Bone histology and growth curve of the earliest ceratopsian <i>Yinlong downsi</i> from the Upper Jurassic of Junggar Basin, Northwest China

<i>Yinlong downsi</i>, the earliest known ceratopsian, is represented by dozens of specimens of different sizes collected from the Upper Jurassic of the Junggar Basin, northwestern China. Here, we present the first comprehensive study on the bone histology of <i>Yinlong downsi</i> based on ten specimens varying in size. Four ontogenetic stages are recognized: early juvenile, late juvenile, subadult, and adult. The reconstructed growth curve suggests that <i>Yinlong</i> may reach sexual maturity at 6 years old, which is earlier than that of the well-studied early-diverging ceratopsian <i>Psittacosaurus</i> (9 years old) but later than ceratopsids (about 3 to 5 years old). This may indicate that sexual maturity begins earlier during the evolution of ceratopsians, and that the giant size of ceratopsids is acquired by accelerating growth rates. The cortex of the tibia mainly consists of fibrolamellar bone tissues, but parallel-fibered bone and lines of arrested growth (LAGs) are very common throughout ontogeny, suggesting a moderate growth rate. Quantitative analysis indicates that <i>Yinlong</i> has a maximum growth rate similar to those of other small-sized dinosaurs such as <i>Psittacosaurus</i>, <i>Dysalotosaurus</i>, and <i>Troodon</i>, and their maximum growth rates are higher than those of extant squamates and crocodiles but lower than those of extant mammals and large dinosaurs. This suggests that body size plays a more important role in growth rate than other factors such as phylogenetic position and/or diet among non-avian dinosaurs.