Orthothecida Explained

The orthothecids are one of the two hyolith orders.^[1]

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-filledintestine – 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.^[2]

Internal taxonomy

Marek ^[2] gives the following diagnoses:

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

Kouchinsky^[3] 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^[4] 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 ^[5]
• Pedunculotheca ^[6]

Notes and References

1. 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.
2. Web site: Machaeridians, Chitons, and Conchiferan Molluscs of the Mojcza limestone . https://web.archive.org/web/20171012202247/http://palaeontologia.pan.pl/Archive/1994-53-1-2_213-252_52-56.pdf . 2017-10-12.
3. Kouchinsky . A. V. . 2000 . Skeletal microstructures of hyoliths from the Early Cambrian of Siberia . Alcheringa . 24 . 2. 65–81 . 10.1080/03115510008619525. 140660142 .
4. Book: Origin and Early Evolution of the Metazoa. 9781489924278. Lipps. Jere H. Signor. Philip W. 2013-11-21.
5. Two new orthothecids from the Cambrian of the Barrandian area (Hyolitha, Skryje-Týřovice Basin, Czech Republic) . Bulletin of Geosciences . 87 . 2 . 2012 . 10.3140/bull.geosci.1142 . 241–248 . Valent . M.. free .
6. 10.1098/rspb.2018.1780 . Hyoliths with pedicles illuminate the origin of the brachiopod body plan. Proceedings of the Royal Society B: Biological Sciences. 285. 1887. 20181780. 2018. Sun. Haijing. Smith. Martin R.. Zeng. Han. Zhao. Fangchen. Li. Guoxiang. Zhu. Maoyan. 30257914. 6170810.