Nanhsiungchelys cf. yangi

Nanhsiungchelys cf. yangi .

Referred specimen. SNHM 1558, a nearly complete carapace and plastron with associated skull and lower jaw.

Locality and horizon. South of the town of Nanxiong, Guangdong Province, China; Upper Cretaceous Zhenshui Formation (latest Campanian to early Maastrichtian).

Description and comparison

Te skull is poorly preserved. Te anterior part of the skull roof is damaged ( Fig. 1A View Fig ). Only the bones in dorsal and left sides of the skull can be identified with some confidence, but the large spaces between the bones limit the understanding of their shape and size ( Fig. 1B, C View Fig ). Te snout is nearly triangular in dorsal view, similar to Nanhsiungchelys yangi ( Ke et al., 2023) , but contrary to Nanhsiungchelys wuchingensis , which has an unusual trumpet shape snout ( Tong & Li, 2019). Most preserved parts of the maxilla are located anterior to the orbit, as in Nanhsiungchelys wuchingensis and Nanhsiungchelys yangi ( Ke et al., 2023; Tong & Li, 2019). Te jugal constitutes the lower rim of the orbit. Due to the poor state of preservation, the shape and size of the cheek emargination cannot be clearly discerned. Te postorbital is elongated anteroposteriorly and forms the posterodorsal and posterior rims of the orbit, as in Nanhsiungchelys wuchingensis and Nanhsiungchelys yangi ( Ke et al., 2023; Tong & Li, 2019). Te skull roof includes a partial prefrontal, a pair of pentagonal frontals, and a pair of parietals ( Fig. 1C View Fig ). Te prefrontal forms part of the dorsal rim of the orbit and contacts the postorbital and frontal posteriorly. Tus, the frontal is entirely excluded from the orbit, similar to other nanhsiungchelyids ( Brinkman et al., 2015; Joyce & Norell, 2005; Ke et al., 2023; Tong & Li, 2019). Te frontals are smaller than the parietals, as in Nanhsiungchelys yangi ( Ke et al., 2023) , but in contrast to Nanhsiungchelys wuchingensis ( Tong & Li, 2019) . Te outline of the temporal emargination cannot be identified with confidence, but likely resembles the shallow condition of Nanhsiungchelys wuchingensis and Nanhsiungchelys yangi ( Ke et al., 2023; Tong & Li, 2019).

Te lower jaw is exposed in ventral view. Te angle between the left and right branches is ~ 70° in SNHM 1558, wider than in Nanhsiungchelys wuchingensis and Nanhsiungchelys yangi (<55°) ( Ke et al., 2023; Tong & Li, 2019). Te unsatisfactory preservation limits more comparison.

Te carapace is 88 cm long and 50 cm wide (maximum width at the level of the sixth costal), with a length to width ratio of ~ 1.76 ( Fig. 1A View Fig ). Te nuchal is large, nearly trapezoidal, and signifiantly wider than the first vertebral scute, forming the central part of the deep nuchal emargination. Te suture between the nuchal and the first neural is straight, as in Anomalochelys angulata and Nanhsiungchelys spp. ( Hirayama et al., 2001; Tong & Li, 2019; Tong et al., 2024), but differing from the curved condition of Xianyuechelys yingliangi ( Ke et al., 2024a). Te preserved neurals (i.e., the first to fourth neurals) are wider than long, as in Nanhsiungchelys spp. ( Tong & Li, 2019; Tong et al., 2024). On the contrary, the first to fourth neurals of Anomalochelys angulata are elongated or as long as wide ( Hirayama et al., 2001), and the other nanhsiungchelyids usually have elongated neurals (e.g., Kharakhutulia kalandadzei ; Sukhanov et al., 2008). Te first to fourth neurals are wide hexagons with short posterolateral sides. Te differentiated neural series that occurs in Nanhsiungchelys sp. (JXGZ(2012)−181 and GMNHF10008, Tong et al., 2024) is not obvious in SNHM 1558. A total of eight pairs of costals could be identified. Te second to the eighth costals are alternating, as in Nanhsiungchelys sp. and Xianyuechelys yingliangi ( Ke et al., 2024a; Tong et al., 2024). A possible suprapygal appears posterior to the eighth costal, but the poor state of preservation limits an accurate identification. Te first peripheral is elongated and forms the anterolateral process. Tese processes are stick-like, as in Nanhsiungchelys sp. (JXGZ(2012)−181, Tong et al., 2024). By contrast, the processes of Nanhsiungchelys wuchingensis , Nanhsiungchelys yangi , and Nanhsiungchelys sp. (GMNHF10008) are flat-like ( Ke et al., 2023; Tong & Li, 2019; Tong et al., 2024). Although Anomalochelys angulata has a pair of similar processes, these elements are only formed by the nuchal ( Hirayama et al., 2001). Te end of the right process was broken and shows a clear cross section, suggesting the cortical bone and a greater proportion of cancellous bone ( Fig. 2 View Fig ).

Te cervical scute is crescent-shaped and estimated to be 3.5 times wider than long. Tis ratio is similar to that of Nanhsiungchelys sp. (JXGZ(2012)−181), but wider than that of Nanhsiungchelys wuchingensis and Nanhsiungchelys sp. (GMNHF10008) ( Tong & Li, 2019; Tong et al., 2024). Te first vertebral scute is roughly triangular with the lateral edges converging anteriorly, as in Anomalochelys angulata and Nanhsiungchelys spp. ( Hirayama et al., 2001; Tong & Li, 2019; Tong et al., 2024). Te second vertebral scute is a wide pentagon. Te shape of the third to fifth vertebral scutes cannot be well identified, but these elements are also wider than long. Te anterior and right margins of the fourth pleural scute could be identified, which is narrow with the lateral sulcus situates on the costals, as in Nanhsiungchelys sp. (JXGZ(2012)−181) ( Tong et al., 2024).

Te plastron is well preserved with most sutures and sulci visible ( Fig. 3 View Fig ). Te plastron is significantly concave in the middle. Te epiplastra are located at the anterior end of the plastron. Teir anterior margin protrudes farther anterior than the nuchal at its emargination. Te left and right epiplastra are disarticulated at the midline, which may be caused by taphonomic compression. Two lines horizontally cross the epiplastra that may represent sutures of additional ossifications (red lines in Fig. 3B View Fig ) ( Ke et al., 2024b). Te entoplastron is large and diamondshaped. Its posterior end is located posterior to the axillary notch. Both hyoplastron and hypoplastron are large and make a similar contribution to the bridge. Te suture between the hyoplastron and the hypoplastron is sinuous, as in Nanhsiungchelys wuchingensis ( Tong & Li, 2019) . Te posterior lobe is narrow and elongate, with a concave posterior rim. Te xiphiplastron is nearly trapezoidal, but the anterior margin is irregular. Te bridge is long, clearly longer than the anterior or the posterior lobe.

Te paired gular scutes are triangular, with the posterior edge extending deeply onto the entoplastron, as in Nanhsiungchelys sp. (CUGW VH272, Ke et al., 2024b). In comparison, Nanhsiungchelys wuchingensis has a single gular scute ( Tong & Li, 2019). Te extragular scute is lacking, as in other Nanhsiungchelys specimens ( Ke et al., 2023, 2024b; Tong & Li, 2019). Te humeropectoral sulcus is straight except the lateral ends, and does not intersect onto the entoplastron, which is unique among nanhsiungchelyids. Other nanhsiungchelyids have an anteriorly convex humeropectoral sulcus extending onto the entoplastron ( Brinkman & Nicholls, 1993; Brinkman et al., 2015; Danilov et al., 2013; Joyce & Norell, 2005; Mallon & Brinkman, 2018; Sukhanov, 2000; Sukhanov et al., 2008; Tong & Li, 2019; Tong et al., 2012, 2016). Te pectoral scute is extremely short, but a small part extends anteriorly to form the rim of the axillary notch, different from that of Nanhsiungchelys wuchingensis ( Tong & Li, 2019) . Te abdominal scute is large and square. Te femoral scute is trapezoidal. Te anal scute is small, wider than long. At least two inframarginal scutes could be identified between the abdominal and marginal scutes, which are wider than those of Nanhsiungchelys wuchingensis ( Tong & Li, 2019) . A small inguinal scute is located near the inguinal notch.