Hyolitha

Hyolitha are an extinct class of marine animals with cone-shaped shells and opercula (lids). They are known from the early Cambrian period to the Permian-Triassic extinction event. Their phylogenetic relationships are debated, with proposed affinities including molluscs, brachiopods, or a stem group to either of these phyla, or even a unique phylum on their own.

Hyoliths typically consist of a conical shell, often composed of calcium phosphate, and a calcareous operculum used to close the shell opening. Some species also possess a pair of curved, rigid structures called helens, the function of which remains uncertain, but are often interpreted as supports, stilts, or anchors.

The conical shells are typically a few millimeters to several centimeters in length. Their cross-sections can be circular, oval, or dorsoventrally flattened. The operculum fits neatly into the shell aperture and is often ornamented. The internal anatomy of hyoliths is poorly understood, but some fossil specimens preserve evidence of soft tissues, providing clues to their lifestyle.

Hyoliths are found in marine sediments worldwide, and their fossils are valuable for biostratigraphy and paleoenvironmental reconstructions. Due to their small size and relatively common occurrence, they are important components of many Cambrian fossil assemblages. The debate surrounding their phylogenetic position continues to drive research into their morphology, taphonomy, and ecological roles within ancient marine ecosystems.