Orthothecida

Orthothecida; Temporal range: Early Cambrian – Early Devonian 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 ;

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.

Internal taxonomy

Marek gives the following diagnoses:

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

Kouchinsky 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^: 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
Pedunculotheca

References

↑ Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology. 83 (1): 147–152. Bibcode:2009JPal...83..147M. doi:10.1666/08-094R.1. S2CID 85924056.
↑ 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.
1 2 "Machaeridians, Chitons, and Conchiferan Molluscs of the Mojcza limestone" (PDF). Archived from the original (PDF) on 2017-10-12.
↑ Kouchinsky, A. V. (2000). "Skeletal microstructures of hyoliths from the Early Cambrian of Siberia". Alcheringa. 24 (2): 65–81. Bibcode:2000Alch...24...65K. doi:10.1080/03115510008619525. S2CID 140660142.
↑ Lipps, Jere H; Signor, Philip W (2013-11-21). Origin and Early Evolution of the Metazoa. Springer. ISBN 978-1-4899-2427-8.
↑ 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. hdl:11104/0217603.
↑ 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.

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[ref_-1] Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology. 83 (1): 147–152. Bibcode:2009JPal...83..147M. 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.

[mach-3] 1 2 "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. Bibcode:2000Alch...24...65K. doi:10.1080/03115510008619525. S2CID 140660142.

[5] Lipps, Jere H; Signor, Philip W (2013-11-21). Origin and Early Evolution of the Metazoa. Springer. ISBN 978-1-4899-2427-8.

[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. hdl:11104/0217603.

[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.