Agmata

Agmata is a proposed extinct phylum of small animals bearing a calcareous, conical shell. They were originally thought to be cephalopods or annelid worms. Living members filled as much as five sixths of their shell with laminae, angled layers built from grains of quartz or calcium carbonate detritus gathered from the environment and cemented together, with larger grains placed near the shell wall and smaller grains toward the center; a very fine tube ran through the shell's core. Although the soft body of the animal is not preserved, it must have been able to find, select, and retrieve grains of specific shapes and materials, ones otherwise rare in the surrounding rock, in order to build these layered structures.