Members of Phylum Hyolitha are enigmatic animals sometimes placed in Superphylum Lophotrochozoa, one of two or three major groups of protostome animals that include brachiopods, annelids, echiura and mulluscs. All members of the Phylum are extinct, and fossils are only known from the Paleozoic. Hyolithids had calcareous shells with a cover (operculum) and two curved supports known as helens. Most were from one to four centimeters in length and their shells are triangular or elliptical in cross section, with some species having rings or striations.

Hyolithids show no obvious resemblance to any extant group making their progenitors and ancestors enigmatic. They were previously placed in their own class in Phylum Mollusca The fossil record gives essentially no information about their internal structures. They were clearly benthic (bottom-dwelling animals), and there is some evidence they were carnivorous.The larval development of hyolithids is inferred to have been closely similar to that of primitive gastropods.

Hyolithids from the Pioche shale in Nevada are sometimes found in association with Olenellid trilobites, testament to their witnessing the radiation of life known as the Cambrian Explosion.