Yorgiidae


Yorgiidae is an extinct family of the class Cephalozoa, which is a part of the wider phylum Proarticulata. They lived from around 571 to 551 Ma.

Description

Like other relatives within the phylum Proarticulata, their body plan consists of isomers arranged in a glide symmetry, meaning they do not have true bilateral symmetry. As with other cephalozoans, their initial right isomer extends right across the body, forming a distinctive 'head', whilst they have a unique feature with their initial left isomer extending slightly to the right side and into the initial right isomer.
This is most prominent in the genus Archaeaspinus, with it having an unpaired left lobe/isomer within its 'head' region, forming a distinctive question mark-like shape.

Distribution

Genera of the family Yorgiidae are most commonly found in the Ust' [Pinega Formation] and Central Urals of Russia, as well as the Flinders Ranges of South Australia.

Taxonomy

Yorgiidae includes the following genera:
Due to the rather small size of known Praecambridium specimens, and the large grain size of the matrix they are commonly found in, the placement of Praecambridium within family Yorgiidae is tentative, as it's hard to discern certain details in such small specimens, although most reconstructions show similarities with juvenile Yorgia, and may even be a larval stage. A more recent study also suggested that Praecambridium may be a juvenile, and possibly a juvenile of another cephalozoan, Spriggina, which occurs in the same beds as Praecambridium.
Andiva was previously assigned to the family Yorgiidae, although a recent study removes it from the family but still placed within the class Cephlazoa.