''Trigonoconcha'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus of

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

in the

family Praenuculidae. The genus is one of eleven genera in the subfamily

Praenuculinae Praenuculinae is an extinct subfamily of prehistoric bivalves in the family Praenuculidae. Praenuculinae species lived from the middle Ordovician through the late Devonian.

. It is one of three Praenuculinae genera known solely from Late Ordivician, Caradoc epoch, fossils found in South America. ''Trigonoconcha'' currently contains a

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accepted species, ''Trigonoconcha acuta''.

Description

''Trigonoconcha acuta'' was first described in 1999 by Teresa M. Sánchez from fossils from sediments of the late Middle Ordivician, Caradocian aged Don Braulio Formation. The formation outcrops on the flank of Sierra de Villicum in the Argentina

precordillera Precordillera is a Spanish geographical term for hills and mountains lying before a greater range, foothills. The term is derived from ''cordillera'' (mountain range)—literally "pre-mountain range"—and applied usually to the Andes. Some places ...

.The Paleobiology database "Sierra de Villicum" entry
accessed 11 February 2012 The shells of ''Trigonoconcha'' have anterior and posterior sides which meet at an angle of no more than sixty-five degrees. While this is similar to the related genera '' Palaeoconcha'' and ''

Similodonta ''Similodonta'' is an extinct genus of early bivalve in the extinct family Praenuculidae. The genus is one of eleven genera in the subfamily Praenuculinae. ''Similodonta'' is known from Middle Ordovician through Middle Silurian fossils foun ...

'', they are both noted for anterior-posterior angles of no less than ninety degrees. The presence of an auricle, a finger-like projection on the underside of the umbo, in ''Palaeoconcha'' also distinguishes specimens from ''Trigonoconcha''. ''Similodonta'' is distinguished from both other genera by an edentulous space, a region on the hinge that lacks teeth. The umbo of ''Trigonoconcha'' is located at the

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formed by the anterior and posterior sides and the ventral margin forms a broad curving edge. On smaller shells, possibly from immature specimens, the ventral margin curves abruptly to meet the anterior and posterior margins creating a triangular shell outline. The anterior edge is host to up to fifteen teeth while the posterior edge has a maximum of ten. The ''Trigonoconcha'' ranges in shell length from and has a height between . When described, the genus name ''Trigonoconcha'' was chosen by Sánchez in reference to the triangle created by the anterior and posterior margins of the shell. Similarly, the species epithet ''acuta'' was coined as a reference to the acute angle formed by the anterior and posterior margins. ''Trigonoconcha'' was described from one holotype specimen, number CEGH-UNC 16158, which is an internal mold of a left valve. A series of twelve other right and left valve molds were also examined and used in the description. The specimens are currently deposited in the Department of Geology paleontology collections in the

National University of Córdoba The National University of Córdoba ( es, Universidad Nacional de Córdoba,) is an institution of higher education in the city of Córdoba, Argentina. Founded in 1613, the university is the oldest in Argentina, the third oldest university of t ...

References

{{Taxonbar, from=Q7841816 Praenuculidae Ordovician bivalves Ordovician animals of South America Ordovician Argentina Fossil taxa described in 1999 Prehistoric bivalve genera