Lancian

[1] By the Lancian, the crested hadrosaurs are no longer the dominant inhabitant of any province of western North America; the only remaining species was Hypacrosaurus.

[2] Lehman records three surviving chasmosaurs, Triceratops, Torosaurus and Nedoceratops, with the possibility of the recently discovered Ojoceratops, Regaliceratops and Bravoceratops.

In the south the transition to the Lancian is even more dramatic, which Lehman describes as "the abrupt reemergence of a fauna with a superficially "Jurassic" aspect.

"[4] The rapid expansion of land and drying of inland climate accompanying a drop in sea level could explain some of the environmental changes occurring Late Cretaceous western North America.

[6] If the geographic range of some dinosaur species were truly as limited as the fossil record suggests, then a rapid rise and in sea level could cause intense pressure even the event was local.

[6] Early Cretaceous deposits in North America reveal that basal neoceratopsians were already present on the continent before their apparent reemergence in the Lancian, so an immigration event from Asia is unnecessary to explain their appearance.

"[6] The faunal turnover may be explained by the descent of more primitive forms existing in upland refugia characterized by conifer-dominated flora into areas that were formerly coastal lowlands as the seas retreated and conditions became more arid.

[8] The decline of mammal diversity in Western North America from the Miocene to the present primarily effected large herbivores and occurred over roughly the same length of time as the Late Cretaceous changes, and so may be parallel.

[9] The mammalian turnover was preceded by an episode of immigration,[9] and was associated with the rapid expansion of terrestrial habitat due to melting glaciers.

[11] At the very least, Lehman argues, the altitudinal life zones would shift, and a change in the distribution of vegetation utilized by herbivorous dinosaurs would have probably resulted.