Admete trigostomoides, Bouchard & Wesselingh & Pouwer & Landau, 2025

Admete trigostomoides n. sp.

( Fig. 13A View FIG 1-A View FIG 3 View FIG , B 1-B View FIG 3 View FIG )

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TYPE MATERIAL. — Holotype: MNHN.F.A91318 , height 5.4 mm, width 2.9 mm, leg. SM . — Paratypes: paratype 1, RGM.1310836 , height 5.9 mm, width 3.3 mm, leg. SM ; paratype 2, RGM.1365263 , height 8.7 mm, width 5.2 mm, leg. AWJ .

OTHER MATERIAL EXAMINED. — Maximum height 8.7, width 5.2. — RGM.1365014 (1), leg. ACJ .

ETYMOLOGY. — Name reflecting the close resemblance to members of the cancellariid genus Trigonostoma .

TYPE LOCALITY. — Selsoif, Saint-Sauveur-le-Vicomte, Manche department, NW France.

STRATUM TYPICUS. — Gelasian, Early Pleistocene

DIAGNOSIS. — Medium sized Admete species, protoconch of 1.5 whorls, teleoconch of four depressed barrel-shaped whorls, subsutural platform canaliculate, sculpture of narrow spiral cords, axials restricted to weak tubercules over cords, last whorl bearing 12 cords, narrow umbilical chink, three subequal columellar folds, columellar callus thickened, parietal callus not developed.

INTRASPECIFIC VARIABILITY. — The Selsoif specimens are relatively consistent in profile and sculpture, although in some the subsutural ramp is slightly slanted giving the profile a more typically Admete - like appearance. The width and concavity of the subsutural ramp is somewhat variable. In one specimen, a further cord runs on the subsutural platform just medial to the shoulder cord on the last two whorls and a fine secondary spiral is intercalated between the second and third primary cords. The subobsolete axial sculpture in this same specimen is slightly stronger, forming poorly defined axial ribs. The small size of the specimens and the unthickened outer lip suggests that they may not represent fully grown adults.

DISTRIBUTION. — Lower Pleistocene: Atlantic, endemic to Selsoif (this paper).

DESCRIPTION

Shell small and relatively thin for genus, with moderate height scalate spire. Protoconch paucispiral, dome-shaped, about 1.5 whorls, surface abraded. Teleoconch of four low, barrel-shaped, strongly tabulate whorls, separated by narrowly impressed, linear suture: subsutural platform broad, increasingly concave and canaliculate abapically, delimited by elevated shoulder cord. Sculpture on early teleconch whorls of three narrow spiral cords, with fourth appearing just above suture. On penultimate whorl single secondary cord intercalated between shoulder cord and mid cord. Axial sculpture subobsolete, represented mainly by indistinct tubercles over cords, and crowded slightly prosocline growth-lines. Last whorl 70% of total height with relatively broad, shallowly canaliculate subsutural platform,sharply angled at shoulder cord, weakly constricted at base, sculptured of four weakly tubercular cords above level of insertion of outer lip,about eight further smooth cords below, becoming closer-spaced over siphonal fasciole; base not delimited; siphonal fasciole weakly developed, slightly rounded with very narrow umbilical chink medially. Aperture 50% of total height, subquadrangular, outer lip not thickened, sharply angled at shoulder; anal sinus lacking, siphonal canal moderately short, open, bent adaxially.Columella excavated in upper third, straight below,twisted at fasciole,bearing three columellar folds of subequal strength,becoming increasingly oblique abapically. Columellar callus thickened, sharply delimited medially by umbilical chink, parietal callus not developed.

COMPARISONS

The strongly scalate spire and deeply canaliculate suture are reminiscent of members of the cancellariid genus Trigonostoma Blainville, 1827 .However, species placed in that group are usually larger and thicker-shelled, have stronger axial sculpture and a broad, deep umbilicus.Moreover, the presence of Trigonostoma was highlighted byLandau et al. (2011:table 1)as a thermophilic marker, which has its maximum diversity in tropical assemblages and does not occur in waters that are colder than subtropical.

Placement in the temperate to cold water genus Admete Möller, 1842 is more likely. Members of that genus are medium-sized, thin-shelled, and show similar sculpture and apertural characters to that seen in the new species. Admete trigostomoides n. sp. is similar to some forms of the extremely variable Pliocene to present-day A.viridula (Fabricius, 1780) that occurs at northern latitudes on both sides of the Atlantic, most notably the form described as A. contabulata Friele, 1879 , which is considered a synonym (see Bouchet & Warén 1985: fig. 684). However, in that form the broad, sloping subsutural ramp is not as flat, and never canaliculate as seen in the French species.Moreover, despite the reasonably wide variation seen in Selsoif discussed above, all specimens have the subsutural ramp flat, or almost so, and none approximate to the more usual rounded whorl profile of A. viridula . The contabulata form was also recorded by Harmer (1920: 524, pl. 49, fig. 10) from the Lower Pleistocene of NE England. The figured specimen has a far steeper subsutural ramp than A. trigostomoides n. sp.