Hyolitha

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

Morphology

The calcareous – probably aragonitic– shells have a cover (operculum) and two curved supports known as helens, all of which grew by marginal accretion. Most are one to four centimeters in length and are triangular or elliptical in cross section. Some species have rings or stripes.

Shell

The orthothecid shell has an internal layer with a microstructure of transverse bundles, and an external layer comprising longitudinal bundles.

Helens

Helens are long structures that taper as they logarithmically coil gently in a ventral direction.

The helens were calcareous, with an organic component, and had an organic-rich central core surrounded by concentric laminae of calcite. They grew by the addition of new material at their base, on the cavity side, leaving growth lines. They were originally described by Walcott as separate fossils under the genus name Helenia, (Walcott's wife was named Helena and his daughter Helen); Bruce Runnegar adopted the name helen when they were recognized as part of the hyolith organism.