TWO TOROSAURUS FRILLS FROM THE GLENROCK EXPOSURE: PARIETAL SIZE VARIATION SUPPORTS TAXONOMIC RETENTION

GMP 212 and GMP 213 are both ceratopsian parietals collected from the Glenrock Exposure of the Lance Formation (Upper Maastrichtian), WY. Both include the dorsal margin of a large parietal fenestra and a surface texture lacking deep and abundant vascular channels – both characteristics historically used to distinguish the controversial chasmosaurine genus Torosaurus from the more common Triceratops. Some previous work has argued that Torosaurus represents an advanced ontogenetic stage of Triceratops and that the former should be synonymized with the latter. Other research has disputed this claim. Across the dorsal margin, GPM 212 has a parietal-midline to lateral-edge width of just over 73 cm, indicating a moderately-sized, but still potentially ontogenetically mature, individual. The equivalent measurement of GPM 213 comes to only 56 cm. GPM 213 is nearly half the size of other Torosaurus specimens (including TMM 41480 and TGM 3145). This small size indicates that GPM 213 is only a subadult and confirms that characteristic Torosaurus cranial morphology was not limited to ontogenetically mature individuals, which, in turn, supports the retention of Torosaurus as a separate genus.