Euthema truncatellina, Balashov & Perkovsky & Vasilenko, 2020

Balashov, Igor A., Perkovsky, Evgeny E. & Vasilenko, Dmitry V., 2020, A mid-Cretaceous land snail Euthema truncatellina sp. nov. (Caenogastropoda Cyclophoroidea, Diplommatinidae) from Burmese amber, Zootaxa 4858 (2), pp. 295-300 : 296-297

Euthema truncatellina sp. nov. ( Fig. 1 )

Types. Holotype in a small plate of amber with sides 11 mm x 7 mm x 3 mm ( SIZK Bu-199, amber collection of I.I. Schmalhausen Institute of Zoology NAS Ukraine, Kiev, Ukraine; the alternative abbreviation IZAN is also sometimes used).

Diagnosis. Shell very small (1.7 mm x 0.9 mm), almost cylindrical, last whorl not constricted, penultimate whorl coiling regularly and much narrower than last whorl, whorls moderately convex, with densely regularly placed ribs that curve diagonally on the ventral part of the last whorl; aperture almost round, without teeth.

Description. Shell dextral, almost cylindrical, consisting of 6.5 moderately convex whorls. Second whorl only half as wide as third whorl, forming a characteristic shape of the first two whorls compared to the following larger part of the shell (similar to other species of Euthema ). Protoconch smooth. Penultimate whorl regularly coiled (not tight), much narrower than the last whorl. Last whorl not constricted. Sculpture of regularly, narrowly spaced radial ribs on entire teleoconch. Weak spiral striation visible on some parts of teleoconch (e.g. near the suture of the penultimate whorl on fig. 1 left). Ventral side of penultimate whorl with ca. 35 ribs. On ventral side of last whorl, ribs specifically diagonally curved (reflects narrowed shape of whorl). Some ribs on dorsal part of last whorl near aperture are weakly S-shaped (sinuous). Ribs with weak lamella-like periostracal parts that more elevated in middle of whorls than near suture. Aperture almost round, its margins strongly reflected. No teeth in aperture. Umbilicus open and wide.

Measurements of holotype. Height of shell 1.70 mm, diameter 0.90 mm; height of aperture 0.60 mm, aperture width 0.55 mm; height of last whorl 0.80 mm, its diameter 0.90 mm, without aperture 0.80 mm; height of visible part of penultimate whorl 0.50 mm, its diameter 0.80 mm; height of visible part of fourth whorl 0.30 mm, its diameter 0.80 mm; height of visible part of third whorl 0.20 mm, its diameter 0.55 mm; diameter of second whorl 0.35 mm, diameter of first whorl 0.25 mm.

Type locality. Hukawng Valley (Kachin State, Myanmar) .

Age. Early Cenomanian.

Etymology. The species is named after the extant genus Truncatellina Lowe, 1852 ( Stylommatophora ), which has a similar shell, acknowledging the convergence in the different lineages of gastropods on land.

Remarks. There is a bubble in the amber in front of the aperture that almost completely covers it. However, the bubble is not pressed closely to the shell and the aperture is largely observable from the sides with distortion. Inside the bubble a diminished reflection of the aperture is visible.

Figures

FIGURE 1. Holotype of Euthema truncatellina sp. nov. in Burmese amber (SIZK Bu-199).