# Similodonta

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{{Short description|Extinct genus of bivalves}}
{{Automatic taxobox
| fossil_range = [Caradoc](/source/Caradoc_(age)) to [Ashgill](/source/Ashgill_(age)) {{Fossil range|460.9|443.7}}
| image = An Ordovician clam called Tellinomya.jpg
| image_caption = A fossil specimen of ''Similodonta'' from the upper Ordovician aged [Sinnipee group](/source/Sinnipee_Group) in [Wisconsin](/source/Wisconsin)
| taxon = Similodonta
| authority = [Soot-Ryen](/source/Helen_Soot-Ryen), 1964
| subdivision_ranks = Species
| subdivision = See text
}}

'''''Similodonta''''' is an [extinct](/source/extinct) [genus](/source/genus) of early [bivalve](/source/bivalve) in the [extinct](/source/extinct) family [Praenuculidae](/source/Praenuculidae).  The genus is one of eleven genera in the subfamily [Praenuculinae](/source/Praenuculinae).<ref name="Sánchez1999">{{cite journal |last1=Sánchez |first1=T.M. |year=1999 |title=New Late Ordovician (Early Caradoc) Bivalves from the Sierra de Villicum (Argentine Precordillera) |journal=Journal of Paleontology |volume=73 |issue=1 |pages=66–76 |doi=10.1017/S0022336000027554 |jstor=1306745|bibcode=1999JPal...73...66S |s2cid=133414164 }}</ref>  ''Similodonta'' is known from [Middle Ordovician](/source/Ordovician) through [Middle Silurian](/source/Silurian) fossils found in Europe and North America.  The genus currently contains eight accepted species, '''''Similodonta ceryx''''', '''''Similodonta collina''''', '''''Similodonta djupvikensis''''', '''''Similodonta magna''''', '''''Smilodonta minor''''',<ref>Wang, Y.; Zhan, R.-B.; Fang, Z.-J. (2026). "New Katian bivalves from the Upper Ordovician Xiazhen Formation, Jiangxi Province, China". Acta Palaeontologica Polonica. 71 (1): 133–153. doi:10.4202/app.01281.2025.</ref> '''''Similodonta recurva''''', '''''Similodonta spjeldnaesi''''', '''''Similodonta wahli'''''<ref name="Hints2000">{{cite journal |last1=Hints |first1=L. |last2=Oraspõld |first2= A. |last3=Kaljo |first3=D. |year=2000 |title=Stratotype of the Porkuni Stage with comments on the Röa Member (Uppermost Ordovician, Estonia) |journal=Proceedings of the Estonian Academy of Sciences, Geology |volume=49 |issue=3 |pages=177–199 |doi=10.3176/geol.2000.3.02 |s2cid=240195751 |url=https://books.google.com/books?id=gEHPcVavLuQC&q=Stratotype+of+the+Porkuni+Stage+with+comments+on+the+R%C3%B6a+Member+(uppermost+Ordovician,+Estonia)&pg=PA177|doi-access=free |url-access=subscription }}</ref> and the type species '''''Similodonta similis'''''.<ref name="Cope1999">{{cite journal |last1=Cope |first1=J.C.W. |year=1999 |title=Middle Ordovician bivalves from Mid-Wales and the Welsh Borderland |journal=Palaeontology |volume=42 |issue=3 |pages=467–499 |doi=10.1111/1475-4983.00081|doi-access=free |bibcode=1999Palgy..42..467C }}</ref>

==Description==
''Similodonta'' is a small bivalve which was first described in 1964 by Helen Soot-Ryen in an ''Arkiv för Mineralogi och Geologi, Kungliga Svenska Vetenskapsakademien'' paper.<ref name="Cope1999"/>  Generally the shells of ''Similodonta'' are rounded on the ventral sides of the shell and triangular on the dorsal sides.  The triangular shape on the dorsal side is formed by the sharp angle at which the anterior and posterior hinge edges meet at the umbo.  ''Similodonta'' species are similar in shape to species of the related genus ''[Palaeoconcha](/source/Palaeoconcha)''.  The two genera can be separated based on the presence of an auricle, a projection on the inside of the shell, in ''Palaeoconcha'' species.  One of the notable features in ''Similodonta'' is the angle at which the anterior and posterior shell edges meet.  In the type description of the genus, Soot-Ryen described the angle as near 80°, subsequent examinations of the type specimens and further fossils have shown the angle to in fact be near 90°.<ref name="Cope1999"/>

The type species for the genus, ''Similodonta similis'', has been found in Late Ordovician, [Ashgill epoch](/source/Ashgill_epoch), sediments of the upper Richmond Group exposed near [Spring Valley, Minnesota](/source/Spring_Valley%2C_Minnesota).  The species was first described in 1892 by  [Edward Oscar Ulrich](/source/Edward_Oscar_Ulrich) as ''Tellinomya similis''.<ref name="Cope1999"/>

''Similodonta ceryx'' was described by John C. W. Cope in 1999 from the internal mold of a single [right valve](/source/Valve_(mollusc)).  The specimen was recovered from a depth of {{convert|108.90|m|ft}} in a borehole near [Llanwrtyd Wells](/source/Llanwrtyd_Wells), [Wales](/source/Wales).<ref name="Cope1999"/>  ''S. ceryx'' is the oldest species to be assigned to the genus, dating from the [Caradocian](/source/Caradocian) Aurelucian Stage.<ref name="Cope1999"/>

Of similar age to the North American species, ''Similodonta collina'' is known from Ashgillian age fossils found in [Scotland](/source/Scotland).  Described in 1946 by Reed, the species is more inflated and larger than ''S. ceryx''.<ref name="Cope1999"/>

''Similodonta djupvikensis'' is one of the youngest species in the genus, dating from the [Wenlock epoch](/source/Silurian) of the Silurian.  Found on the [Swedish](/source/Sweden) island of [Gotland](/source/Gotland) in the Baltic Sea, the species was described by Soot-Ryen in 1964.  As with ''S. similis'', ''S. recurva'', and ''S. spjeldnaesi'',  ''S. djupvikensis'' is stouter and has a more robust hinge plate then that seen in ''S. ceryx''.<ref name="Cope1999"/>

Another united kingdom species, ''Similodonta magna'' was described in 1946 by Lamont and is known from Ashgillian fossils found near the Scottish coastal town of [Girvan](/source/Girvan).  While similar in morphology to ''S. ceryx'', ''S. magna'' is distinguished by the stronger curve in the posterior edge and muscle scars that are not as distinct.<ref name="Cope1999"/>

Also in the Richmond group of North America is the species ''Similodonta recurva''.  This species was named in the same 1892 paper by Ulrich as ''S. similis''.<ref name="Cope1999"/>

''Similodonta spjeldnaesi'' is a late Ordovician species found in and described from the Upper Chasmops Shale, [Ringerike](/source/Ringerike_(traditional_district)), [Norway](/source/Norway).  The species was described in 1960 by Helen Soot-Ryen and Tron Soot-Ryen 
as noted, the species has a more robust hinge plate and is stouter then ''S. ceryx''.<ref name="Cope1999"/>

Dating from the Latest Ordovician, the species ''Similodonta wahli'' was described in 1991 by Isakar from fossils found in the Ärina Formation of Northern Estonia.<ref name="Hints2000"/>

==References==
{{Reflist}}

{{Taxonbar|from=Q7517830}}

Category:Praenuculidae
Category:Ordovician bivalves
Category:Silurian bivalves
Category:Fossil taxa described in 1964
Category:Silurian animals of Europe
Category:Paleozoic life of Quebec
Category:Prehistoric bivalve genera

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Adapted from the Wikipedia article [Similodonta](https://en.wikipedia.org/wiki/Similodonta) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Similodonta?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.