Proposed factors in its extinction include climatic change reducing viable habitat,[11] as well as human hunting of herbivore prey causing a trophic cascade.

[7] A replica of the jaw of the first American lion specimen to be discovered can be seen in the hand of a statue of famous paleontologist Joseph Leidy, currently standing outside the Academy of Natural Sciences in Philadelphia.

In the 1890s in the "Cueva del Milodon" in southern Chile, fossil collector Rodolfo Hauthal collected a fragmentary postcranial skeleton of a large felid that he sent to Santiago Roth.

In 1904, Roth reassessed the phylogenetic affinities of "Iemish" and named it Felis listai and referred several cranial and fragmentary postcranial elements to the taxon.

In 1934, Felis onca mesembrina was named by Angel Cabrera based on that partial skull from "Cueva del Milodon" and the other material from the site was referred to it.

[17] Further material, including feces and mandibles, was referred to as F. onca mesembrina from Tierra del Fuego, Argentina and other southern sites in Chile.

[20] The American lion was initially considered a distinct species of Pantherinae, and designated as Panthera atrox, which means "cruel" or "fearsome panther" in Latin.

It was proposed that it arose from pantherines that migrated to North America during the mid-Pleistocene and gave rise to American lions and jaguars (Panthera onca).

[3] Another study grouped the American lion with P. leo and P. tigris, and ascribed morphological similarities to P. onca to convergent evolution, rather than phylogenetic affinity.

[47][48][49] On the other hand, fossils of a large felid from late Pleistocene localities in southern Chile and Argentina traditionally identified as an extinct subspecies of jaguar, Panthera onca mesembrina, have been considered by some authors actually represent remains of the American lion, though this interpretation is highly controversial, with many authors favouring a jaguar attribution for these remains.

[50] At La Brea tar pits, recent isotopic analysis suggests that American lions preferred forested-dwelling prey much like the contemporary S.

Panthera atrox has the highest proportion of canine breakage in La Brea, suggesting a consistent preference for larger prey than contemporary carnivores.

[28] The fragment of a femur from a gray wolf from the La Brea Tar Pits shows evidence of a violent bite which possibly amputated the leg.

American lions likely descended from Panthera spelaea, which was possibly a solitary animal, based on fossil evidence and several isotopic studies.

[59] However, Van Valkenburgh and Sacco, in their 2002 paper, warned that high levels of sexual dimorphism isn’t a reliable way to determine group behavior.

However, the authors of the paper admit small sample size of remains prior to 14 kya does not support or refute evidence of forming prides.

[61] Some scientists suggested if American lions were gregarious, they likely had a larger brain to body mass than to solitary predators found at the La Brea tar pits.

[63] The remains of American lions are not as abundant as those of other predators like Smilodon fatalis or dire wolves (Aenocyon dirus) at the La Brea Tar Pits.

But its rarity in the tar pits would suggests that it was possibly more solitary than Smilodon and Aenocyon or was gregarious but lived in low densities similar to African wild dogs.

[67] A 2017 study suggested that the viable habitat for the American lion in North America had been greatly reduced over the course of the Last Glacial Period, which would have made it more vulnerable to extinction.

[11] Other authors have suggested that the extinction of the American lion and other competing carnivores like dire wolves, and the sabertooth cats Smilodon and Homotherium may have been due to trophic cascade effects caused by Paleoindian hunting of herbivores.

These authors suggested that the herbivores already probably existed at low population numbers prior to Paleoindian arrival due to their abundance being limited by predators, rather than being at the carrying capacity of the ecosystem based on food resources.