forelimbs of Protoceratops could sprawl laterally but not for quadrupedal locomotion, which was accomplished with the elbows tucked in. In 2010 Alexander Kuznetsov and Tereshchenko analyzed several vertebrae series of Protoceratops in order to estimate overall mobility, and concluded that Protoceratops had greater lateral mobility in the presacral (pre-hip) vertebrae series and reduced vertical mobility in the cervical (neck) region. The fossilized footprint associated with the specimen ZPAL Mg D-II/3 described by Niedźwiedzki in 2012 indicates that Protoceratops was digitigrade, meaning that it walked with its toes supporting the body weight. In 2019 however, Słowiak and team described the limb elements of ZPAL Mg D-II/3, which represents a sub-adult individual, and noted a mix of characters typical of bipedal ceratopsians such as a narrow glenoid with scapular blade and an arched femur. The absence of these traits in mature individuals indicates that young Protoceratops were capable of facultative bipedal locomotion and adults had an obligate quadrupedal stance. Even though adult Protoceratops were stocky and quadruped, their tibia-femur length ratio—the tibia being longer than femur, a trait present in bipedal ceratopsians—suggests the ability to occasionally stand on their hindlimbs. Słowiak and team also suggested that the flat and wide hand unguals (claw bone) of Protoceratops may have been useful for moving on loose terrain (such as sand) without sinking. Digging behavior Longrich in 2010 proposed that Protoceratops may have used its hindlimbs to dig burrows or take shelter under bushes and/or scrapes in order to escape the hottest temperatures of the day. A digging action with the hindlimbs was likely facilitated by the strong caudofemoralis muscle and its large feet equipped with flat, shovel-like unguals. As this behavior would have been common in Protoceratops, it predisposed individuals to become entombed alive during the sudden collapse of their burrows and high energy sand-bearing events—such as sandstorms—and thus explaining the standing in-situ posture of some specimens. Additionally, Longrich suggested that a backward burrowing could explain the preservation of some specimens pointing forward with curved tails. In 2019 Victoria M. Arbour and David C. Evans cited the robusticity of the ulna of Ferrisaurus as a useful feature for digging, which may have been also true for Protoceratops. Tail function Gregory and Mook in 1925 suggested that Protoceratops was partially aquatic because of its large feet—being larger than the hands—and the very long neural spines found in the caudal (tail) vertebrae. Brown and Schlaikjer in