Circothecidae are a family of

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

problematica, sometimes attributed to the

Hyolitha Hyoliths are animals with small conical shells, known as fossils from the Palaeozoic era. They are at least considered as lophotrochozoan, and possibly being lophophorates, a group which includes the brachiopods, while others consider them as be ...

, though some authors suggest (on the basis of no specified evidence) that they're definitely not.Webers, G. F., & Yochelson, E. L. (1989). Late Cambrian molluscan faunas and the origin of the Cephalopoda. Geological Society, London, Special Publications, 47(1), 29–42. doi:10.1144/gsl.sp.1989.047.01.04

References

Cambrian molluscs Prehistoric animal families Hyolitha {{Paleo-mollusc-stub