Thylacosmilus (Thylacosmilus atrox)

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First described in 1934 by Elmer Riggs, Thylacosmilus atrox was a large saber-toothed predator that lived and hunted on the plains of prehistoric South America. What is known about the creatures is derived mostly from two partial skeletons found in Pliocene deposits of Argentina, which up to now remain the most complete evidence ever found. The Thylacosmilus ('pouch-knife') and its prey lived and thrived during the late Miocene and early Pliocene and are believed to have succumbed to extinction around 2 million years ago. Often lumped into the category of other saber-toothed predators, mainly the "saber-toothed cats" of the true cat family, the Thylacosmilus atrox was neither a cat nor related to cats at all but rather a highly specialized species of prehistoric pouched marsupial. It was more closely related to the opossum and kangaroo than to any feline or other placental mammalian group.

TheThylacosmilus is an excellent example of parallel evolution. It had long dagger-shaped teeth that could stab through the thickest of hides. Out of all the saber-toothed puncture killers, the saber teeth of Thylacosmilus were the longest and most specialized of them all. The fangs were tapered to an edge on both the front and the back cusps. In addition, unique among he saber-toothed tigers, the sabers continued to grow continuously throughout life to compensate for wear on the tips. These highly specialized features may have arisen as a countermeasure to the thick and sometimes armored coats that evolved among the contemporary herbivores that were its prey.

About the size and build of a modern jaguar, Thylacosmilus atrox was squat and powerfully built. Short, heavily muscled limbs indicates that it may have been an ambush hunter. Unlike its feline counterparts though, Thylacosmilus lacked retractable claws, an indication that it may have hunted in a very different way. Examination of the forelimbs shows an articulation and musculature that would have made it more than capable of grasping and subduing prey in a catlike manner.

Another similarity to the placental saber-tooths is the independent development of a long powerful neck. A long neck provides vertebrae with enlarged processes for muscle attachment. This provides for strong head-depressing musculature necessary for the stabbing action needed to successfully penetrate the hide and flesh of their prey.

Although anatomically similar in build and possibly in life-style as well, the Thylacosmilus would have looked uniquely distinct from other saber-toothed predators. They would have possessed marsupial characteristics not present in other animal groups. Color patterns, identifying marks, and social habits are indeterminable from fossil remains but may have parallels among modern marsupial predators. Such distinctions include subtle anatomical differences in the skulls of the marsupial saber-tooths compared to the skulls of their feline counterparts. The most distinct feature are the elongated flanges that jut down and gently slope inward from the lower jaw. These flanges would have acted like a natural sheath by serving to protect the teeth when the jaws were closed. The skull of Thylacosmilus atrox also possessed a postorbital bar, a very rare structure in carnivores that may have served as an area of attachment for temporalis fibers, and the aponeurosis, which overlays the entire muscle. It would have also shielded and secured the eyes from pressure by the contracting jaw muscles.

Thylacosmilus atrox would have reproduced in the same way as all marsupials, nursing their young to maturity by way of an external pouch as seen in kangaroos and wombats. The pouches may have opened to the back as seen in the recently extinct thylacines. Such an adaptation would have served to better protect the young while the mother raced through brush in pursuit of prey.

Thylacosmilus and related species are believed to have evolved into hunters from primitive marsupial insectivores that became isolated when South America drifted away from the rest of the continents. Isolation allowed marsupial as well as placental evolution to follow unique paths and fill unexploited niches left open after the demise of the dinosaurs. The result was many diverse, geographically distinct creatures that existed nowhere else on Earth. Thylacosmilus and their type are collectively known as borhyaenids ['meat/food-hyaenids']: the lions, tigers, and bears of their time. The doglike borhyaenids and saber-toothed thylacosmilids are placed into separate families, the Borhyaenidae and Thylacosmilidae respectively, within the superfamily Borhyaenoidea.

Thylacosmilus and its relatives survived unchallenged until around 2 million years ago when they ultimately became extinct. There are a couple theories attributed to their demise. The first of the two describes a great upheaval that completely changed the makeup of South American fauna. The upheaval followed a temporary end to the continent's isolation, when sea levels dropped and a land bridge emerged to join North and South America. Highly adapted mammals from the north would have migrated southward across the newly formed bridge. This "northern invasion" may have introduce new types of predators such as jaguars and pumas, which may have pressured and competed with the Thylacosmilus to the point of extinction. Another theory also suggests that the Thylacosmilus was outcompeted, not by other mammals though but by large, fast-running ground birds that used massive beaks to stab and rip their prey. These ground birds known as phororhacoids ('thief-rags-like', also phorusrhacoids) were more like latter-day carnivorous dinosaurs than birds. Their power and speed may have proved superior to the thylacosmilids and borhyaenids, winning out in the role for top predators. More recently there has come to light a new theory that suggests that the Thylacosmilus and other contemporary fauna may have fallen to extinction as a result of a meteor impact, much in the same way as the dinosaurs. Although it would have been on a much smaller scale, a localized impact could have upset the ecosystem causing the demise of entire groups of animals, especially if the species were geographically distinct to the blast area. Evidence for this is based on a stratum of rock and sediment containing natural glass formed of sand from intense heat. The intense heat is believed to have occurred as a result of a meteor impact, with the logic being that below the glass layer fossils of Thylacosmilus and other contemporary animals can be found but not above the layer, an intriguing theory. Clearly more evidence and further study is needed, and an exact cause for the extinction may never be known.