My home state of New Jersey is the epitome of suburban sprawl, McMansions and cul de sacs being about as common as the White-Tailed Deer that take advantage of the grass and brush on the side of the Garden State Parkway year round. There is seemingly no place you can go in the state where the rumble and roar of traffic cannot be heard, although the sprawling network of impervious surface does allow for easy travel to almost anywhere in the “Garden State.” On these roads, usually on warm summer nights, you’re likely to see what appears to be a large white rat shuffling across the lanes. While there are no hackneyed jokes that I know of about the Virginia Opossum crossing the road (“To eat your garbage” would be the most realistic answer), the critters turn up as roadkill quite often, not a very dignified end for the only marsupial mammal to live north of the Rio Grande in North America. While the scruffy Virginia Opossum represents the whole of marsupial mammals in the U.S., it has many close relatives throughout South America (Order Didelphimorphia), and is a bit more distantly related (but still close) to the Australian “possums” (Suborder Phalangeriformes), the marsupial forms of “the island continent” being perhaps the most familiar and oddly charismatic of any members of the Infraclass Marsupialia.
A Red Kangaroo (Macropus rufus) at the Philadelphia Zoo (taken in February, 2007). The Red Kangaroo is probably the world’s most recognizable living marsupial mammal.
Much like any group of living mammals, however, the fossil record of marsupial mammals is full of bizarre forms that have left no living descendants. We should not regard such lines of extinct fauna as somehow inferior or flawed, however. As famed paleontologist Stephen Jay Gould once wrote in his book Wonderful Life;
First, in an error that I call “life’s little joke”, we are virtually compelled to the stunning mistake of citing unsuccessful lineages as classic “textbook cases” of “evolution.” We do this because we try to extract a single line of advance from the true topology of copious branching. In this misguided effort, we are inevitably drawn to bushes so near the brink of total annihilation that they retain only one surviving twig. We then view this twig as the acme of upward achievement, rather than the probable last gasp of richer ancestry.
I can scarcely think of a better example of this notion of the spectacular diversity of past life than the extinct marsupial Australian Megafauna, and the carnivorous Thylacoleo carnifex would remind any fossilist that just because an animal is extinct, such status does not imply that it was not a terror in its heyday. Deemed the “Marsupial Lion” Thylacoleo carnifex developed many of the predatory adaptations we seen in living big cats (hence the “leo”, meaning “lion”, in the genus name), and despite the superficially rodent-like appearance of it’s front teeth, it was certainly a powerful predator.
In order to understand why Thylacoleo was such a formidably hunter we first need to understand something about living Carnivora (civets, otters, cats, dogs, bears, etc.) and the way their teeth were arranged. While their have been many large carnivorous mammals since in the past 65 million years, carnivores are set apart by their carnissal, or “scissor”, teeth. If we look at the massive skull of the predatory mesonychid Andrewsarchus of the Eocene, for example, the front teeth appear useful for piercing but the teeth further back in the jaw a large and a bit blunted. While useful in tearing flesh from bone and crushing, they were not especially well-adapted to cutting slicing flesh and such creatures probably ate a fair amount of bone (and possibly had problems with bone splinters in their gastrointestinal tracts) as well.
The only known skull of Andrewsarchus, on display at the American Museum of Natural History in New York City. Note the large, blunted teeth towards the back of the jaw.
The likely ancestors of today’s extant carnivores had their start long before Andrewsarchus was roaming what is present-day Asia. Miacids were weasel-like mammals and are known from the Paleocene and Eocene epochs, and are the first mammal group known to have teeth called “carnissals.” These are the teeth that group all living carnivores together, robust and pointed teeth that seem to be essential to consuming flesh. Another group of mammals, the creodonts (the first of which were discovered by E.D. Cope), also possessed carnissal teeth, but their line died out about 8 million years before the present. Still, the success of the carnivorous mammals seemed to depend on the specialization of the some of the premolar and molar teeth into a sharp, cutting edge, commiting many of the group to a strictly carnivorous lifestyle. Cats are the most specialized today, as they have lost some teeth in the front of their jaw in order to allow their dagger-like canines to have the maximum effectiveness and they no longer have flattened molars at the back of their jaw like canids (dogs) have, allowing those animals a little bit more of a diverse diet in tough times. Indeed, overspecialization in a predatory niche, called “hypercarnivory,” can often put a species at risk if they cannot effectively process other food sources if prey stocks dwindle (such a hypothesis has been put forward about the recent “bone-crunching wolf” discovered in Alaska).
A replica of the skull of Thylacoleo, on display at the American Museum of Natural History.
Thylacoleo, however, was an entirely different branch of the mammalian tree, but it seems that its skull and jaws were adapted to similar ends (although arguably were more extreme in their modifications). As easily seen from the skull of Thylacoleo, this marsupial predator was adapted to have it’s own fearsome shearing teeth. The premolars essentially became laterally-compressed blades, more high-ridged and pointed at the front, yet still sharp all the way down their length. These teeth in the upper and lower jaw even helped to sharpen each other as they slid past, allowed the predator to retain a sharp edge. Flattened teeth that might be useful for grinding or processing other foods are entirely absent behind the premolars, showing the Thylacoleo was a specialist of the highest order, having much more scissor-like teeth than the placental carnivores on other continents. Such a gape would have been absolutely fearsome, as exemplified by this recent reconstruction by Jeanette Muirhead;
Thylacoleo carnifex, used with permission of artist Jeanette Muirhead.
What is even more surprising than the blade-like teeth of Thylacoleo, however, is how strong its jaws were for a creature of its size. A recent study by Wroe, McHenry, and Thomason found that Thylacoleo, a predator that was less than four-feet long and probably weighed only 220 pounds, had the a bite force equivalent to a modern lion twice its size. The unusual dental arrangement of its jaw might have mitigated this somewhat and technical trials still have to be carried out, but if what the researchers found holds then Thylacoleo could probably have preyed upon most animals living in its range up to sub-adult size on its own, perhaps being the fiercest mammalian predator ever known.
How did Thylacoleo attain such high bite forces? The answer might have to do with the brain and skull differences between marsupials and placental mammals. Many carnivores have relatively large brains in comparison with marsupials, lessening the amount of bone they can devote to massive muscle attachments to enhance bite force. Thylacoleo, by contrast, seems to have had stronger muscle attachments and a smaller brain, and it’s skull superficially resembles that of a big cat. While canids often have elongated skulls, cats have foreshortened ones, and oddly enough Wrote and his colleagues seem to have found that carnivorous mammals that are known to be bone crunchers (primarily dogs or dog-like carnivores) appeared to have overall weaker bite forces than those that did not have the same osteophagous tendencies. This may have to do with the actual killing of prey, big cats and similarly-designed predators depending on strong bite forces in order to choke their prey to death or tear out a large chunk of the prey’s neck with a jugular bite. There are exceptions to this, the bite of saber-toothed cats often being calculated as relatively weak, but overall it seems that a shorter skull with a deep mouth is better of achieving high bite forces than a longer and narrower one. Hence, Thylacoleo actually is not a bad name for the “pouched lion”; it seems to share a large amount of convergences with its modern-day namesake, although it may have been less bright (and less sociable) with a more powerful bite.
Ventral view of the skull of Thylacoleo. From E.D. Cope’s “The Tertiary Marsupialia” in The American Naturalist, Vol. 18, No. 7. (Jul., 1884), pp. 686-697.
The predatory affinities of this animal did not always seem so obvious, however. Paleontologist E.D. Cope, in a paper entitled “The Tertiary Marsupiala,” recaps some of the controversy about the feeding habits of Thylacoleo that formed in the late 1800′s;
The discussion between Professor Owen on the one side, and Messrs. Falconer, Krefft and Flower on the other, as to the nature of the food of Thylacoleo, is known to paleontologists. From the form of the teeth alone, Professor Owen inferred the carnivorous nature of the food of this genus, while his opponents inferred a herbivorous diet from the resemblance between the dentition and that of the herbivorous Hypsiprymnus. I have pointed out that the comparison of Thylacoleo with Hypsiprirnnus is weakened by two considerations :
First, the cutting teeth in the two genera are not homologous ; second, the grinding series of molars, complete in Hypsiprymnus, is almost wanting in Thylacoleo. It evidently does not follow that because Hypsiprymnus is herbivorous Thylacoleo is so also. Professor Flower refers to the reduction of the molars in Thylacoleo as slightly complicating the problem, and concludes that the food of that animal may have been fruit or juicy roots, or even meat. It is difficult to imagine what kind of vegetable food could have been appropriated by such a dentition as that of Ptilodus and Thylacoleo. The sharp, thin, serrate or smooth edges are adapted for making cuts and dividing food into pieces. That these pieces were swallowed whole is indicated by the small size and weak structure of the molar teeth, which are not adapted for crushing or grinding anything but very small and soft bodies. It is not necessary to suppose that the dentition was used on the same kind of food in the large and the small species… In Thylacoleo carnifex it might have been larger eggs, as those of the crocodiles, or even the weaker living animals. The objection to the supposition that the food consisted of vegetables, is found in the necessity of swallowing the pieces without mastication. In case it should have been of a vegetable character the peculiar premolar teeth would cut off pieces of fruits and other soft parts as suggested by Professor Flower, but that these genera could have been herbivorous in the manner of the existing kangaroos, with their full series of molars in both jaws, is clearly an inadmissible supposition.
I have to agree with Cope; it is hard to imagine what sort of vegetable matter Thylacoleo would be eating with its specialized dentition. It would have been able to chop plants, surely, but whatever food was not inside the mouth would fall into the ground, that inside the mouth would have to be swallowed whole. This would results in Thylacoleo chewing much more low-quality plant food than other animals with teeth adapted to herbivory, and I doubt that Thylacoleo had a caecum or a habit of swalling smooth stones to aid in the digestion of the hypothetical greenery. Even in 1969, however, there seemed to be some doubt as to whether Thylacoleo was a herbivore, an omniovore, a carnivore, or a hypercarnivore. Leigh Van Valen, in the paper “Evolution of Dental Growth and Adaptation in Mammalian Carnivores”, wrote;
The jaw musculature of Thylacoleo was generally similar to but more powerful than that of Trichosurus, but whether all this increase was an adaptation to greater size is unknown. The question of the diet of Thylacoleo is unresolved. If Thylacoleo was carnivorous, it was in several respects a relatively inefficient carnivore. However, large carnivores were rare in Australia, and the condition of Thylacoleo is what would be expected if a Trichosurus-like phalanger became carnivorous. But the herbivorous diet advocated especially by Flower, Krefft, Lydekker, Charles Anderson, and Gregory remains a real possibility. A decision on this matter will probably not be possible until there is adequate knowledge of the appendicular skeleton.
The initial descriptions of Thylacoleo by Richard Owen were more certain of the carnivorous habits of the marsupial, however. One December 16, 1858, Owen’s paper “On the Fossil Mammals of Australia. Part I. Description of a Mutilated Skull of a Large Marsupial Carnivore (Thylacoleo carnifex, Owen), from a Calcareous Conglomerate Stratum, Eighty Miles S. W. of Melbourne, Victoria” was read before the Royal Society. It states;
The evidence of a large carnivorous marsupial, from pliocene formations in Australia, reached me not many years after my determination of the still larger herbivorous marsupial, Diprotodon australis, which first suggested the idea of the coexistence. The evidence was received in the year 1846…
The fragmentary skull of Thylacoleo from Owen’s paper “On the Fossil Mammals of Australia. Part I. …”
The initial fragmented skull of Thylacoleo carnifex (Owen), pictured above, was obtained and sent to the great naturalist by Dr. Hobson of Melbourne. Upon receiving the fossils, it seems that Owen almost immediately recognized the convergences in the skull with modern carnivores, the extant lion (Panthera leo) being his foil for the characters in the new skull. Owen describes the deterministic state and characters of the skull as follows;
The ‘skull’ consisted of the cranial part, similar in size and in the development of the temporal ridges and fossae to that of a Lion. The ‘incisor’ was a large tooth with a trenchant or incisive crown, implanted, with a small tubercular tooth, in a portion of the right superior maxillary bone, including part of the orbit and lacrymal bone. The latter specimen gave decisive confirmation of the carnivorous character of the fossil, the ‘incisor’ tooth answering in shape and function to the great sectorial or ‘carnassial’ and the tubercular tooth to the small tubercular molar of the Lion; being situated, as in that animal, on the inner side of the back part of the sectorial tooth.
Indeed, the bladelike teeth alone were enough to convince Owen of the ferocious nature such an animal must have possessed, writing;
In existing carnivorous mammals the ferocity of the species is in the ratio of the ‘carnassiality’ of the sectorial molar, i. e, of the predominance of the ‘blade’ over the ‘tubercle;’ and this ratio is shown more particularly in the upper sectorial, in which, as the tubercular part enlarges, the species becomes more of a mixed feeder, and is less devoted to the destruction of living prey. From the size and form of the carnassials of Thylacoleo, especially of the upper one, we may infer that it was one of the fellest and most destructive of predatory beasts.
A second, more complete skull of Thylacoleo carnifex, from Owen’s paper “On the Fossil Mammals of Australia. Part II. Description of an Almost Entire Skull of the Thylacoleo carnifex, Owen, from a Freshwater Deposit, Darling Downs, Queensland”
Owen’s assertions did not go unchallenged, however. In a later 1886 paper “Additional Evidence of the Affinities of the Extinct Marsupial Quadruped Thylacoleo carnifex (OWEN),” the anatomist includes a quite humorous remark in response to one of his critics. As noted before, some scientists believed that Thylacoleo was essentially a living Cuisinart specializing in cutting up fruit, no more terrifying than some of the arboreal relatives of the Virginia Opossum noted above. Owen, in classic style, writes;
These eminent authors received the support, in reference to objections to my conclusions, of the (then) Curator of the Australian Museum, Sydney, Mr. GERARD KREFFT, who, in his contribution to the ‘Annals and Magazine of Natural History,’ series 3, vol. 18, 1866, p. 148, records his opinion that “the famous marsupial Lion was not much more carnivorous than the Phalangers of the present time.”
The species of carnivorous Phalanger is not named. No evidence of such by fossil specimens has reached me, nor have I found such exceptional habit of an existing species of Phalangista elsewhere noted.
Lower jaw (outside view) of Thylacoleo carnifex, as seen in Plate I of Owen’s “Additional Evidence of the Affinities of the Extinct Marsupial Quadruped Thylacoleo carnifex (OWEN)”
Indeed, armed with a more complete lower jaw of the animal, Owen even further extrapolated its carnivorous habits, postulating that it had been the “check” on the large herbivores known from the same period in Australia. All the large forms, in Owen’s view, ceased to exist when “bimanous” forms came to the continent, either eliminating Thylacoleo or putting it out of a job through competition, although the wholesale slaughter of Australia’s megafauna by the people who would become the Aborigines is not an open and shut case. Even beyond the skull, Owen was provided with a claw complete with retractable teeth, now known to occupy the “thumb” position of this carnivore. Some have speculated that its size, ferocity, and retractable claw allowed it to climb trees like a leopard, although others have doubted this an account of how robust Thylacoleo probably was (being twice the weight of modern leopards), which 1) would have made it difficult to climb trees, and 2) would have allowed it to chase off most of the competing predators/scavengers of it’s day. I doubt that there were many creatures that would have crossed the path of Thylacoleo and survived if the “pouched lion” was hungry or territorial.
Despite it’s fearsome reputation, Thylacoleo seems to have disappeared from the land “down under” around 40,000 years ago, probably the very last of its lineage. Indeed, while I have primarily focused on Owen’s T. carnifex here, there were many other earlier species and related genera, each showing different aspects of the skull and form. Why these bizarre creatures, once so prominently disputed, have disappeared from the public understanding of paleontology I cannot say, but it is probably to the relief of living kangaroos and other Australian fauna that they are long gone.
Partial skull elements, most notably the incisors, from Owen’s paper “On the Fossil Mammals of Australia. Part IV. Dentition and Mandible of Thylacoleo carnifex, with Remarks on the Arguments for Its Herbivority”