Hyoliths

Hyoliths are an extinct group of marine invertebrates that lived from the Early Cambrian to the Permian periods. Their classification has been a subject of debate for a long time, but they are now generally considered to be a distinct phylum, Hyolitha, or possibly related to mollusks. The name "hyolith" comes from the Greek words "hyo" meaning "cup" or "bowl" and "lithos" meaning "stone," referring to their characteristic shell.

The most recognizable feature of a hyolith is their conical shell, often referred to as a theca. This shell is typically straight and tapers to a point, but some species have curved shells. Hyoliths also possessed a lid-like operculum that sealed the opening of the shell when the animal retracted. This operculum had a complex shape, often with a crescentic lip. Extending from the shell were one or two slender, stiff appendages called helens, which are thought to have been used for respiration or anchoring.

These ancient creatures were sessile or slow-moving, likely feeding on organic detritus or plankton filtered from the water. They are found as fossils worldwide, preserved in a variety of marine sedimentary rocks. The diversity of hyoliths is considerable, with over 200 genera described. Their extinction at the end of the Permian period, coinciding with the largest mass extinction event in Earth's history, marks the end of their long evolutionary history.