Hyolitha

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 striations.

Taxonomy & ecology
Because hyoliths are extinct and do not obviously resemble any extant group, it is unclear who their closest relatives are. Some authors treat them as molluscs, but it is generally agreed that there is no very strong argument to do so. 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.