Structure of the manus and pes has long been a source of confusion in ankylosaurs, owing to the imperfect preservation or complete lack of these parts of the skeletons in most specimens, and the fact that many species appear to have undergone a reduction in numbers of digits and phalanges. New specimens of Pinacosaurus from Alag Teeg in Mongolia confirm that the phalangeal formula of the manus is 2−3−3−3−2. However, there are only three toes in the pes, which has a phalangeal formula of X−3−3/4−3/4−X. Importantly, the number of phalanges in the third and fourth pedal digits can vary between either three or four per digit, even within the same specimen. The Alag Teeg site has yielded as many as a hundred skeletons of the ankylosaur Pinacosaurus, most of which were immature when they died. Each skeleton is preserved in an upright standing position, with the bones of the lower limbs often in articulation. The remainder of the skeleton, including the upper parts of the limbs, is generally disarticulated and somewhat scattered. Based on the presence of large numbers of juvenile Pinacosaurus specimens at Alag Teeg, as well as other Djadokhta−age sites (Ukhaa Tolgod in Mongolia, Bayan Mandahu in China), it seems juvenile Pinacosaurus were probably gregarious.

Key words: Ankylosauridae, Pinacosaurus, manus, pes, Cretaceous, Alag Teeg, Mongolia.

Philip J. Currie [Philip.Currie@ualberta.ca] and Eva B. Koppelhus [ebk@ualberta.ca], University of Alberta, Department of Biological Sciences, CW405 Biological Sciences Building, Edmonton, Alberta T6G 2E9, Canada; Demchig Badamgarav [dbadamgarav@yahoo.com]. Paleontological Center, Mongolian Academy of Sciences, Box 260, Ulaan Baatar 210351, Mongolia; Matthew Vickaryous [mvickary@uoguelph.ca], University of Guelph, Department of Biomedical Sciences, 50 Stone Road, Guelph, Ontario N1H 4Y1, Canada.