Tristeirostoma, DeVries, 2019

DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), Journal of Natural History 53 (25), pp. 1533-1584 : 1572-1573

Genus Tristeirostoma gen. nov.

Type species: Tristeirostoma saxaquadrata sp. nov. Early Paleogene, southern Peru. Species included: Tristeirostoma saxaquadrata sp. nov.

Diagnosis

Shell small, stoutly fusiform. Spiral sculpture of beaded cords; beads aligned axially in sinuous pattern, interrupted by opisthocline axial ribs. Columella with three parallel keeled folds.

Description

Shell length less than 15 mm, stoutly fusiform, thick-shelled. Spire angle about 35°. Protoconch unknown. Teleoconch with six to seven whorls; whorls weakly convex, sutures impressed. Shoulder evenly rounded. Spire whorls with five beaded spiral cords, equally strong and spaced. Anterior and base of last whorl with seven additional equally spaced, beaded, spiral cords and one basal unbeaded spiral cord. Axial sculpture of 10 opisthocline, broadly rounded axial ribs weakly crossed by beaded spiral cords; axial interspaces twice as wide as ribs; ribs end half way between shoulder and base. Aperture elongate, lozenge-shaped. Parietal deposits absent; columella with three equally spaced, keeled, parallel folds descending from aperture at angle of 30°; anterior keel adjacent to poorly preserved spatulate outer lip or weakly developed anterior canal, weaker than two posterior columellar folds. Outer lip partly missing, inner surface smooth.

Remarks

The genus Tristeirostoma gen. nov. is distinguished by its three columellar keels and a distinctive pattern of smoothed axial ribs set against a backdrop of axially and spirally aligned beads. While axial sculpture is rare among pyramidelloids, pyramidellid species of Otopleura Fischer, 1885 , do have both spiral and axial sculpture. As is the case for many pyramidellids, however, the posteriormost columellar fold of Otopleura is stronger, unequally spaced, and not parallel with the two anterior folds. The species from the Caballas Formation, in contrast, has three columellar folds that are equally spaced and parallel to one other. The two folds across the columellar midsection are equally strong, whereas the anterior fold, which borders a poorly preserved anterior declivity that may be a spatulate portion of a continuous outer lip or a shallow anterior canal, is weaker.

The three parallel columellar folds are arranged in a manner typical of most species of Acteonellidae Gill, 1871, a group of heterobranch gastropods that became extinct by the end of the Cretaceous ( Sohl and Kollman 1985; Kollman 2014). Acteonellids are abundantly present in an Upper Cretaceous limestone of the La Mesa Formation in the Sechura Basin of northern Peru ( Olsson 1944; Jaillard et al. 2005). Acteonellids, which include the North American Maastrichtian Mexicotrochacteon Akopian, 1972, and Acteonella d ’ Orbigny, 1843, are usually medium to large sized, smooth shelled, and often have a subsutural excavation inside the aperture, characters not exhibited by Tristeirostoma .

Etymology

Greek, meaning ‘ tri-keeled aperture ’.

Occurrence

Lower Paleogene, Cuenca Member, Caballas Formation, East Pisco Basin, southern Peru.