The skull of the pouch-knife clearly shows the huge extensions of the mandible that protected the long canines. The long root of the canines can be seen extending beyond the eye. Very few remains of this animal are known. (Ross Piper)

Knife Predator
Pouch-Knife—A pair of pouch-knife marsupials prepare to go hunting after a long rest. This unusual predator probably used ambush tactics and strength to catch and subdue its prey. (Renata Cunha)

Scientific name: Thylacosmilus atrox Scientific classification: Phylum: Chordata Class: Mammalia Order: Sparassodonta Family: Thylacosmilidae When did it become extinct? The pouch-knife became extinct around 4 million years ago. Where did it live? The remains of this animal are only known from Argentina.

The skull of the pouch-knife clearly shows the huge extensions of the mandible that protected the long canines. The long root of the canines can be seen extending beyond the eye. Very few remains of this animal are known. (Ross Piper)

Today's land-dwelling, large mammal fauna is a shadow of what it was in prehistory. Since the disappearance of the dinosaurs, almost every landmass has been home to a changing roll call of large mammals. Of all the large mammals, the herbivores have attained the greatest sizes, and this, along with thick skin, horns, tusks, and antlers, has given them a lot of protection from potential predators. However, evolution always finds a way, and over the last 50 million years or so, there have been at least four separate mammal groups that have evolved a weapon to dispatch large, thick-skinned prey. The weapon is the saber tooth, and we have already been introduced to two types of extinct cat that were able to kill their prey with massively modified canine teeth (see the entries "Saber Tooth Cat" and "Scimitar Cat" in chapter 5).

When South America was rafted away from the other landmasses that formed the supercontinent of Gondwanaland, it carried an unusual assemblage of mammals quite distinct from the inhabitants of the other continents. There were the forerunners of the sloths, ant-eaters, and armadillos we know today as well as less familiar types. Along with Australia, South America was also a marsupial stronghold, and for a while, these pouched mammals were very successful predators on this southern continent. For much of the time, South America was isolated, and the only large predators were the marsupials and giant, flesh-eating birds. Evolution even shaped members of this marsupial stock into an animal very similar to the more familiar saber tooth cats. This animal was the pouch-knife, and it is a very enigmatic creature.

This animal was first described in 1934 by the paleontologist Elmer Riggs, of the Field Museum in Chicago, from two incomplete skeletons discovered in Argentina. In terms of size, the pouch-knife was probably as large as a jaguar, though it had shorter legs. The preserved skulls of this extinct marsupial have been slightly distorted by fossilization, but they, with fragments of unearthed skeletons, are still the only decent fossil evidence of the pouch-knife. It is amazing that the skull of the pouch-knife is so superficially similar to those of the saber tooth cats, even though marsupials and cats sit on very different branches of the mammalian family tree. Again, this is another excellent example of convergent evolution and goes to show how nature can come up with similar solutions to the same problem in very different locations.

The skulls of saber tooth cats and the pouch-knife may be very similar at first glance, but there are many major differences, which show that the pouch-knife was a very different mammal. Its sabers were enormous, relatively larger than those of Smilodon populator, and they also grew throughout the animal's life, which was very useful as the tips and cutting edge always remained sharp. As the pouch-knife's teeth grew continuously, they could not be fixed in the jaw with a bulbous anchor like those of the saber tooth cats. Instead, they grew from long roots that extended to a position well behind and above the pouch-knife's eyes. Also, when the mouth was closed, these massive canines were protected by scabbardlike outgrowths of the pouch-knife's chin. These scabbards were equipped with tough pads that may have sharpened the teeth as the jaws were opened and closed.

As the fossil record for the pouch-knife is so scant, we only have a very limited idea of how it lived. It seems that this pouched predator lived in a savannahlike environment, sharing this open habitat with the other strange denizens of South America, including the numerous types of large, native ungulate; the extinct relatives of the sloths and armadillos; numerous types of rodent (some of them huge); and the giant, predatory terror birds (see the entry later in this chapter). The pouch-knife was undoubtedly a predator as the canines are suited to killing and the shearlike cheek teeth are like those in the skull of a big cat—ideal for slicing flesh from a carcass. Not only was this extinct marsupial equipped with impressive teeth, but the region of the skull that once housed its hearing organs is well developed, indicating that this sense was probably acute. Along with sabers and a good sense of hearing, the pouch-knife's neck muscles and forelimbs must have been very strong. Powerful forelimbs allowed the marsupial to get a firm grip on prey, while the muscular neck allowed the stabbing canines to be driven through the tough hide of the victim into the soft tissues beneath. The hip joint of this animal is also very flexible, and some experts think it may have been capable of moving on its hind legs over short distances, much like the thylacine (see the entry in chapter 1). This may have been important in reaching up to the neck of its prey to deliver the killer bit. Exactly what prey the pouch-knife killed and ate is unknown, but it may have been a specialist predator of the numerous small- to medium-sized herbivores that once roamed South America. As it was short-legged and quite sturdy, it is doubtful that the pouch-knife was capable of pursuing its prey over any great distance. It probably opted for an ambush strategy, concealing itself behind pampas vegetation before it launched a lightning lunge at its victim. We may only be able to guess at the feeding behavior of this extinct predator, but we know much more about how it reproduced. As it was a marsupial, it probably had a pouch, and if the thylacine is a good example of a predatory marsupial, the female pouch-knife may have had a pouch that faced backward so that dirt and vegetation did not get into the furry pocket that cosseted her developing young. You can imagine a young pouch-knife, its sabers still small and developing, slipping from its mother's pouch to investigate the outside world.

The pouch-knife is a mysterious animal, and the fossil record of the group of animals to which it belongs is far from complete, but this is due to the fact that fossilization is very rare, and finding what's left of these long-dead animals is very difficult and often relies on sheer luck. What we do know is that the ancestors of the pouch-knife lived around 13 to 14 million years ago. What caused the demise of the pouch-knife? One unlikely theory is that an asteroid impact in South America caused the local extinction of many animal species, including the pouch-knife. There is some limited evidence for an impact event, but it is impossible to say if it was disastrous enough to kill off some of the South American fauna. It is more likely that the Great American Interchange led to the demise of the pouch-knife (see the "Extinction Insight" in chapter 2). This began around 3 million years ago as a result of the formation of the Isthmus of Panama—a land bridge that fully connected North and South America for the first time. Land and freshwater animals freely traversed this bridge, and the mammals of South America were exposed to an influx of North American animals. At the time of this event, the predatory marsupials were already on the decline, and we know from recent extinctions in Australia that when predatory marsupials come into direct competition with placental mammals, they often lose. The dwindling pouch-knife may have never been very abundant, and in their last few thousand years, these marsupials may have been pitted against the much larger saber tooth cats, which migrated into South America from the north. These felines may have been more efficient at dispatching their thick-skinned prey, contributing to the extinction of the pouch-knife.

♦ As the skull of the pouch-knife has been distorted by fossilization, the big canines are actually splayed, and it was once thought that this is how the living animal must have looked. This idea is now rejected as such large, splayed teeth jabbed into a victim would have generated skull-splitting force.

♦ In the marsupials we know today, the young become independent as soon as they finish taking their mother's milk. However, the pouch-knife young may have stayed with their mother for extended periods of time to learn and develop the specialized killing technique used by this species.

♦ Victorian paleontologists came up with all sorts of ideas for how the pouch-knife used its impressive teeth. One of the more amusing theories is that the marsupial used its canines and scabbards like can openers to open the domed carapaces of glyptodonts (see the entry in chapter 5). Even if a pouch-knife was foolish enough to gnaw the bony shell of one of these animals, it would have quickly found itself with a pair of broken canines.

Further Reading: Argot, C. "Evolution of South American Mammalian Predators (Borhyaenoidea): Anatomical and Palaeobiological Implications." Zoological Journal of the Linnean Society 140 (2004): 487-521.

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