Hyolitha are enigmatic animals with small conical shells known from the Palaeozoic Era.

Shell morphology

The calcareous shells have a cover (operculum) and two curved supports known as helens. Most are one to four centimeters in length and are triangular or elliptical in cross section. Some species have rings or stripes.

Taxonomy & ecology

Because hyoliths are extinct and do not obviously resemble any extant group, it is unclear which living group they are most closely related to. Some authors treat them as molluscs, but it is generally agreed that this argument is not strongly supported. Fossil traces showing a twisted, looped, intestine bear some resemblance to the gut of sipunculan worms.

Despite the fact that hyolithid shells are quite common as fossils, next to nothing is known about their ancestry and internal structures. They were obviously benthic (bottom-dwellers), and there is some evidence that they were carnivores.

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