Edmontonian

In southern North America, little changed in the transition from the Judithian to the Edmontonian.[1] However, the northern biome experienced a general trend in reduction of centrosaurines, with leaving Pachyrhinosaurus as one of the few surviving species.[2] Likewise among lambeosaurs, only the single genus Hypacrosaurus remained.[3] Inland faunas of the age are distinguished by a Saurolophus-Anchiceratops association while more coastal areas were characterized by Pachyrhinosaurus and Edmontosaurus.[3] Pachyrhinosaurus occurred as far north as Alaska.[3] "Archaic" elements such as hypsilophodonts like Parksosaurus and the "(re)appearance" of basal neoceratopsians like Montanoceratops begin characterizing inland faunas.[3] Paleontologist Thomas M. Lehman described the Edmontonian Arrhinoceratops as a likely ancestor for the Lancian Triceratops.[3]

Ecological disturbance brought them to an end during the Edmontonian.[4] Relative sea levels fell very rapidly due to the Laramide orogeny.[4] Opportunistic generalist herbivores filled the vacated niches that were once filled by a diverse number of specialist forms.[4] The newly formed ecosystems tended to be dominated by a single herbivorous species each.[4] The new dominant herbivores were usually less ornamented and probably represent "survivors from indigenous lineages" rather than immigrants from other areas.[4] Gradually however "relict" dinosaurs such as protoceratopsids and sauropods began expanding into lower altitude areas as sea-levels fell.[4]

• Cifelli, Richard L.; Eberle, Jaelyn J.; Lofgren, Donald L.; Lillegraven, Jason A.; Clemens, William A. (2012). "Mammalian Biochronology of the Latest Cretaceous". In Woodburne, Michael O. (ed.). Late Cretaceous and Cenozoic mammals of North America biostratigraphy and geochronology. New York: Columbia University Press. ISBN 9780231503785.
• Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004). Mammals from the age of dinosaurs origins, evolution, and structure. New York: Columbia University. pp. 19–108. ISBN 9780231509275.
• Lehman, T. M., 2001, Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.
• Lucas, Spencer G, ed. (1991). "Dinosaurs and Mesozoic biochronology". Modern Geology. 16: 127–138. Reprinted in Halstead, L.B. (1991). Dinosaur studies : commemorating the 150th anniversary of Richard Owen's Dinosauria. Philadelphia: Gordon and Breach. ISBN 9782881248337.