by Tim Flannery
A large collection is held in the Australian Museum in Sydney, and one day in 1981 I found myself examining this treasure-trove with an American scientist. Fred Szalay, of the City University of New York, has a stellar reputation in the field of mammalian evolution, and as a young student I was honoured, even a little intimidated, to be in his company.
Fred and I had both come to the conclusion that the evolution of various animal groups could best be approached via the study of their feet. This may sound unorthodox, but for creatures with such a strange method of locomotion as the kangaroos it felt natural to focus on feet for enlightenment. Fred had spent decades studying the feet of everything from opossums to pangolins. He taught me that a single foot-bone is, in many cases, sufficient to identify a species—it’s astonishing how distinctive the foot-bones of otherwise similar-looking creatures can be. On that day we sorted the ankle-bones into piles—grey kangaroo, giant wallaby, short-faced kangaroo—when Fred unexpectedly stopped, a peculiar ankle-bone in his hand.
It was, he said, unmistakably that of a large tree-dwelling kangaroo. Did I know of such a creature? A glance was enough to confirm his diagnosis, and to see that this was no normal tree-kangaroo, for it was twice the size of the largest known species. It is sometimes said that an extinct animal can be reconstructed from a single bone, but in truth, often only broad outlines of the species’ ecology can be established. In the case of the owner of the ankle-bone, various facets revealed that the foot could be twisted inwards so that the soles of the feet could oppose each other. Such are the stresses on the foot of terrestrial kangaroos that twisting would lead to a crippling injury, but in tree-kangaroos it is essential, for without the ability to grasp a tree-trunk between its feet it could not climb.
I can only assume that Bohra, as we called the creature, lived in relict rainforest thickets that grew around Wellington Caves a million or more years ago, though nothing else suggestive of a rainforest surfaced among the thousands of bones we examined. Bohra was a rare creature, for nothing more than a few leg- and foot-bones—all from the Wellington Caves—have ever been recovered. Judging from them, it must have weighed thirty to forty kilograms—as much as a female orang-utan—and was doubtless quite a sight in the treetops.
Bohra is only one rare species among dozens of kangaroos that became extinct in Australia towards the end of the ice age. Had you visited the continent then you might not have noticed the red and grey kangaroos that are so eye-catching today, for larger and more striking species then existed, among them a multitude of short-faced kinds. These creatures had short and thick tails, pot-bellies, short necks, a single toe on each hindfoot (as opposed to the four in most living kangaroos), and blunt, deep muzzles. Their teeth indicate that most were leaf-eaters, and their arms were specialised into great hooks, with two very long digits tapering to long, straight claws which were probably used as rakes to draw foliage to the mouth.
No fewer than thirty species of these kangaroos have been recorded, and they form the dominant fossils in many ice-age deposits. Remarkably, the lineage seems to have come out of nowhere, for only a few fragments of teeth have been found in Hamilton-age deposits; yet by a million years ago they had become the dominant type of kangaroo over most of southern Australia. Their flowering, however prolific, was brief, for they vanished even more rapidly than they appeared.
Short-faced kangaroos were found in almost all habitats, from desert to tall forest, and up to half-a-dozen species could co-exist. The smallest were the size of a female grey kangaroo, while the largest were the largest kangaroos that ever lived. When I look at caves filled with their fossilised bones, and at their leaf-slicing teeth, I cannot help but wonder at the land that fed them. Today, over much of the country where they were once found, grass predominates, a food unsuitable for them. True, there are huge areas of saltbush and bluebush in southern Australia that could have provided suitable fodder, but the short-faced kangaroos were far more widespread than the saltbush plains. Southern Australia must have changed in ways that we are yet to fully fathom since their time. It is as if an entire biome has vanished.
The true giants of the short-faced kangaroo tribe were not, according to their teeth, leaf-eaters. Known as Procoptodon, these short-faced monsters are a specialty of, as well as a convenient marker for, the Pleistocene period some 2.4 million to 10,000 years ago. While completing this book, Kris Helgen and I devised a simple and elegant method to estimate the weight of these vanished giants. Our preliminary analysis suggests that kangaroos are so proportioned that their weight can be gauged from the dimensions of their femur—the bone of the upper leg. If you measure its circumference in millimetres, then subtract 30, you now have an approximation of the long-vanished creature’s weight in
An ice-age mystery—the skull of Procoptodon goliah. Its similarity to a human skull is unmistakable. (1 eye socket; 2 nasal cavity.)
kilograms. From this method, we infer that the largest kangaroo that ever lived—a short-faced species known as Procoptodon goliah—weighed around 130 kilograms. This is a little less than previous ‘guesstimates’, and I suspect that as our study unfolds we will find that the weight of many extinct kangaroo species has been overestimated—an important point to keep in mind when we try to imagine the fauna and environment of Australia’s ice age.
Judging from the bones that support it, Procoptodon’s belly was of Friar Tuck-like proportions. Its fore-limbs were exceptionally long and powerful, with rake-like claws on two of its digits that were longer than human fingers. Its neck was short and its head, with its ridiculously
The largest kangaroo, Procoptodon goliah, as it may have appeared in life. Its short face, elongated claws on the hands and hoof-like claw are unlike anything seen in living kangaroos. It became extinct around 46,000 years ago, about the time humans arrived in Australia.
shortened muzzle and heavy jaws, almost box-like. Standing well over two metres tall, Procoptodon would have been an impressive sight. But what would you have seen had you looked into that strange face?
From what we can reconstruct, you might have discerned something similar to a gorilla, a lemur, or even yourself—a resemblance wrought by binocular eyes, a distinct chin, and short nose. Overall, the bones are striking in their hominid resonance. The teeth also have an eerie human quality; the molars are heavy and wrinkled, like those of the extinct African man-ape Australopithecus, and the enamel is extremely hard. Perhaps the diet of Procoptodon was similar to that of the robust australopithecines, consisting of seeds, roots and a little grass. But mystery still surrounds these beasts, for there are so few palaeontologists in Australia that detailed studies of their teeth and the chemistry of their bones (to reveal their diet) are yet to be undertaken.
A second ubiquitous but now vanished ice-age group were the giant wallabies. Belonging to the genus Protemnodon they were more widespread than the short-faced kangaroos but less flamboyant in their evolutionary branchings. By a million years ago their heyday had passed, and just three species survived in Australia. The wetter regions were home to a graceful giant known as Protemnodon anak. Named for a giant of the Bible, it was around twice the bulk of a living grey kangaroo. It resembled an overgrown swamp wallaby, though its arms were large and heavy, and its neck, at over twice the length of any living kangaroo, made it the nearest approximation to a marsupial giraffe that ever existed. Few details of its ecology are available, which is a shame because exceptionally well-preserved remains including stomach contents and skin impressions have been available for over twenty years, yet remain unstudied.
The inland and parts of the coast were home to two small-bodied but very large-headed Protemnodon species. Little is known of them, but it is possible that one had a specialised nose, for its nasal bones are pulled backward in a manner similar to that of a tapir, though not so extreme. The most extraordinary Protemnodon of all, however, inhabited New Guinea. The ice-age megafauna of that rainforest-covered island included three species of Protemnodon (the only ice-ag
e megafaunal kangaroos known from the island) that were distributed from lowlands to alpine mountain summits.
In 1983 I was fortunate enough to name the first of these to be discovered, calling it Protemnodon tumbuna in honour of the first New Guineans (tumbuna meaning ‘ancestors’ in Melanesian pidgin). Then it was known from a single jawbone and a few loose teeth excavated from a cave in Chimbu Province, but around 1990 a near-complete skeleton of this grey-kangaroo-sized creature turned up in a swamp near Tari in the Southern Highlands.
What it revealed transformed scientific opinion, for its head was large in relation to its body, its hind legs absurdly short and stumpy for a kangaroo, and its fore-limbs very powerful. Details of its shoulder and lower arm revealed that its fore-limbs functioned differently from other kangaroos, and had been modified to assist in bandicoot-like bounding. Protemnodon tumbuna, it seems, was the only kangaroo ever to relinquish that invaluable gift of its ancestors—the ability to hop. Instead it made its way through New Guinea’s dense rainforests on all fours, rather like a bandicoot or rabbit.
Although the protemnodons are extinct, the rainforests of New Guinea are still home to a generalised, dwarfed version of these creatures. Known as forest wallabies, Dorcopsis (meaning ‘gazelle-faced’) still abound in lowland rainforests throughout the island. There are three species, which replace each other in a ring pattern around the
Protemnodon anak (upright), a giant wallaby that until around 46,000 years ago inhabited eastern Australia. It was the closest thing the kangaroo family ever produced to a giraffe. Protemnodon tumbuna inhabited New Guinea’s mountains and was the only kangaroo ever to relinquish hopping.
lowlands: one with a bold white stripe down its back in the north, a yellowish and ash-coloured one in the southeast and a chocolate-coloured one in the southwest. Around the size of a swamp wallaby, they all have fine, silken fur and long faces. The powerful arms of the males (three times as big as the females) may be used to ‘wrestle’ for mating opportunities. But it is their teeth that are most unmistakably Protemnodon-like, for their premolars are two-centimetre-long blades, while the molars are low-crowned and entirely unsuited to eating grass. Scientists suspect that Dorcopsis wallabies use their premolars like scissors to cut up leaves and fruit, but some New Guineans think otherwise. In 1984 I asked a Mianmin hunter from the Upper Sepik region what the forest wallabies ate. He insisted that they consumed insects, found by turning over stones on riverbanks.
The behaviour of these wallabies in the wild is almost entirely unstudied, which is a great pity, for they are very odd creatures. Their tail is never laid flat on the ground, but carried in an arch, with just its tip making contact with the earth. It looks like a cumbersome way of getting about, and no one knows quite why they do it, but it has been suggested that leeches are so plentiful in the rainforest that they could suck the blood from a wallaby in no time if it were to lay its tail on the ground. The tail has a cornified tip which may be very useful in pushing the animal along when held in an arch. Interestingly, some Protemnodons appear to have lacked this specialisation.
Only one more extinct genus remains in this catalogue of giant ice-age kangaroos—Propleopus—and here we have no living species to guide us in our thinking. This is an extremely ancient lineage—I would argue the most ancient of all, and was probably already separate by Lake Pinpa times, 20 to 40 million years ago. For creatures with such a long tenure on our planet they have left us far too few remains—a tooth here and a jaw fragment there—with no site, no matter how rich in fossils, yielding more than a handful of pieces. In 1981 as part of my doctoral studies, I gathered together most of the known fragments, which fitted into a shoebox packaging and all, to see what I could learn about them.
There were three ice-age species which although similar in size (around forty kilograms), had rather different teeth. The most widespread species, Propleopus oscillans, once occupied much of southeastern Australia, from Queensland’s Darling Downs to the southeast of South Australia. It had a very stout and sharp, almost dagger-like, forward-pointing lower incisor, four crushing molars, and a modest-sized buzz-saw-shaped premolar. A second species, Propleopus wellingtonensis, which is known only from a single jaw found in the Wellington Caves, had somewhat larger premolars; while the third, Propleopus chillagoensis, whose remains were found in the Chillagoe Caves of northeastern Queensland, had massive, serrated premolars and rear molars reduced to rounded bumps.
I wanted to know how these creatures lived. With only teeth and jaws as testimony to their existence, I thought that I should start with diet. After all, you are what you eat. The answer at first seemed straightforward. The musky rat-kangaroo of Queensland’s tropical rainforests has similar teeth and eats seeds, fruit and insects. Perhaps, I reasoned, this is what Propleopus did.
But the bones of Propleopus were often associated with creatures that inhabited woodlands and plains. In such places fruit and nuts are uncommon, certainly not existing in sufficient quantity to sustain a forty-kilogram kangaroo. And the idea that any creature of that size could find enough tough-skinned insects (such as beetles) to satisfy it is ludicrous. As I pondered these conundrums I wondered why the remains of Propleopus were so widely distributed, yet so rare that an avid fossil hunter like myself should consider it ‘probably hopeless’ that I’d ever unearth one.
I decided to go straight to the teeth themselves to determine if the food they had once bitten had left any traces. Almost as soon as I loaded the first Propleopus premolar onto the platform of a scanning electron microscope the answer leaped out. The worn surface of the blade was gouged by deep, parallel grooves. The teeth of just two other marsupial species—the thylacine and the marsupial lion—show similar marks. Tooth enamel is tough stuff, and the exteriors of neither nuts nor insects could do such damage. You need a mineral to gouge such grooves in tooth enamel, and in this case the mineral was probably quartz, in the form of sand grains that had been caught in the coats of the victims of flesh-eaters.
Suddenly the mystery of the Propleopus made sense. Those stout incisors had been used to stab flesh and the buzz-saw-shaped premolars to slice it. And of course predators are always fewer in number than their prey, explaining the rarity of the creatures in the fossil record. But was it really possible that carnivorous kangaroos once roamed Australia?
Although the evidence fitted the theory splendidly, at first I resisted the idea of a carnivorous kangaroo. On reflection, however, I realised that herbivorous lineages have on occasion given rise to carnivores; the marsupial lion is a carnivorous wombat-relative, while us humans are carnivorous descendants of herbivorous apes.
Then there were a delicious couple of days when, as I worked on my theory without telling anyone else, I was the only person on Earth who knew that great, carnivorous kangaroos once stalked Australia. The hypertrophied premolars and reduced molars of Propleopus chillagoensis indicate that it was the most devoted flesh-eater of the trio, while its more widespread cousin Propleopus oscillans was, judging from its large
The killer kangaroo, Propleopus oscillans, was a meat-eater that once roamed much of eastern Australia.
molars and smaller premolars, an omnivore. Long after I wrote up these findings with my doctoral supervisor, Michael Archer, even more intriguing pieces of these mysterious kangaroos were discovered, including a near-complete skull of a smaller, related Riversleigh species which had a high sagittal crest atop the skull that anchored powerful biting muscles; and a partial skull of Propleopus oscillans from a cave in South Australia, with a fine, dagger-like upper canine—all the better to bite you with, we might say of this wolf in kangaroo’s clothing.
Despite these breakthroughs, the likelihood of a more complete understanding of Propleopus seems as distant as ever. The only bone from behind the skull that has ever been found is a humerus, or upper arm bone of Propleopus oscillans, and it is only attributed to the species because, in its resemblance to a human arm bone, it is too weird to belong to anything else. Beyon
d that, all we have is mystery.
So what can I tell you of Propleopus by way of summary? It was around the size of a female grey kangaroo and I doubt that it hopped, for the lineage diverged too early from the kangaroo mainstream to benefit from that innovation. Instead, it probably bunny-hopped like a bandicoot or rabbit. Judging from the large attachment points on the skull, the head was muscular—somewhat in the style of a Tasmanian devil. Its body could have been lithe—designed to run down the likes of rat-kangaroos, birds and bandicoots—or may have been stout, muscle-bound and tail-less, a creature given to tearing into the bodies of dead and dying ice-age giants. Until we find a skeleton, we cannot know more.
18
Is the Answer 46?
Ever since George Rankin’s downfall led to the discovery of Australia’s megafauna, scientists have speculated on what made these animals extinct. Once there were giants. Now there are none. When I wrote an ecological history of Australasia (The Future Eaters) in 1994, I tried to determine what might have happened to the giant kangaroos and other megafauna, and to imagine the consequences of their extinction on the environment as a whole. Then, the data needed to solve the mystery was limited, and I had to work from first principles rather than the fossil record. My study pointed towards the megafauna having been hunted quickly to extinction by humans, and that the extinction event led to a long series of dramatic consequences for the entire Australian environment.