Trigonoconcha
Trigonoconcha is an extinct genus of bivalve in the extinct family Praenuculidae. The genus is one of eleven genera in the subfamily Praenuculinae. It is one of three Praenuculinae genera known solely from Late Ordivician, Caradoc epoch, fossils found in South America. "Trigonoconcha" currently contains a single 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. 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", 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 vertex 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.