Gyraulus sp.

Hsu, Chia-Hsin, Osipova, Diana & Lin, Chien-Hsiang, 2025, Fossil freshwater mollusks from the Early Pleistocene (Calabrian) of northern Taiwan, Geodiversitas 47 (20), pp. 721-737 : 728

Gyraulus sp.

( Fig. 6H)

MATERIAL EXAMINED. — Taiwan • 2 specimens; Shuiniukeng (= Water Buffalo Valley ), New Taipei City; Tananwan Formation; Early Pleistocene, Calabrian; ASIZF0100920 , ASIZF0100941 .

DESCRIPTION

Shell discoid, pseudodextral, small (WSh: 3.5 mm) with rounded whorl periphery, slightly angulated. Almost imperceptible keel in the middle of the whorl. DIWb slightly longer than LWWb. WA longer than HA. Examined specimen with at least 3.3 whorls. Shell surface smooth, glossy, closer to aperture with evident growth lines. Whorl wide, convex, umbilicus deep, and concave, protoconch is visible. Suture deep. Aperture rounded.

REMARKS

The members of family Planorbidae are known for their heterogeneous shell morphology, making species discrimination difficult ( Hubendick 1955). Based on the rounded periphery of the sole well-preserved specimen, slightly visible keel in the middle of the shell, and the DIWb being much larger than in other genus, the specimen has been here assigned to the genus Gyraulus . Considering the small size ( 3.5 mm) of two obtained specimens, it is potentially subadult individuals, as general size of Taiwanese planorbids is 5−6 mm. The genus Gyraulus is represented by one species in Taiwan — Gyraulus spirillus (Gould, 1859) , occurring with at least two morphotypes ( Chen 2011). Although the studied specimen appears to be less angulated, it is possible that it belongs to one of the morphotypes of G. spirillus . Gyraulus has occurred in Asia since the Cretaceous ( Isaji 2010), and has been reported from the Cenozoic deposits in China ( Yen 1943).