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level could explain some of the environmental changes occurring Late
Cretaceous western North America. The wetland habitat enjoyed by many dinosaurs would have shrunk and fragmented. Since many species had very limited geographic ranges its plausible that some of the fragments would be smaller than the area needed to support the species. However, there's no direct evidence for the shrinking of wetland environments. Lehman contends that the actual area of coastal lowlands within 150m of the shoreline must have actually increased significantly. Further, dinosaurs that inhabited inland or arid environments were among the most prevalent in the Lancian. The
428:. This faunal turnover coincides with the Laramide orogeny and the uplift of the central Rockies. Strata exhibit changes in lithology and the direction of paleocurrents, and a severe drop in relative sea level. 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. By the end of the Judithian, North America had 7.7 million km of land area, but by the end of the Lancian it had reached 17.9 million km, nearly the modern value of 22.5 million km.
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The extreme changes occurring in the make-up of herbivore communities during the faunal turnover suggests that a change in the ecosystems' flora was "the most immediate cause...though perhaps not the ultimate one." The rapid expansion of land and drying of inland climate accompanying a drop in sea
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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. They have many commonalities, including the replacement of diverse
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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. The major potential immigrants represent archaic forms that probably
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was an immigrant from Asia. Inhabitants of upland environment are more likely to be endemic than coastal species, and tend to have less of an ability to cross bodies of water. Further, early
Cretaceous titanosaurs were already known, so North American potential ancestors for
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Terrestrial sedimentary strata from the
Judithian to the Lancian are generally regressive throughout the entire sequence, so the preserved changes in fossil communities represent not only phylogenetic changes but ecological zones from the
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being the only known survivors. 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." These faunas are dominated by
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to the south). The most spectacular and specialized forms became extinct. The mammalian turnover was preceded by an episode of immigration, and was associated with the rapid expansion of terrestrial habitat due to melting
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In the southern biome, by
Lancian time sauropods had replaced both hadrosaurs and ceratopsians. In the north, both were still present although hadrosaurs were demoted to a "subordinate" role in dinosaur ecosystems.
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wouldn't have directly competed with the disappearing forms anyway. Lehman described the evidence for immigration as a driving force in
Lancian dinosaur faunal turnovers as "not particularly compelling."
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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.).
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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.
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was the dominant northern hadrosaurid. At the end of the Cretaceous, most ecosystems were dominated by a single herbivore. The northern biome was dominated by
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also had precursors in North America and its apparent range expansion may represent the expansion of its preferred habitat rather than an immigration event.
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By the Lancian, the crested hadrosaurs are no longer the dominant inhabitant of any province of western North America; the only remaining species was
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co-occurred in Asia as well as North America. Potential Asian immigrants were especially common in upland environments. The appearance of
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126:. It was the final stage of the Cretaceous period in North America, lasting from approximately 70.6 to 66 million years ago.
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may have represented an immigration event from South America. Some taxa may have co-occurred on both continents, including
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The faunal turnover may be explained by the descent of more primitive forms existing in upland refugia characterized by
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were some of the known surviving hadrosaurs. All lacked the elaborate ornamentation of their predecessors, the
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appears and achieves dominance in its environment very abruptly. Some scientists speculated that
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227:. The ankylosaurs had been reduced to down to a handful of species, with ankylosaurs like
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Dinosaur studies : commemorating the 150th anniversary of Richard Owen's Dinosauria
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Late Cretaceous and Cenozoic mammals of North America biostratigraphy and geochronology
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created selective pressure as coastal lowlands were swallowed up the sea, resulting in
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association probably represent semi-arid inland plains. In previous research
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Lucas, Spencer G, ed. (1991). "Dinosaurs and Mesozoic biochronology".
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548:"Competition from Invaders Hypothesis," Lehman (2001); pages 321-322.
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Mammals from the age of dinosaurs origins, evolution, and structure
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Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004).
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557:"Descent from the Highlands Hypothesis," Lehman (2001); page 322.
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584:"A Recent Analog?" Lehman (2001); pages 323-324.
304:speculated that a rise in sea level during the
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139:habitats to near-sea level coastal habitats.
461:"Lancian Turnover," Lehman (2001); page 317.
51:introducing citations to additional sources
575:"A Recent Analog?" Lehman (2001); page 323.
479:"What Happened?" Lehman (2001); page 319.
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315:The appearance or reappearance of basal
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41:Relevant discussion may be found on the
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610:"Conclusions," Lehman (2001); page 324.
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358:already existed.
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103:August 2015
743:Categories
716:: 127–138.
617:References
310:anagenesis
275:in Texas.
248:Edmontonia
193:Torosaurus
162:Torosaurus
137:submontane
73:newspapers
432:Footnotes
342:Avisaurus
62:"Lancian"
43:talk page
402:glaciers
754:Lancian
406:cervids
393:caribou
372:conifer
130:Geology
122:of the
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