Platyzona gradata, Ketwetsuriya & Karapunar & Charoentitirat & Nützel, 2020

Ketwetsuriya, Chatchalerm, Karapunar, Baran, Charoentitirat, Thasinee & Nützel, Al- Exander, 2020, Middle Permian (Roadian) gastropods from the Khao Khad Formation, Central Thailand: Implications for palaeogeography of the Indochina Terrane, Zootaxa 4766 (1), pp. 1-47 : 35-36

Platyzona gradata sp. nov.

( Fig. 23 )

Etymology. From Latin gradata, because of the gradate spire.

Holotype. Only one specimen: ESKU-19-LP 11 .

Dimensions (mm): ESKU-19- LP 11: height = 11.6; width = 8.3; apical angle = c. 72 º.

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. Moderately high-spired, turbiniform shell consisting of 6 whorls (apex missing) with a step-like whorl profile; early teleoconch whorls (first three whorls) rounded, convex, with an ornament of faint spiral threads (maybe due to preservation); upper whorl surface of later whorls developing wide, steeply inclined ramp; ramp slightly convex to straight ornamented by c. 8 to 10 equally spaced spiral threads; selenizone wide and flat, approximately 0.8 mm wide, covering almost half of whorl height of body-whorl, situated below ramp; adapical border of ramp at about mid-whorl, forming also angulated border of ramp; outer whorl surface of selenizone almost straight, parallel to shell axis, bordered by two spiral threads followed by strong spiral cords; selenizone with curved transverse lunulae; adapical spiral rib forming periphery; abapical spiral somewhat above lower suture; whorls embracing somewhat below abapical spiral rib; suture distinctly impressed; base flatly convex with numerous evenly spaced spiral threads, shallowly phaneromphalous; aperture not well-preserved, seemingly with straight columellar lip.

Remarks. Platyzona is reported from Thailand for the first time. Several species of this genus have been reported from other Permian assemblages from Asia e.g., from Cambodia ( Mansuy 1913 a; Delpey 1941), Perak, Malaysia ( Batten 1972), South China ( Pan & Erwin 2002) and from the Akasaka Limestone, Japan (Nützel & Nakazawa 2012). It has also been reported from the USA ( Batten 1989).

The single specimen at hand resembles the paratype of Platyzona dongluoensis ( Pan & Yu, 1993) from the Upper Permian Changxing Formation of south China ( Pan & Yu 1993, pl. 8, fig. 3a–c). However, the holotype of this species differs considerably from the paratype and from the present specimen in having a narrow selenizone at mid-whorl and in other characters ( Pan & Yu 1993, pl. 8, fig. 2a–b). The paratype of P. dongluoensis represents the genus Platyzona but the holotype and hence the species itself does not, the species reported by Pan & Erwin (2002) as of P. dongluoensis from the Late Permian of China resembles the misidentified paratypes of this species and P. gradata sp. nov. and could be conspecific.

P. nodohumerosa Batten, 1972 from the Permian of Malaysia resembles P. gradata sp. nov. but has a nodular ornament and distinct spiral cords or lirae on the selenizone. P. eulkaiensis ( Reed, 1927) as illustrated by Batten (1972) from the Permian of Malaysia and by Nützel & Nakazawa (2012) form the Permian of Japan is also similar but has a narrower selenizone that is lower on the whorls (just above the suture) and a weaker spiral ornament.

Three Late Permian species from South China have been assigned to Platyzona by Pan & Erwin (2002): P. pulchella Pan & Erwin, 2002 lacks spiral cords and has more pronounced growth-lines. P. nitella Pan & Erwin, 2002 is broader and has more pronounced lunulae. P. luculenta Pan & Erwin, 2002 is based on an early juvenile specimen (protoconch and 1.5 teleoconch whorls). In contrast to P. gradate sp. nov., it has strong spiral cords on the earliest teleoconch and the selenizone is higher on the whorls.

Four Permian species from SW USA have been assigned to Platyzona by Batten (1989): P. rotunda Batten, 1989 is broader and has entirely rounded whorls. P. cancellata Batten, 1989 has a much narrower selenizone and a cancellate ornament. P. pagoda Batten, 1989 is more slender and has a concave selenizone and has entirely rounded whorls. P. anguispira Batten , has uncoiled whorls.

Figures

FIGURE 23. Platyzona gradata sp. nov., A–F, ESKU-19-LP 11, holotype. Scale bars represent: 2 mm (A–B, E–F); 1 mm (C–D).

FIGURE 1. Geological map of the study location at Erawan Hill, Chong Sarika sub-district, Phatthana Nikhom district, located about 13 km east of Lopburi Province, Central Thailand.