extinct
Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of early
bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
in the
extinct
Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
family
Praenuculidae
Praenuculidae is an extinct family of prehistoric bivalves in the superfamily Nuculoidea. Praenuculidae species lived from the early Ordovician, Arenig stage through the Early Devonian Emsian stage.
. The genus is one of eleven genera in the subfamily
Praenuculinae
Praenuculinae is an extinct subfamily (biology), subfamily of prehistoric bivalves in the family (biology), family Praenuculidae. Praenuculinae species lived from the middle Ordovician through the late Devonian. ''Similodonta'' is known from
Middle Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (milli ...
through
Middle Silurian
Middle or The Middle may refer to:
* Centre (geometry), the point equally distant from the outer limits.
Places
* Middle (sheading), a subdivision of the Isle of Man
* Middle Bay (disambiguation)
* Middle Brook (disambiguation)
* Middle Creek (d ...
fossils found in Europe and North America. The genus currently contains eight accepted species, ''Similodonta ceryx'', ''Similodonta collina'', ''Similodonta djupvikensis'', ''Similodonta magna'', ''Similodonta recurva'', ''Similodonta spjeldnaesi'', ''Similodonta wahli'' and the type species ''Similodonta similis''.
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. 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''. 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°.
The type species for the genus, ''Similodonta similis'', has been found in Late Ordovician, Ashgill epoch, sediments of the upper Richmond Group exposed near
Spring Valley, Minnesota
Spring Valley is a city in Fillmore County, Minnesota, Fillmore County, Minnesota, United States. The population was 2,479 at the 2010 United States Census, 2010 census.
History
Spring Valley was laid out in 1855, and named for a spring near the ...
. The species was first described in 1892 by
Edward Oscar Ulrich
Edward Oscar Ulrich (1 February 1857, in Covington, Kentucky – 22 February 1944, in Washington, D.C.) was an invertebrate paleontologist specializing in the study of Paleozoic fossils.
Biography
Ulrich was educated at Wallace College and the Oh ...
as ''Tellinomya similis''.
''Similodonta ceryx'' was described by John C. W. Cope in 1999 from the internal mold of a single right valve. The specimen was recovered from a depth of in a borehole near
Llanwrtyd Wells
Llanwrtyd Wells ( "church of St Gwrtud") is a market town and community (Wales), community in Powys, Wales, in the historic counties of Wales, historic county of Brecknockshire (Breconshire) on the Afon Irfon. The town is on the A483 road, A483 ...
,
Wales
Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
. ''S. ceryx'' is the oldest species to be assigned to the genus, dating from the Caradocian Aurelucian Stage.
Of similar age to the North American species, ''Similodonta collina'' is known from Ashgillian age fossils found in
Scotland
Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
. Described in 1946 by Reed, the species is more inflated and larger than ''S. ceryx''.
''Similodonta djupvikensis'' is one of the youngest species in the genus, dating from the
Wenlock epoch
The Wenlock Epoch (sometimes referred to as the Wenlockian) is the second epoch of the Silurian. It is preceded by the Llandovery Epoch and followed by the Ludlow Epoch. Radiometric dates constrain the Wenlockian between and million years ago. ...
of the Silurian. Found on the
Swedish
Swedish or ' may refer to:
Anything from or related to Sweden, a country in Northern Europe. Or, specifically:
* Swedish language, a North Germanic language spoken primarily in Sweden and Finland
** Swedish alphabet, the official alphabet used by ...
island of
Gotland
Gotland (; ; ''Gutland'' in Gutnish), also historically spelled Gottland or Gothland (), is Sweden's largest island. It is also a Provinces of Sweden, province/Counties of Sweden, county (Swedish län), Municipalities of Sweden, municipality, a ...
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''.
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
Girvan (, "mouth of the River Girvan") is a burgh and harbour town in Carrick, South Ayrshire, Scotland. Girvan is situated on the east coast of the Firth of Clyde, with a population of about 6,450. It lies south of Ayr, and north of St ...
. 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.
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''.
''Similodonta spjeldnaesi'' is a late Ordovician species found in and described from the Upper Chasmops Shale, Ringerike,
Norway
Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...
. 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''.
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.