Orthothecida

The orthothecids are one of the two hyolith orders.^[2]

Orthothecida Temporal range: Early Cambrian – Early Devonian[1] PreꞒ Ꞓ O S D C P T J K Pg N
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

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.^[3]

Internal taxonomy

Marek ^[3] gives the following diagnoses:

• Orthothecidae: Flat or concave bottom surface of conical shell. Transverse aperture.

Kouchinsky^[4] lists the following taxonomic criteria:

• Circothecidae: 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^[5]: 243 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 ^[6]
• Pedunculotheca ^[7]