the dentary. It was a large and somewhat rounded bone that complemented the curvature of the dentary. On its inner surface it was attached to the . The articular was a smaller bone and had a concavity on its inner surface for the articulation with the quadrate. Protoceratops had leaf-shaped dentary and maxillary teeth that bore several denticles (serrations) on their respective edges. The crowns (upper exposed part) had two faces or lobes that were divided by a central ridge-like structure (also called "primary ridge"). The teeth were packed into a single row that created a shearing surface. Both dentary and maxillary teeth presented marked homodonty—a dental condition where the teeth share a similar shape and size. P. andrewsi bore two small, peg to spike-like teeth that were located on the underside of each premaxilla. The second premaxillary tooth was larger than the first one. Unlike dentary and maxillary teeth, the premaxillary dentition was devoid of denticles, having a relatively smooth surface. All teeth had a single root (lower part inserted in the alveoli). Postcranial skeleton The vertebral column of Protoceratops had 9 cervical (neck), 12 dorsal (back), 8 sacral (pelvic) and over 40 caudal (tail) vertebrae. The centra (centrum; body of the vertebrae) of the first three cervicals were coossified together (, and third cervical respectively) creating a rigid structure. The neck was rather short and had poor flexibility. The atlas was the smallest cervical and consisted mainly of the centrum because the (upper, and pointy vertebral region) was a thin, narrow bar of bone that extended upwards and backwards to the base of the axis neural spine. The capitular facet (attachment site for chevrons; also known as cervical ribs) was formed by a low projection located near the base of the neural arch. The anterior facet of the atlas centrum was highly concave for the articulation of the of the skull. The neural arch and spine of the axis were notably larger than the atlas itself and any other cervical. The axial neural spine was broad and backwards developed being slightly connected to that of the third cervical. From the fourth to the ninth all cervicals were relatively equal in size and proportions. Their neural spines were smaller than the first three vertebrae and the development of the capitular facet diminished from the fourth cervical onwards. The were similar in shape and size. Their neural spines were elongated