Scientific name: Th ylacosmilus atrox Scientific classification:
Phylum: Chordata Class: Mammalia Order: Sparassodonta Family: Thylacosmilidae
When did it become extinct? Th e pouch-knife became extinct around 4 million years ago.
Where did it live? Th e remains of this animal are only known from Argentina.
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 fi nds 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. Th e 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 modifi ed 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 super-continent of Gondwanaland, it carried an unusual assemblage of mammals quite distinct from the inhabitants of the other continents. Th ere 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 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)
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)
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, fl esh-eating birds. Evolution even shaped members of this marsupial stock into an animal very similar to the more familiar saber tooth cats. Th is animal was the pouch-knife, and it is a very enigmatic creature.
Th is animal was fi rst 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. Th e 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 superfi cially similar to those of the saber tooth cats, even though marsupials and cats sit on very diff erent branches of the mammalian family tree. Again, this is another excellent example of convergent evolu-tion and goes to show how nature can come up with similar soluevolu-tions to the same problem in very diff erent locations.
Th e skulls of saber tooth cats and the pouch-knife may be very similar at fi rst glance, but there are many major diff erences, which show that the pouch-knife was a very diff erent 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 fi xed 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 scabbard-like outgrowths of the pouch-knife’s chin. Th ese 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 armadil-los; numerous types of rodent (some of them huge); and the giant, predatory terror birds (see the entry later in this chapter). Th e 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 fl esh 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 fi rm 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. Th e hip joint of this animal is also very fl exible, 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). Th is 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 be-havior 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.
Th e 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 fi nding what’s left of these long-dead animals is very diffi cult 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 spe-cies, including the pouch-knife. Th ere 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). Th is began around 3 million years ago as a re-sult of the formation of the Isthmus of Panama—a land bridge that fully connected North and South America for the fi rst time. Land and freshwater animals freely traversed this bridge, and the mammals of South America were exposed to an infl ux 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. Th e dwindling pouch-knife may have never been very abundant, and in their last few thousand years, these mar-supials may have been pitted against the much larger saber tooth cats, which migrated into South America from the north. Th ese felines may have been more effi cient 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. Th is 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 fi n-ish 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.