Bone histology has provided valuable information on the life history of dinosaurs, and the presence of growth lines provides useful information for age estimation, growth variation, and the reconstruction of paleobehavior. Here, we present new data recovered from five individuals of the non-iguanodontian ornithopod dinosaur Jeholosaurus shangyuanensis from the Early Cretaceous Jehol Biota. These specimens, ranging in body length from 16 to 62 cm, represent early juvenile, late juvenile, and subadult ontogenetic stages. The bones of Jeholosaurus mainly consist of fibrolamellar tissue, which is similar to that of other non-iguanodontian ornithopods; however, parallel-fibered bone and lamellar bone tissues were also deposited in early juvenile through subadult individuals, suggesting relatively slow growth rates. Parallel-fibered bone is only regionally present in the juvenile but is well developed throughout the outermost cortex of the subadult. Skeletochronology indicates that these specimens range in age from one to five years old. Analyzing bone tissue distribution and lines of arrested growth (LAGs) in these specimens, we estimate that Jeholosaurus reached sexual maturity at two to four years old. The largest individual (IVPP V15939) displays an apparently higher growth rate during the first two years, which is abruptly reduced in the following years, suggesting a distinct growth pattern that may be related to sexual dimorphism or variable environmental conditions. Finally, the largest specimen displays parallel-fibered bone tissue but lacks an external fundamental system (EFS) near the periphery, suggesting that it was still growing but was approaching somatic maturity at death.
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