers, often making a "hairband" like appearance. (example cephalozoans include YorgiaYorgia
Yorgia waggoneri is a member of the Ediacara biota, and resembles a cross between the organisms Dickinsonia and Spriggina. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of...
, Praecambridium, Andiva, Archaeaspinus, Ivovicia, Spriggina
Spriggina
Fossils of Spriggina are known from the Ediacaran period, around . The segmented organism reached about 3 cm in length and may have been predatory...
, Marywadea
Marywadea
Marywadea is a genus of Ediacaran biota shaped like an oval with a central ridge. It is a bilaterian organism as evidenced by its symmetry. The fossil has an asymmetrical first chamber of the quilt. It has transverse ridges away from the central axis that may be gonads. The head is shaped as a...
and Cyanorus. Some cephalozoans from the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
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.
Dipleurozoa
The dipleurozoan body is subradial divided by isomers entirely (e.g., DickinsoniaDickinsonia
Dickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A...
, Epibaion
Epibaion
Epibaion is an Ediacaran Proarticulatan imprint, often occurring in chains, that is interpreted as a feeding trace; some chains terminate in a body fossil, allowing their maker to be identified. Several specimens are known; E. waggoneris was produced by Yorgia waggoneri; E. costatus by Dickinsonia...
and Phyllozoon
Phyllozoon
Phyllozoon is an Ediacaran imprint that resembles a proarticulatan and has been interpreted as a feeding trace. It usually occurs in long chains of imprints formed, presumably, as the making organism moved....
). Juveniles Dickinsonia show an undivided anterior area but this region was reduced in the course of ontogeny, and in adult Dickinsonia-like proarticulates changed so strongly that became almost indistinguishable from isomers.