This allowed mastodons to niche partition with other members of Proboscidea in North America, like gomphotheres and the Columbian mammoth, who had shifted to mixed feeding or grazing by the late Neogene-Quaternary.

Jefferson referenced the theory of American social degeneracy by Georges-Louis Leclerc, Comte de Buffon, countering it by using extant and extinct animal measurements, including those of "mammoths," as proof that North America faunas were not "degenerative" in size.

[16][17] In 1799, laborers recovered a thighbone while digging a marl pit at John Masten's farm in Newburgh, New York, and subsequent excavations were observed by a crowd of over a hundred people.

Author Keith Stewart Thomson argued that the promotion of the "mastodon" skeleton made it a symbol of the strength of American nationalism and that "mammoth" as a term became associated with gigantism.

Scottish writer Robert Kerr erected the species name Elephas americanus in 1792 based on fossil tusks and "grinders" from the Big Bone Lick locality.

Kerr was unsure about the taxonomic affinities of the molars and referenced that Thomas Pennant supposed that they belong to an unknown species within the genus Elephas, giving the common name "American elephant".

[69][48] In 1933, Childs Frick named the species Mastodon raki from the locality of Truth or Consequences, New Mexico based on differences on the heel and M3 tooth from M. americanus, otherwise having proportions similar to it.

[96] The skull of M. americanum has many plesiomorphies (or ancestral traits) that can be observed, namely the low and flat brain case, a slightly vertical basicranium, a narrow nasal aperture inlet of the nose with no step-like perinasal fossa, and a backside infraorbital foramen.

[48] Mammutids also exhibited evidences of horizontal tooth displacement where milk teeth were gradually replaced by permanent molars, mirroring elephantidans in an instance of parallel evolution.

[109] In terms of postcranial anatomy, M. pacificum differs from M. americanum by the presence of six as opposed to five sacral vertebrae and the femur having a larger diameter of the middle shaft (or main cylindrical area).

Of note is that whereas mammutids of Eurasia went extinct by the early Pleistocene in association with more seasonal climates, Mammut survived in North America and became abundant, although the reason for the latter faunal trend does not have any offered explanation.

[117] The browsing specialization of Mammut is supported further by the coprolites (or fossil dung) of M. americanum, which are large-sized similar to extant elephants and predominantly consist of consumed woody contents but no grass.

[118] Of the Pleistocene New World proboscideans, the American mastodon appears to have been the most consistent in browsing rather than grazing, consuming C3 as opposed to C4 plants, and occupying closed forests versus more open habitats.

[119] The mastodon commonly browsed on woody plants (i.e. twigs) and fruits, occupying dense coniferous forests made up of spruces (Picea) and pines (Pinus) within most of eastern North America.

In stark contrast, the contemporary gomphothere Stegomastodon showed progressive developments in response to increasingly arid and extensive grasslands from the Blancan up to the early Irvingtonian, with molar complexities resembling those of Mammuthus.

[132] The range of most species of Mammut is unknown as their occurrences are restricted to few localities, the exception being the American mastodon (M. americanum), which is one of the most widely distributed Pleistocene proboscideans in North America.

M. americanum fossil sites range in time from the Blancan to Rancholabrean faunal stages and in locations from as far north as Alaska, as far east as Florida, and as far south as the state of Puebla in central Mexico.

[48] Coexistent with the mammutid species were a large variety of other mammals, namely those of the Artiodactyla (antilocaprids, camelids, tayassuids), Carnivora (canids, felids, mustelids, ursids), Eulipotyphla (talpids), Lagomorpha (leporids), Perissodactyla (equids, rhinocerotids), and Rodentia (aplodontiids, castorids, geomyids, heteromyids, cricetids, mylagaulids, and sciurids).

[144] The latest Hemphillian of Florida based on the Palmetto Fauna of the Bone Valley Formation records the coexistence of M. matthewi with similar types of faunas, namely Pilosa (megalonychids), Eulipotyphla (talpids), Lagomorpha (leporids), Carnivora (borophagine canids, canine canids, ursids, procyonids, mustelids including lutrines, feline felids, machairodontine felids), Proboscidea (gomphotheres), Perissodactyla (tapirs, rhinocerotids, hipparionine equids), and Artiodactyla (tayassuids, protoceratids, camelids, "pseudoceratines," cervids, antilocaprids).

[145] North America in the late Neogene is understood to have undergone a long-term decline in large mammal diversity (i.e. the Dromomerycidae, "Blastomerycinae," Rhinocerotidae) as a result of C4 grassland expansion, cooler climates, and increased seasonality.

[148] M. raki from the Palomas Formation of Truth or Consequences in New Mexico is recorded with a few other mammalian faunas, namely the megalonychid ground sloth Megalonyx, the pocket gopher Geomys, the cricetid Sigmodon, the equin Equus, the hipparionine Nannippus, and the camelid Camelops.

[149] A late Blancan locality known as the Fish Springs Flat Fauna in Nevada reveals that fossils of M. americanum were found with those of the leporid Hypolagus, lutrine Satherium, equid Equus, camelid Gigantocamelus, gopher Thomomys, and the ground squirrel Spermophilus.

[153] The Port Kennedy Bone Cave of Pennsylvania is of Irvingtonian age (Middle Pleistocene) and reveals that during this time, M. americanum was present with the megalonychid Megalonyx wheatleyi, the tremarctine bear Arctodus pristinus, the jaguar (Panthera onca), the felid Miracinonyx inexpectatus, and the machairodontine Smilodon gracilis.

[150] The Big Bone Lick locality in Kentucky, which dates to the latest Pleistocene (Rancholabrean), indicates the coexistence of the American mastodon with the extant reindeer (Rangifer tarandus) along with various other extinct megafauna like ancient bison (Bison antiquus), the caprine bovid Bootherium bombifrons, mylodontid ground sloth Paramylodon harlani, megalonychid Megalonyx jeffersoni, true deer Cervalces scotti, equid Equus complicatus, and the Columbian mammoth.

[161][162] According to the American paleontologist Daniel C. Fisher, the "Heisler mastodon" site in Calhoun County, Michigan, which recovered about 50% of the skeleton, was proof of meat caching in a pond by Paleoindians in the late Pleistocene.

If true, they stated, the site would imply evidence of now-extinct species of Homo in North America during the Marine Isotope Stage 5 (MIS 5e) temporal range of the early late Pleistocene.

Mammut initially occupied the region during the Last Interglacial (~125,000-75,000 years ago) back when suitable forested habitats were present there but was subsequently extirpated in correlation with environmental changes from the Wisconsin glaciation (MIS 4).

[138] The latest Pleistocene of North America records a large extinction phase that resulted in the disappearances of over 30 genera of mammals, the majority of which are considered "megafauna" (~45 kg (99 lb) or larger).

[171] As a result, the extinctions that occurred in the latest Pleistocene of North America have been mainly attributed to human hunting, climate change, or some combination of the two (there are alternate but lesser-supported hypotheses).

[171] In 2018, Jack M. Broughton and Elic M. Weitzel calculated populated dynamics of some of the North American late Pleistocene megafauna based on summed probability distributions (SPDs) using calibrated radiocarbon dates.