Parvitrigonia, Halligan, 2023

Halligan, William Keith, 2023, Washington State (USA) trigoniids (Bivalvia) from the conglomerate of Patterson Lake (Early Cretaceous), PaleoBios 40 (8), pp. 1-15 : 9-10

PARVITRIGONIA n. gen.

Zoobank LSID— urn:lsid:zoobank.org:act:5CF78024-4AE3-478D-9F13-C9F55C98403A

Diagnosis— Very small, moderately inequilateral (umbo in anterior third), elongate-pyriform, and inflated with a narrow, nearly pointed posterior terminus. Marginal angulation conspicuous, rounded, gently concave, coursing from beak to posterior terminus creating an angle of almost 90° between the surfaces of the flank and area. Sunken area/escutcheon forms a platform that mimics the concave curve of the marginal angulation; platform unornamented except for fine growth lines.Area rises up smoothly to form a nearly straight posterodorsal margin. Flank ribs subcommarginal, flat-topped, wavy ventrally, and attenuate as they approach the posterior terminus.

Type species— Parvitrigonia cooperi n. sp. Original description below.

Etymology— A rutitrigoniid named after its small size: Parvus (Latin for “small”).

Discussion— There are no clear criteria for the creation of a new genus; for an in-depth presentation of the issues involving trigoniid genera in general and rutitrigoniids in particular, see Leanza (1993) and Cooper (2015b). Given the provincial nature of rutitrigoniids ( Cooper 2015b), the first place to look for an appropriate genus assignment for this fossil is a rutitrigoniid genus that is from the Cordilleran province, the province that includes Washington State. The only rutitrigoniid genus found in the Cordilleran province is Earlpackardia Cooper, 2015b . Cooper (2015b) describes the genus as: “Moderately large, robust,…with…subparallel ventral and posterodorsal margins and broadly-rounded respiratory margin.” In contrast, Parvitrigonia is very small, inflated, pyriform with a narrow, nearly pointed respiratory margin.

Considering rutitrigoniid genera from other provinces that span the middle Albian, there are three possibilities: Rutitrigonia van Hoepen, 1929 , Lycettitrigonia Cooper, 2015b , and Balaklavella Cooper, 2015b . While there are some similarities between these genera and the fossil from this study, the following excerpts from Cooper (2015b) demonstrate significant differences that argue against its inclusion in any of those genera.

Rutitrigonia is “pandemic to the Ethiopian ( South Africa) and Andean ( Argentina) provinces” and described as: “Small…;…respiratory margin subrounded to obliquely subtruncate, posterodorsal margin shallowly concave;…area with commarginal costellae in early growth continuous from flank, occasionally persisting; flank costellae fine, dense, subcommarginal in early growth, later cutting obliquely across growth striae anteriorly and curving downwards posteriorly where they may become crowded.” In contrast, Parvitrigonia is very small, with a narrow, nearly pointed respiratory margin; area is unornamented except for fine growth lines; flank costellae are wider (not fine), flat topped, less crowded, and do not curve downward posteriorly.

Lycettitrigonia is “pandemic to the European ( England, France, Switzerland), Middle East ( Libya, Sinai,? Jordan) and northern Ethiopian ( India) provinces, perhaps extending into the American (?Texas) Province.” The genus is described as: “Small to moderately large;

trigonally ovate to subpyriform…; escutcheon barely sunken, poorly discriminated with indistinct rim;…; fine subcommarginal flank costellae.” Posterior margin is smoothly rounded. In contrast, Parvitrigonia is very small, elongate-pyriform, with narrow, nearly pointed posterior, sunken area/escutcheon, and conspicuous marginal angulation creating a nearly 90° angle between the surfaces of the flank and area. Flank costellae are more coarse, flat-topped, and wavy ventrally.

Balaklavella is “endemic to the Central Asian (Turkistan) Province” and described as: “Small to medium-sized…with concave posterodorsal margin… conspicuously bipartite area…and fine subcommarginal flank costellae which curve strongly upwards posteriorly so as to meet the area almost at right angles.” In contrast, Parvitrigonia is very small with nearly straight posterodorsal margin. Area not bipartite; flank costellae more coarse, wavy, flat-topped, and do not curve upwards posteriorly.

There being no existing rutitrogoniid genus that satisfactorily describes the morphology of the fossil in this study, a new genus, Parvitrigonia , is proposed.