Orthothecida

Orthothecida Temporal range: Early Cambrian – Early Devonian[1] PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Brachiopoda
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

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

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]

References

1. Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology. 83: 147–152. doi:10.1666/08-094R.1. S2CID 85924056.
2. Malinky, J.M.; Skovsted, C.B (2004). "Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland". Acta Palaeontologica Polonica. 49 (4): 551–578.
3. "Machaeridians, Chitons, and Conchiferan Molluscs of the Mojcza limestone" (PDF). Archived from the original (PDF) on 2017-10-12.
4. Kouchinsky, A. V. (2000). "Skeletal microstructures of hyoliths from the Early Cambrian of Siberia". Alcheringa. 24 (2): 65–81. doi:10.1080/03115510008619525. S2CID 140660142.
5. Lipps, Jere H; Signor, Philip W (2013-11-21). Origin and Early Evolution of the Metazoa. ISBN 9781489924278.
6. Valent, M. (2012). "Two new orthothecids from the Cambrian of the Barrandian area (Hyolitha, Skryje-Týřovice Basin, Czech Republic)". Bulletin of Geosciences. 87 (2): 241–248. doi:10.3140/bull.geosci.1142.
7. Sun, Haijing; Smith, Martin R.; Zeng, Han; Zhao, Fangchen; Li, Guoxiang; Zhu, Maoyan (2018). "Hyoliths with pedicles illuminate the origin of the brachiopod body plan". Proceedings of the Royal Society B: Biological Sciences. 285 (1887): 20181780. doi:10.1098/rspb.2018.1780. PMC 6170810. PMID 30257914.