Hyolithes cerops, Spence Shale, Idaho (Middle Cambrian)
The orthothecid shell has an internal layer with a microstructure of transverse bundles, and an external layer comprising longitudinal bundles.
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.
The operculum closes perfectly over the aperture of the shell, leaving two gaps through which the helens can protrude. It comprises two parts: the cardinal shield, a flat region at the top of the shell; and the conical shield, the bottom part, which is more conical. The inside of the shell bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.
Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.
The Orthothecida are somewhat more problematic, and probably contain a number of non-hyoliths simply because they are so difficult to identify with confidence, especially if their operculum is absent. They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets.
Orthothecids fall into two groups: one, the orcothecida sensu stricto, is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms. All were sessile and benthic; some may have been filter feeders.
Because hyoliths are extinct and do not obviously resemble any extant group, it is unclear which living group they are most closely related to. They may be molluscs; authors who suggest that they deserve their own phylum do not comment on the position of this phylum in the tree of life. Their grade of organization is considered to be of the 'mollusc-annelid-sipunculid' level, consistent with a Lophotrochozoan affinity, and comparison is usually drawn with the molluscs or sipunculids. Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).
They were probably benthic (bottom-dwellers).
In the Cambrian, their global distribution shows no sign of provinciality, suggesting a long-lived planktonic larval life stage; but by the Ordovician distinct assemblages were becoming evident.
The first hyolith fossils appeared about Nemakit-Daldynian Stage of Siberia and in its analogue the Paragloborilus subglobosus–Purella squamulosa Zone of the 梅树村阶（Meishucunian Stage，寒武纪第一个阶） of China。 Hyolith abundance and diversity attain a maximum in the Cambrian, followed by a progressive decline up to their Permian extinction.in the Purella antiqua Zone of the
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