Editing Great Plains (section)

===Paleontology===
[[File:Daemonelix burrows (Harrison Formation, Middle Miocene; Agate Fossil Beds National Monument, northwestern Nebraska, USA) (15545351982).jpg|thumb|Excavation of a fossil [[Palaeocastor|Daemonelix]] burrow at Agate Fossil Beds National Monument. ]]
During the [[Cretaceous]] Period (145–66&nbsp;million years ago), the Great Plains were covered by a shallow [[Inland sea (geology)|inland sea]] called the [[Western Interior Seaway]]. However, during the [[Late Cretaceous]] to the [[Paleocene]] (65–55&nbsp;million years ago), the seaway had begun to recede, leaving behind thick marine deposits and a relatively flat terrain which the seaway had once occupied.<ref name="slattery">{{cite journal |title=Early Cretaceous to Paleocene paleogeography of the Western Interior Seaway: the interaction of eustasy and tectonism |first1=Joshua S |last1=Slattery |first2=William A |last2=Cobban |first3=Kevin C |last3=McKinney |first4=Peter J |last4=Harries |first5=Ashley L |last5=Sandness |journal=Wyoming Geological Association Guidebook |volume=68 |pages=22–60 |date=2013 |url=https://www.researchgate.net/publication/280641436 |doi=10.13140/RG.2.1.4439.8801}}</ref>

During the [[Cenozoic|Cenozoic era]], specifically about 25&nbsp;million years ago during the [[Miocene]] and [[Pliocene]] epochs, the continental climate became favorable to the evolution of grasslands. Existing forest biomes declined and grasslands became much more widespread. The grasslands provided a new [[Ecological niche|niche]] for mammals, including many [[ungulate]]s and [[glires]], that switched from browsing diets to grazing diets. Traditionally, the spread of grasslands and the development of grazers have been strongly linked. However, an examination of mammalian teeth suggests that it is the open, gritty habitat and not the grass itself which is linked to diet changes in mammals, giving rise to the "[[Grit, not grass hypothesis|grit, not grass]]" hypothesis.<ref>{{cite journal |first1=Phillip E. |last1=Jardine |first2=Christine M. |last2=Janis |first3=Sarda |last3=Sahney |first4=Michael J. |last4=Benton |title=Grit not grass: Concordant patterns of early origin of hypsodonty in Great Plains ungulates and Glires |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=2012 |volume=365–366 |pages=1–10 |doi=10.1016/j.palaeo.2012.09.001 |bibcode=2012PPP...365....1J |url=https://www.sciencedirect.com/science/article/pii/S0031018212004919}}</ref>

[[Paleontology|Paleontological]] finds in the area have yielded bones of [[mammoth]]s, [[Smilodon|saber-toothed cats]] and other ancient animals,<ref>"[http://www.museum.state.il.us/exhibits/larson/ice_age_animals.html# Ice Age Animals] {{Webarchive|url=https://web.archive.org/web/20110808030355/http://www.museum.state.il.us/exhibits/larson/ice_age_animals.html |date=August 8, 2011 }}". Illinois State Museum.</ref> as well as dozens of other [[megafauna]] (large animals over {{cvt|100|lb|disp=sqbr}}) – such as [[Ground sloth|giant sloths]], [[Evolution of the horse|horses]], [[mastodon]]s, and [[American lion]] – that dominated the area of the ancient Great Plains for thousands to millions of years. The vast majority of these animals became extinct in North America at the end of the [[Pleistocene]] (around 13,000 years ago).<ref>"[https://www.sciencedaily.com/releases/2006/10/061002154043.htm A Plan For Reintroducing Megafauna To North America] {{Webarchive|url=https://web.archive.org/web/20180628125128/https://www.sciencedaily.com/releases/2006/10/061002154043.htm |date=June 28, 2018 }}". ScienceDaily. October 2, 2006.</ref>

A number of significant fossil sites are located in the Great Plains including [[Agate Fossil Beds National Monument]] ([[Nebraska]]), [[Ashfall Fossil Beds]] ([[Nebraska]]), [[Clayton Lake State Park]] ([[New Mexico]]), [[Dinosaur Valley State Park]] ([[Texas]]), [[Hudson-Meng Bison Kill]] (Nebraska), [[Makoshika State Park]] (Montana), and [[The Mammoth Site]] ([[South Dakota]]).