Orthothecida

{{Automatic taxobox

| fossil_range = {{Fossil range | Early Cambrian | Early Devonian | ref={{Cite journal | last1 = Malinky | first1 = J. M. | doi = 10.1666/08-094R.1 | title = Permian Hyolithida from Australia: The Last of the Hyoliths? | journal = Journal of Paleontology | volume = 83 | pages = 147–152 | year = 2009 | issue = 1 | bibcode = 2009JPal...83..147M | s2cid = 85924056 }} }}

| taxon = Orthothecida

| authority = Marek, 1966

| subdivision = {{Linked taxon list

| Allathecidae | Rozanov et al., 1969

| Circothecidae | Rozanov et al., 1969

| Gracilithecidae | Syssoiev, 1972

| Orthothecidae |Syssoiev, 1957

| Tetrathecidae | Sysoyev, 1968

| incomplete = true

}}

The orthothecids are one of the two hyolith orders.{{Cite journal | author1=Malinky, J.M. | author2=Skovsted, C.B| year=2004 | title = Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland | journal = Acta Palaeontologica Polonica |volume=49 |issue=4 |pages=551–578}}

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.{{Cite web | url=http://palaeontologia.pan.pl/Archive/1994-53-1-2_213-252_52-56.pdf | title=Machaeridians, Chitons, and Conchiferan Molluscs of the Mojcza limestone | archive-url=https://web.archive.org/web/20171012202247/http://palaeontologia.pan.pl/Archive/1994-53-1-2_213-252_52-56.pdf | archive-date=2017-10-12}}

Internal taxonomy

Marek gives the following diagnoses:

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

Kouchinsky{{cite journal | last1 = Kouchinsky | first1 = A. V. | year = 2000 | title = Skeletal microstructures of hyoliths from the Early Cambrian of Siberia | journal = Alcheringa | volume = 24 | issue = 2| pages = 65–81 | doi=10.1080/03115510008619525| bibcode = 2000Alch...24...65K | s2cid = 140660142 }} 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{{Cite book | url= https://books.google.com/books?id=CQzyBwAAQBAJ&q=Tetrathecidae&pg=PA243| title= Origin and Early Evolution of the Metazoa| isbn= 9781489924278| last1= Lipps| first1= Jere H| last2= Signor| first2= Philip W| date= 2013-11-21| publisher= Springer}}{{rp|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 {{Cite journal | url = https://www.researchgate.net/publication/259192183 |journal= Bulletin of Geosciences |volume= 87 |issue= 2 |year=2012 |doi=10.3140/bull.geosci.1142 |title=Two new orthothecids from the Cambrian of the Barrandian area (Hyolitha, Skryje-Týřovice Basin, Czech Republic) |pages=241–248 | last1 = Valent | first1 = M.|doi-access=free }}
Pedunculotheca {{cite journal | doi=10.1098/rspb.2018.1780 | title=Hyoliths with pedicles illuminate the origin of the brachiopod body plan| journal=Proceedings of the Royal Society B: Biological Sciences| volume=285| issue=1887| pages=20181780| year=2018| last1=Sun| first1=Haijing| last2=Smith| first2=Martin R.| last3=Zeng| first3=Han| last4=Zhao| first4=Fangchen| last5=Li| first5=Guoxiang| last6=Zhu| first6=Maoyan| pmid=30257914| pmc=6170810}}

References

Category:Paleozoic invertebrates

Category:Prehistoric brachiopod orders

Category:Hyolitha