Mongolocypris shai Wang, 2025, sp. nov.

Mongolocypris shai sp. nov. Figs. 2, 3C–O. 1985 Cypridea (Pseudocypridina) globra —Zhang: pl. 12, figs. 8a–b. ?1999 Langtonia setina rectidorsata —Nikolaeva & Neustrueva: pl. 9, fig. 8. ?2005 Langtonia setila rectidorsata —Neustrueva et al.: p. 37, pl. 13, fig. 8. (misspelled) Etymology. Named for the world-renowned Chinese geologist and paleontologist, Professor Jingeng Sha, in recognition of his significant contributions to our understanding of Mesozoic bivalves and the correlation of marine and non-marine Cretaceous stratigraphy. Additionally, he has been a longstanding mentor to YQW. Holotype. NIGP 206909 (Fig. 3D–E), complete carapace, sample 2024-XNG-11 from the Guanghuanjie-Yumuqiaozi section of the Xiaonangou Formation, L: 1.52 mm, H: 0.90 mm, W: 0.74 mm. Paratypes. NIGP 206910 (Fig. 3F–G), complete carapace, sample 2024-XNG-11 from the Guanhuanjie-Yumuqiaozi section of the Xiaonangou Formation, L: 1.46 mm, H: 0.81 mm, W: 0.62 mm; NIGP 206912 (Fig. 3J– K), complete adult carapace, sample 2024-XNG-10A from the Guanhuanjie-Yumuqiaozi section of the Xiaonangou Formation, L: 1.52 mm, H: 0.90 mm, W: 0.71 mm; NIGP 206915 (Fig. 3N–O), complete adult carapace, sample 2024-XNG-11 from the Guanhuanjie-Yumuqiaozi section of the Xiaonangou Formation. Type locality. Guanhuanjie-Yumuqiaozi section of the Baishan Basin, Xiaonangou Formation, Jilin, China (41°56'34.4"N, 126°34'18.6"E). Type horizon. Lower part of the Guanhuanjie-Yumuqiaozi section, Xiaonangou Formation, Baishan Basin, Jilin, China. Material. More than one hundred carapaces, but most of them compressed laterally and partially covered with sediment. Dimensions. L: 1.36–1.52 mm, H: 0.76–0.96 mm, W: 0.56–0.80 mm. Diagnosis. A species with elongated oval shape in lateral view; both anterior and posterior margins rounded and equicurvate; ventral margin nearly parallel to dorsal margin. Description. Carapace very large size with thick shell, elongated oval shape in lateral view, elongated ovate in dorsal view. Maximum height at anterior cardinal angle, maximum length at mid-height, maximum width at 2/3 of length. LV> RV; LV overlapping RV along all margins except for hinge margin. Anterior margin rounded and equicurvate, anterior cardinal angle rounded but remarkable, around 130–145°. Posterior margin equicurvate, posterior cardinal angle rounded, 130–145°. Dorsal margin nearly straight, gently sloping towards both anterior and posterior margins. Ventral margin nearly parallel to dorsal margin, slightly convex in LV. Rostrum short but visible, strongly bending backwards, its point acute and considerably overreaching ventral margin of the RV. Alveolar notch well-developed and triangle shape, more distinct in RV. Alveolar furrow absent. Hinge margin slightly incised, forming very shallow dorsal furrow. Surface smooth. Remarks. Based on the very large carapace size (larger than 1.2 mm) and the development of the rostrum, the studied specimens are assigned to the genus Mongolocypris. Mongolocypris shai sp. nov. differs from other Mongolocypris species by its more visible rostrum in the RV and the ventral margin nearly parallel to the dorsal margin. Mongolocypris shai sp. nov. is the dominant species in the Xiaonangou Formation of the Baishan Basin. Cypridea (Pseudocypridina) globra Hou, 1958 as identified by Zhang (1985), differs from the original description by Hou (1958) in having a nearly straight dorsal margin and anterior and posterior margins of almost equal width, with a less pronounced backward curvature of the rostrum. However, these carapace characteristics fully match the diagnosis of M. shai sp. nov. of this study, and thus it is assigned to M. shai sp. nov. Langtonia setina rectidorsata Sylvester-Bradley, 1949 reported from Mongolia is questionably included [The genus Langtonia is considered as a synonym of Cypridea (Pseudocypridina) by Sames (2011a)]. Although the illustrations of Mongolian specimens in Nikolaeva and Neustrueva (1999) and Neustrueva et al. (2005) show a close resemblance to M. shai sp. nov., those authors did not provide a detailed description and only illustrated the left valve. Therefore, the relationship between L. setina rectidorsata and M. shai sp. nov. requires further investigation. Mongolocypris shai sp. nov. differs from Cypridea (Pseudocypridina) limpida Zhang, 1985 [The subgenus Cypridea (Pseudocypridina) is considered as a synonym of Mongolocypris by Hou et al. (2002)] by having parallel dorsal and ventral margins with a more rounded posterior margin. Mongolocypris yanjiensis Gou, 1983 from the Albian Dalazi Formation of the Yanji Basin is similar to M. shai sp. nov., but the former features a supracurvate posterior margin and more pronounced cardinal angles. Distribution. Xiaonangou Formation (this study) and upper part of the Fuxin Formation of western Liaoning (Zhang 1985).