The orthothecids are one of the two

hyolith Hyoliths are animals with small conical surface, conical animal shell, 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, w ...

orders. Marek diagnoses the order thus: Conchs with a flat or concave ventral surface — opercula with large, flat cardinal processes but without clavicles – tightly sigmoidal, sediment-filled intestine – helens absent. Sometimes the Circothecidae and Tetrathecidae are split out into a separate order 'Circothecida', which is defined by the bottom surface not being flat, the cardinal processes being pronounced, and a circular rim sometimes showing hints of differentiation into clavicles.

Internal taxonomy

Marek gives the following diagnoses: *

Orthothecidae The orthothecids are one of the two hyolith orders. Marek diagnoses the order thus: Conchs with a flat or concave ventral surface — opercula with large, flat cardinal processes but without clavicles – tightly sigmoidal, sediment-filled intes ...

: Flat or concave bottom surface of conical shell. Transverse aperture. Kouchinsky lists the following taxonomic criteria: *

Circothecidae Circothecidae are a family of Cambrian 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 ...

: circular cross-section; same shape and sculpture on upper and lower surfaces. * Turcurthecidae: oval or lenticular cross-section; top and bottom identical * Allathecidae: top side flat or slightly convex; bottom side convex Elsewhere is stated: * Tetrathecidae: polyhedral cross-section; shell axially twisted * Novitatidae: bottom side concave * '' Lenatheca'': aperture heart-shaped. Additional genera are not assigned to a family: * '' Probactrotheca'' * ''

Pedunculotheca ''Pedunculotheca'' is a genus of orthothecid hyolith known from the Chengjiang biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known f ...

References

Paleozoic invertebrates Prehistoric animal orders Hyolitha {{paleo-protostome-stub