Plesiosaurs were a long-lived and widespread group of marine reptiles, with a worldwide distribution and a temporal range from the Late Triassic to the Late Cretaceous. Most occur in marine deposits, but some occur in low-salinity, brackish to freshwater environments. We report plesiosaurs from the freshwater fluvial deposits of the mid-Cretaceous (?Albian–Cenomanian) Kem Kem Group of Morocco. Remains include numerous shed teeth, vertebrae, and a humerus. The humerus represents a young juvenile; vertebrae likely belong to sub-adults. Teeth show heavy wear, similar to teeth of co-occurring spinosaurids. While coeval plesiosaurs from the Bahariya Formation of Egypt are members of Polycotylidae, the Kem Kem fossils show features of Leptocleididae, small-bodied plesiosaurs that were widely distributed in nearshore and non-marine settings in the Early Cretaceous. These fossils are the first freshwater plesiosaurs from Morocco, and are among the youngest representatives of Leptocleididae. The Kem Kem leptocleidids could have been infrequent visitors from the sea, freshwater-tolerant, or even freshwater-adapted, as in modern river dolphins. The abundance of shed teeth in the Kem Kem Group supports the hypothesis that they had some degree of freshwater tolerance. Furthermore, leptocleidids occur almost exclusively in shallow nearshore, brackish, or freshwater environments, suggesting adaptation to shallow, low-salinity environments. Other plesiosaur groups and other Mesozoic marine reptiles, including teleosaurids and mosasaurids, also occur in freshwater settings, suggesting plesiosaurs and other marine reptiles frequently exploited non-marine environments.

蛇颈龙是一种长寿且分布广泛的海洋爬行动物群，分布于世界各地，时间范围从三叠纪晚期到白垩纪晚期。大多数出现在海洋沉积物中，但也有一些出现在低盐度、微咸水至淡水环境中。我们报告了摩洛哥中白垩纪（？Albian-Cenomanian）Kem Kem 群淡水河流沉积物中的蛇颈龙。遗骸包括许多脱落的牙齿、椎骨和肱骨。肱骨代表年轻的少年；椎骨很可能属于亚成人。牙齿磨损严重，类似于同时出现的棘龙的牙齿。虽然来自埃及 Bahariya 组的同时代蛇颈龙是多子叶科的成员，但 Kem Kem 化石显示出 Leptocleididae 的特征，白垩纪早期广泛分布在近岸和非海洋环境中的小型蛇颈龙类。这些化石是来自摩洛哥的第一批淡水蛇颈龙，是蛇颈龙科最年轻的代表之一。Kem Kem leptocleidids 可能是来自海洋的不常游客，它们可以耐受淡水，甚至可以适应淡水，就像现代河豚一样。Kem Kem 群中大量脱落的牙齿支持了它们具有一定程度的淡水耐受性的假设。此外，leptocleids 几乎只出现在浅近岸、半咸水或淡水环境中，这表明它们适应了浅层、低盐度的环境。其他蛇颈龙类群和其他中生代海洋爬行动物，包括远程龙科和沧龙科，也出现在淡水环境中，