Orthothecida

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

Orthothecida
Temporal range: Early Cambrian – Early Devonian[1]
Scientific classification
Kingdom: Animalia
Clade: Lophophorata
Clade: Brachiozoa
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 [4] gives the following diagnoses:

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

Kouchinsky[5] 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[6]: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:

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. http://palaeontologia.pan.pl/Archive/1994-53-1-2_213-252_52-56.pdf
  4. "MACHAERIDIANS, CHITONS, and CONCHIFERAN: MOLLUSCS of the MOJCZA LIMESTONE" (PDF). Cite journal requires |journal= (help)
  5. 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.
  6. Lipps, Jere H; Signor, Philip W (2013-11-21). Origin and Early Evolution of the Metazoa. ISBN 9781489924278.
  7. 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.
  8. 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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