Thylacosmilus atrox was a large sabretoothed predator of
South America in the
Miocene and
Pliocene periods. It was however a
marsupial, so it was not closely related to the more famous North American sabretoothed
felids such as
Smilodon fatalis. When
Thylacosmilus first existed the two continents were separate; about 3.5 million years ago the
Isthmus of Panama was created, and placental mammals from the north came down and outcompeted a lot of the old fauna.
Information is a bit hard to come by. It existed; it was unique, in that this great elongation of teeth has not otherwise happened among marsupials (large predatory marsupials existed in Australia before humans arrived and out-preyed them); and no complete skeleton is known. Evidence mainly comes from two partial skeletons unearthed in Pliocene deposits in Argentina. It was first described by Elmer Riggs in 1934.
The weight of the largest known individual has been estimated at 116 kg, which makes it about the size and shape of a jaguar. It had short, muscly legs, so it might have gone in for surprise rather than trying to run down its prey. Some of the larger South American animals are slow and armoured, so Thylacosmilus's very sharp teeth, edged on both front and back, might have been for winkling the Miocene ancestors of armadillos out of their shells. (Or even the enormous extinct Glyptodon, though that would have been like eating a Volkswagen Beetle.) A long neck gave them more room for stabbing-muscles.
Its main competitors at the top of the food chain were Phorusrhacidae, a kind of crane or coot that had swollen up to ostrich size to fill the velociraptor niche. The Thylacosmilus was related to a group of hyena-like marsupials called Borhyaenidae. The modern cats did not exist in South America then, until they came down over the new land bridge, in what's known as the Great American Interchange. It's possible the phorusrhacids did in for the borhyaenids, and it was just a series of unfortunate events that meant the land bridge opened up about the same time.
Another possibility is indicated by the fact that remains only occur under a layer of glass. An impact from space large enough to affect the continent could have weakened their ecosystem and either extinguished them outright or left them vulnerable to takeover.
The genus name is from the Greek for pouch + chisel/knife. There was another species, T. lentis, which as far as the Web goes is just a name: it's T. atrox that gets the fame and the models made of it.
http://www.paleocraft.com/thylacosmilus.html
http://dml.cmnh.org/2002Jan/msg00210.html
http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Synapsida/Metatheria/Notometatheria/Sparassodonta/Sparassodonta.htm