Orthothecida
Extinct order of shelled animals
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The orthothecids are one of the two hyolith orders.[2]
Marek, 1966
| Orthothecida Temporal range: | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Class: | †Hyolitha |
| Order: | †Orthothecida Marek, 1966 |
| Families | |
| |
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]
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