Helicoplacus gilberti
Early Cambrian
Poleta Formation
Inyo County, CA, USA

Two specimens 22mm total on 66mm slab

Helicoplacus is a genus of early echinoderms of the Class Helicoplacoidea.  Isolated plates of helicoplacoids have been found at a few different sites around the world but the group is known from complete specimens in the White Mountains, California.

It seems the helicoplacoids appeared during the Early Cambrian and died out before the Middle Cambrian.

See: https://en.wikipedia.org/wiki/Helicoplacoidea (Opens in new tab)