The first juvenile specimen of Manchurochelys manchoukuoensis from the Early Cretaceous Jehol Biota

A small juvenile turtle is described from the Early Cretaceous Jehol Biota, shedding light on the juvenile morphology and ontogeny of Manchurochelys manchoukuoensis. Several juvenile features are uncovered, such as a small and circular carapace (less than half of the adult), wide vertebral scales, and lateral carapacial fontanelles. In contrast to the adult morphology, which has an oval carapace, closed lateral fontanelles, and longer vertebrals 2–4, the juvenile of M. manchoukuoensis is more comparable to that of Sinemys lens, except for earlier occurrence of the well-ossified carapace of the latter. Differs from Changmachelys bohlini, and Ordosemys liaoxiensis, in which the circular carapace is relatively independent of ontogenetic age, and the lateral fontanelles are only closed in adult stage of O. liaoxiensis. Therefore, the trajectory of ontogenetic change appears to be highly diversified in the sinemydids.