| Description |
Ceratopsian dinosaurs are one of the most diverse dinosaur groups in the Cretaceous, and an outstanding question is how growth strategies of this group evolved in relation to their shift from small bipedal basal ceratopsians to larger quadrupedal ceratopsids. Previous bone histology studies have investigated several basal ceratopsians and centrosaurine ceratopsids (e.g., Centrosaurus, Pachyrhinosaurus, Einiosaurus), but no chasmosaurine ceratopsids have been investigated. I conducted histological analysis of humeri, ulnae, femora, tibiae, ribs, and ossified tendons from multiple specimens of two species of chasmosaurine ceratopsid dinosaurs from the late Campanian (Upper Cretaceous) Kaiparowits Formation of southern Utah, Kosmoceratops richardsoni and Utahceratops gettyi, to examine bone microstructure indicators of growth rate and maturity. I also reexamined the long-bone histology of the ceratopsian dinosaurs Psittacosaurus mongoliensis, Protoceratops andrewsi, and Centrosaurus apertus. All elements of Utahceratops and Kosmoceratops examined are dominated by densely vascularized tissue, indicative of sustained fast growth. Radially-oriented vascular canals as well as dense osteocytes from throughout ontogeny are further indicators of rapid growth. I identified juvenile (UMNH VP 20444 & UMNH VP 20454), subadult (UMNH VP 16681) and adult (UMNH VP 16860, UMNH VP 16861, UMNH VP 12198) specimens of Utahceratops, and two subadult to adult specimens (UMNH VP 17000 & UMNH VP 21339) of Kosmoceratops. I conclude that basal ceratopsians grew more slowly than the large quadrupedal ceratopsids, as evidenced by a generally higher number of definitive growth lines prevalent throughout development. In contrast, the presence of dense osteocytes, and reticular and radially-oriented vascular canals are rapid growth indicators shared by all sampled large ceratopsids, and imply an elevated metabolism for all ceratopsians. Sampled specimens of Utahceratops and Kosmoceratops do not preserve any evidence of annual lines of arrested growth (LAGs). Placed in context with the number of LAGs observed in Alaskan Pachyrhinosaurus, Centrosaurus from Alberta, and Einosaurus from Montana, these data suggest a latitudinal gradient in the number of LAGs, which suggests that bone growth is reacting to the climate. |
