Orthothecida

Orthothecida
Temporal range:
Scientific classification
Kingdom: Animalia
Class: †Hyolitha
Order: †Orthothecida
Marek, 1966
Families
  • Allathecidae Rozanov et al., 1969
  • Circothecidae Rozanov et al., 1969
  • Gracilithecidae Syssoiev, 1972
  • Orthothecidae Syssoiev, 1957
  • Tetrathecidae Sysoyev, 1968
Not to be confused with Orthonectida.

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 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.

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