Smilodon
Distribution and habitat

Smilodon lived during the Pleistocene epoch (2.5 mya–10,000 years ago), and was perhaps the most recent of the saber-toothed cats. S. fatalis lived in a variety of habitats, being able to inhabit open grassland and parkland, marginal woodland-grassland settings, and closed forests. Fossils of the genus have been found throughout the Americas. The northernmost remains of the genus are S. fatalis fossils from Alberta, Canada, with the southernmost remains being known from the far south of Patagonia. The habitat of North America varied from subtropical forests and savannah in the south, to treeless mammoth steppes in the north. The mosaic vegetation of woods, shrubs, and grasses in southwestern North America supported large herbivores such as horses, bison, antelope, deer, camels, mammoths, mastodons, and ground sloths. North America also supported other saber-toothed cats, such as Homotherium and Xenosmilus, as well as other large carnivores including dire wolves, short-faced bear (Arctodus simus) and the American lion. Competition from such carnivores may have prevented North American S. fatalis from attaining the size of South America's S. populator. The similarity in size of S. fatalis and the American lion suggests niche overlap and direct competition between these species, and they appear to have fed on similarly sized prey.

S. gracilis entered South America during the early to middle Pleistocene, where it probably gave rise to S. populator, which lived in the eastern part of the continent. S. fatalis also entered western South America in the late Pleistocene, and the two species were thought to be divided by the Andes mountains. However, in 2018, a skull of S. fatalis found in Uruguay east of the Andes was reported, which puts the idea that the two species were allopatric (geographically separated) into question. The American interchange resulted in a mix of native and invasive species sharing the prairies and woodlands in South America; North American herbivores included proboscideans, horses, camelids and deer, South American herbivores included toxodonts, litopterns, ground sloths, and glyptodonts. Native metatherian predators (including the saber-toothed thylacosmilids) had gone extinct by the Pliocene, and were replaced by North American carnivores such as canids, bears, and large cats.

S. populator was very successful, while Homotherium never became widespread in South America. The extinction of the thylacosmilids has been attributed to competition with Smilodon, but this is probably incorrect, as they seem to have disappeared before the arrival of the large cats. The phorusrhacid "terror birds" may have dominated the large predator niche in South America until Smilodon arrived. S. populator may have been able to reach larger size than S. fatalis due to a lack of competition in Pleistocene South America; S. populator arrived after the extinction of Arctotherium angustidens, one of the largest carnivores ever, and could therefore assume the niche of mega-carnivore. S. populator preferred large prey from open habitats such as grassland and plains, based on evidence gathered from isotope ratios that determined the animal's diet. In this way, the South American Smilodon species was probably similar to the modern lion. S. populator probably competed with the canid Protocyon there, but not with the jaguar, which fed primarily on smaller prey. On the other hand, morphometry points to S. populator being best adapted for more closed environments.
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