Four skull specimens (MPC-D 100/537,100/538,100/539 and100/540) of Protoceratopsidae from the Upper Cretaceous in Udyn Sayr, Mongolia are described, and their ontogenetic stage and expression of sexual dimorphism are estimated. These specimens are identified as Protoceratops andrewsi (MPC-D 100/537, 100/539), P. cf. andrewsi (MPC-D 100/538), and Protoceratops sp. (MPC-D 100/540), respectively. MPC-D 100/537 and 100/539 are attributed to subadult “female” and MPC-D 100/538 to subadult “male”. MPC-D 100/540 is adult with unknown sex.
Based on the frill morphologies, the Udyn Sayr specimens are classified into three types: type 1 (MPC-D 100/539), well developed ridge on the lateral surface of the squamosal; posteriorly projected posterior margin of the squamosal; type 2 (MPC-D 100/537), posteriorly rounded posterior margin of the squamosal; developed ridge on the posterior margin of the parietal; and type 3 (MPC-D 100/540), large size; posteriorly curved posterior margin of the squamosal; the rugose surface texture on the dorsal side of parietal. MPC-D 100/538 could not be categorized because the specimen's frill is not preserved. These frill morphologies differ from those of Protoceratops from the Djadokhta Formation in the adjacent dinosaur locality Tugrikin Shire. The morphological differences among the Udyn Sayr specimens may indicate intraspecific variation of Protoceratops.