Hyolithids are a poorly understood group of critters. They first appeared in the Cambrian and became extinct during in the Great Dying(Permian-Triassic extinction) at the end of the Paleozoic. The Great Dying was the most severe crisis for the biosphere that the planet has ever experienced… about 95% of all species perished!
There are numerous hypothesis to explain the extinctions(google it), but it was probably a combination of several catastrophic changes that killed off most life.
A few hyolithids that were recovered from the Millersburg mb:
Someday, I will devote a post to the extinction event.