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Description
A size chart for the 3 species of Smilodon, along with a rival predator for each one. All silloutes are taken from Prehistoric Predators, sans the jaguar, which is just a stock image.

Part of the smilodontin tribe within the machairodontine felids, this group first reached the New World around 4.5 million years ago, during the early Pliocene. Following a local extinction event at the end of the Hemphillian stage (Late Miocene) that saw the disappearance of the previous apex predator guild (like Amphimachairodus, Barbourofelis, and Epicyon), many Old World predators started colonizing North America over the Bering Strait, and among them was Megantereon hesperus, part of highly sucesful genus that ranged all across Africa and Eurasia, thoug it never grew much bigger than a lioness.

Around 2.5 million years ago, at the start of the Pleistocene epoch, when Earth experience the first ice age, the American Megantereon evolved into Smilodon gracilis, a jaguar-sized animal that is known to have coexisted with a variety of predators, and although a formidable hunter, it was not the largest one in its environment, coexisting with the larger homotherin Xenosmilus (who was similarly stocky in build), the grizzly bear-sized tremarctine Arctodus pristinus, and of course, an invader from South America, the terror bird Titanis walleri, whose fossils suggest a height range of 1.5 to 2 meters.

By the end of the Blancan stage, 2 million years ago, Titanis dies out while S. gracilis subsequently shows a gradual increase in size, and at some point in the lower Irvingtonian, Xenosmilus vanishes as well, likely allowing S. gracilis to evolve into the larger Smilodon fatalis in North America, who was around a meter high at the shoulder but still easily weighed as much as a full-grown tiger due to its brawny physique. Though dwarfed by the massive Arctodus simus (who could have weighed up to 900 kg) and being shorter (but certainly more muscular) than the athletic Homotherium serum, S. fatalis was the largest cat and hypercarnivore in North America until about 350,000 years ago, when a population of cave lions (Panthera spelaea) migrated south of Beringia and evolved into the massive American lion (Panthera atrox), who was one of the largest cats known.

Rivaling it in size though was the South American descendent of S. gracilis, Smilodon populator, who stood up to 1.3 meters at the shoulder and regularly weighed over 300 kg, with one large specimen from Uruguay pointing towards a max size of 435 kg! Unlike its smaller cousins though, S. populator was actually the largest carnivore in its environment, with even the lion-sized Pleistocene jaguars being notably smaller. When S. gracilis first entered South America, it had to contend with the massive tremarctine Arctotherium angustidens, who rivaled A. simus in size, but for unknown reasons, around 800,000 years ago, Arctotherium species started a shrinking in size, right around the same time S. populator showed up, going from 500 kg to being no larger than the spectacled bear by the end of the Pleistocene, while S. populator remained a monster of a cat until just 10,000 years ago, when it, S. fatalis and H. serum vanished, leading to the end of the machairodonts.

Description
A size chart for the 3 species of Smilodon, along with a rival predator for each one. All silloutes are taken from Prehistoric Predators, sans the jaguar, which is just a stock image.

Part of the smilodontin tribe within the machairodontine felids, this group first reached the New World around 4.5 million years ago, during the early Pliocene. Following a local extinction event at the end of the Hemphillian stage (Late Miocene) that saw the disappearance of the previous apex predator guild (like Amphimachairodus, Barbourofelis, and Epicyon), many Old World predators started colonizing North America over the Bering Strait, and among them was Megantereon hesperus, part of highly sucesful genus that ranged all across Africa and Eurasia, thoug it never grew much bigger than a lioness.

Around 2.5 million years ago, at the start of the Pleistocene epoch, when Earth experience the first ice age, the American Megantereon evolved into Smilodon gracilis, a jaguar-sized animal that is known to have coexisted with a variety of predators, and although a formidable hunter, it was not the largest one in its environment, coexisting with the larger homotherin Xenosmilus (who was similarly stocky in build), the grizzly bear-sized tremarctine Arctodus pristinus, and of course, an invader from South America, the terror bird Titanis walleri, whose fossils suggest a height range of 1.5 to 2 meters.

By the end of the Blancan stage, 2 million years ago, Titanis dies out while S. gracilis subsequently shows a gradual increase in size, and at some point in the lower Irvingtonian, Xenosmilus vanishes as well, likely allowing S. gracilis to evolve into the larger Smilodon fatalis in North America, who was around a meter high at the shoulder but still easily weighed as much as a full-grown tiger due to its brawny physique. Though dwarfed by the massive Arctodus simus (who could have weighed up to 900 kg) and being shorter (but certainly more muscular) than the athletic Homotherium serum, S. fatalis was the largest cat and hypercarnivore in North America until about 350,000 years ago, when a population of cave lions (Panthera spelaea) migrated south of Beringia and evolved into the massive American lion (Panthera atrox), who was one of the largest cats known.

Rivaling it in size though was the South American descendent of S. gracilis, Smilodon populator, who stood up to 1.3 meters at the shoulder and regularly weighed over 300 kg, with one large specimen from Uruguay pointing towards a max size of 435 kg! Unlike its smaller cousins though, S. populator was actually the largest carnivore in its environment, with even the lion-sized Pleistocene jaguars being notably smaller. When S. gracilis first entered South America, it had to contend with the massive tremarctine Arctotherium angustidens, who rivaled A. simus in size, but for unknown reasons, around 800,000 years ago, Arctotherium species started a shrinking in size, right around the same time S. populator showed up, going from 500 kg to being no larger than the spectacled bear by the end of the Pleistocene, while S. populator remained a monster of a cat until just 10,000 years ago, when it, S. fatalis and H. serum vanished, leading to the end of the machairodonts.