taller when approaching adulthood. In 2017 Mototaka Saneyoshi with team analyzed several Protoceratops specimens from the Djadokhta Formation, noting that from perinate/juvenile to subadult individuals, the parietal and squamosal bones increased their sides to posterior sides of the skull. From subadult to adult individuals, the squamosal bone increased in size more than the parietal bone, and the frill expanded to a top direction. The team concluded that the frill of Protoceratops can be characterized by these ontogenetic changes. In 2018 paleontologists Łucja Fostowicz-Frelik and Justyna Słowiak studied the bone histology of several specimens of P. andrewsi through cross-sections, in order to analyze the growth changes in this dinosaur. The sampled elements consisted of neck frill, femur, tibia, fibula, ribs, humerus and radius bones, and showed that the histology of Protoceratops remained rather uniform throughout ontogeny. It was characterized by simple fibrolamellar bone—bony tissue with an irregular, fibrous texture and filled with blood vessels—with prominent woven-fibered bone and low bone remodeling. Most bones of Protoceratops preserve a large abundance of bone fibers (including Sharpey's fibres), which likely gave strength to the organ and enhanced its elasticity. The team also find that the growth rate of the femur increased at the subadult stage, suggesting changes in bone proportions, such as the elongation of the hindlimbs. This growth rate is mostly similar to that of other small herbivorous dinosaurs such as primitive Psittacosaurus or Scutellosaurus. Movement In 1996 Tereshchenko reconstructed the walking model of Protoceratops where he considered the most likely scenario to be Protoceratops as an obligate quadruped given the proportions of its limbs. The main gait of Protoceratops was probably trot-like mostly using its hindlimbs and it is unlikely to have used an asymmetric gait. If trapped in a specific situation (like danger or foraging), Protoceratops could have employed a rapid, facultative bipedalism. He also noted that the flat and wide pedal unguals of Protoceratops may have allowed efficient walking through loose terrain, such as sand which was common on its surroundings. Tereshchenko using speed equations also estimated the average maximum walking speed of Protoceratops at about 3 km/h (kilometres per hour). Upon the analysis of the forelimbs of several ceratopsians, Phil Senter in 2007 suggested that the hands of Protoceratops could reach the ground when the hindlimbs were upright, and the overall forelimb morphology and range of motion may reflect that it was at least a facultative (optional) quadruped. The