Early Cambrian

Poleta Formation

White Mountains, Inyo County, CA, USA

60mm slab with 3 and 2 partials. Average size 7mm (r11)

Helicoplacoid.

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.