Worthenia humiligrada Ketwetsuriya & Karapunar & Charoentitirat & Nützel 2020, sp. nov.

Worthenia humiligrada sp. nov.

(Fig. 14 A–I)

Etymology. From Latin humilis, meaning low, and Latin gradus, meaning step, referring to the low-stepped whorl profile.

Holotype. ESKU-19-LP 22.

Paratypes. ESKU-19- LP 23, 33, 34, 35, 74

Dimensions (mm): ESKU-19- LP 22: height = 6.8; width = 7.0; apical angle = 98º. ESKU-19- LP 23: height = 10.5; width = 12.7. ESKU-19- LP 33: height = c. 16.0; width = 13.8; apical angle = 92º. ESKU-19- LP 34: height = c. 9.0; width = 11.3; apical angle = 110º. ESKU-19- LP 35: height = c. 10.9; width = 13.6; apical angle = 110º. ESKU- 19- LP 74: height = c. 1.7; width = c. 3.1.

Type locality and stratigraphical range. Erawan Hill, Chong Sarika sub-district, Phatthana Nikhom district, located about 13 km east of Lopburi Province, Central Thailand (Fig. 1), Khao Khad Formation, Saraburi Group, Middle Permian, Roadian.

Description. Shell low-spired for genus, gradate, trochiform; first whorl planispiral; early teleoconch whorls (about first two or three whorls) low-spired, smooth, convex, rounded; second and third whorls with fine spiral and axial ribs; suture distinct and impressed; later whorls sharply angulated at about mid-whorl face, the middle angulation separates subsutural ramp from subvertical lower whorl face; subsutural ramp slightly convex near adapical suture to concave on the remaining part; ramp inclining at an angle of 35–50°; subsutural ramp ornamented with up to eight spiral ribs and equally spaced prosocyrt fine collabral ribs or strengthened growth lines; spiral ribs and collabral ribs form nodes at intersections, forming fine reticulate pattern; ornamentation more prominent near adapical suture; lower whorl face subvertical, concave, ornamented with up to six strong spiral cords and numerous faint prosocyrt collabral threads or growth lines; mid angulation forms crest-like periphery of whorl where selenizone situated; selenizone sharply angular covered by v-shaped lunulae; lunulae form nodes when intersecting with angulation; selenizone bordered with fine spiral ribs, upper rib represents lowest spiral rib of subsutural ramp, while lower rib represents uppermost spiral rib of the subvertical lower whorl face; intersections of lunulae and angulation nodular; basal edge sharply angulated; base convex, ornamented by several evenly spaced spiral ribs and very weak opisthocyrt collabral threads; suture shallow, indistinct; narrowly phaneromphalous; aperture unknown.

Remarks. The convex early teleoconch whorls, the position and the ornamentation pattern of the selenizone suggest that this species belongs to the genus Worthenia de Koninck, 1883. The distinguishing characters of Worthenia humiligrada sp. nov. are the relatively low spire, wide umbilicus, weak subsutural convexity of the ramp, sharp angulation at the basal edge and nodular selenizone. The combination of these characters distinguishes W. humiligrada sp. nov. from the other species belonging to the genus Worthenia.

There are some similar Permian species in Southeast Asia which have been assigned to Worthenia, for instance W. multicarinata (Mansuy, 1912) from the Carboniferous of Yunnan, China (Mansuy 1912) and the Permian of Cambodia (Delpey 1941) and Malaysia (Batten 1972, p. 32, figs 36–38) is very similar in shape but it has more prominent subsutural nodes on the upper whorl face, a distinctly impressed suture and its basal edge is rounded angular and ornamented with nodes. The type specimens of W. schirjaevensis (Stuckenberg, 1905) from the Upper Carboniferous of Samara, Russia and the specimens assigned to W. schirjaevensis from the Permian of Cambodia (Delpey 1941, fig. 27) has a very similar whorl profile and ornamentation but they differ from W. humiligrada sp. nov. by having a higher spire. According to Batten (1972, fig. 38), W. schirjaevensis (Stuckenberg, 1905) lacks nodes on the selenizone. W. arizonensis Winters, 1963 from the Permian of Arizona is another species with planispiral early whorl but differs from W. humiligrada sp. nov. by having a narrower upper whorl face, a very prominent subsutural convexity, by lacking the sharp angulation at the basal edge and by having a very narrow or no umbilicus. W. crenulata Batten, 1989 from the Permian of southwestern United States, with steeper upper whorl face and more prominent nodes on its selenizone. The specimens assigned to W. corrugata Chronic, 1952 by Batten (1989) differ from W. humiligrada by having a stronger spiral ornament, a sharper collabral ornament and by the presence of axial folds on its upper whorl face. The specimens assigned to W. corrugata by Kulas & Batten (1997) from the Permian of Wyoming differ by having a narrow umbilicus and faint lunulae.