Isomer (Proarticulata)

Isomer (Greek isos = "equal", méros = "part") is an element of transverse body articulation of the bilateral fossil animals of the Phylum Proarticulata from the Ediacaran (Vendian) period.

[1][2][3] However, this type of symmetry is not unique to Proarticulata, a modern lancelets have analogous asymmetric arrangement of myomeres and somites of larvae.

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.

[1][5][7][8] These proarticulatans demonstrate incomplete segmentation, as the anterior zone is free of isomers, often making a "hairband" like appearance.

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.

Yorgia waggoneri demonstrate obvious asymmetry of left and right parts of the body
The Dickinsonia costata is subradial divided by isomers entirely, the anterior part (head) of the body on the right-bottom
Examples of the classes of Proarticulata, including reconstructions of Vendia sokolovi , Dickinsonia costata and Yorgia waggoneri .