by Tim Flannery
Taken as a whole this astonishing system represents a reproductive ‘conveyor belt’ which is poised, forever ready with another ‘product’ to roll off at the earliest opportunity. It is perhaps the major factor in the success of the kangaroos, but there are other anatomical features that have also helped them triumph.
The final ‘kangaroo essentials’ relate to diet, and are manifest in the teeth, jaws and stomach. This is fortunate for palaeontologists because most vertebrate fossils are teeth, which permits easy identification of kangaroos in the fossil record. Kangaroo jawbones are perforated by a large hole, through which a cheek muscle enters to anchor far forward and deep inside the bone. This allows great force to be delivered in a part of the jaw which in primitive kangaroo species is occupied by a blade-like and finely ridged premolar. The entire system appears to have evolved to deal with food that needs considerable force to break into, but which is soft inside. The serrated premolars have been lost in many kangaroo lineages, but the hole in the jaw invariably remains, and is a sure guide that the bone belongs to a kangaroo.
The food that those premolars broke up then made its way to a rather odd stomach. All living kangaroos possess large stomachs in which different ‘sections’ can be discerned (albeit less distinctly in more primitive species). This compartmentalisation was to become a most important development in those lineages of kangaroos that evolved to eat grass, for it permitted a highly efficient form of digestion.
Because the specialisations listed above are common to all kangaroos, or somewhat modified in a few lineages, they must have been present in a common ancestor. Using them we can reconstruct some of the challenges and opportunities that this creature, which evolved over 30 million years ago and for which we have almost no fossil evidence, must have faced. They indicate that the ancestral kangaroo was spending more time on the ground. Was this because forests were vanishing, or because new opportunities were opening up at ground level? Perhaps it was a bit of both, but the opportunities on the ground surely included some foods difficult to break into. For an animal not much larger than a pygmy possum, many foodstuffs can be hard to crack: seeds, nuts, insects with tough shells and even snails are all possible candidates.
And the unique method of kangaroo reproduction suggests that there were threats in this new environment as well as opportunities. You can tell a lot about the risks an animal faces from its method of reproduction. Species with high predation rates typically have many young, while those that face fewer predators tend to invest more in raising fewer offspring. Because the ancestral kangaroo bore a single young upon which it lavished much care, we can infer that once the perils of weaning were passed the joey had a fair chance of reaching old age. For an omnivorous creature the size of a rat this is rather surprising, as there are plenty of predators capable of killing such an animal in most ecosystems.
But what of the delayed development of the embryo? Some scientists believe that in the early stages of its evolution the process served to extend pregnancy (and thus care of the young), and to increase energy efficiency (by allowing the mother to be pregnant when lactation was at its height, thus minimising the time needed to maintain a high body temperature). Nevertheless, by the time the ancestor of the living kangaroos had evolved, the process was probably reducing delays between births, and into that we can read several evolutionary meanings. Having an embryo waiting, quiescent in the uterus, is useful to animals that stand a fair chance of losing their young before weaning, for it allows them to rapidly replace any losses. Despite the rare occasion upon which a mother ejects an immature young from her pouch, most pouch-young are safe from being eaten as long as their mother survives. Starvation is the greatest killer, for young, growing bodies demand food of far higher quality than that required by adults. Today, moderate droughts will kill innumerable young kangaroos, which raises the possibility that drought may have been a feature of the Australian environment at least 30 million years ago, when the first kangaroos appeared. If upheld by further research this is an important discovery, for many of Australia’s plants and animals tolerate drought, yet enigmatically there has been little evidence of drought in the fossil record before around 8 million years ago.
The delayed development of the embryo implies one further feature of the environment that shaped the ancestral kangaroo: it was not strongly seasonal, for to state the obvious, species in seasonal environments give birth seasonally, not when an older sibling ceases suckling. Furthermore, those few kangaroos that breed seasonally today have had to modify the original pattern. The absence of strong seasonality is interesting when one considers that, before 30 million years ago, Australia was located far south of its present position. Indeed, until around 40 million years ago it was still attached to Antarctica and a part of the ancient supercontinent of Gondwana.
So, was the first kangaroo grey, woolly and with ‘pride inordinate’ as Kipling would have it, and did it dance upon an outcrop in central Australia? We still do not know when and where kangaroos first arose, but we can look to modern environments that have some of the features that our analysis suggests the first kangaroos experienced. The high mountains of New Guinea, with their frigid, mossy forests, offer a good starting point. Firstly, they are composed of ancient Gondwanan plant species of a type that we know existed in Australia in times past, and many of these bear hard seeds and nuts. Secondly, these forests have very few predators capable of eating a large rat-sized creature. Only the New Guinea quoll and a few owls fall into this category, for the environment is far too cold for large reptiles.
At altitude, towards the tree-line, there are also open spaces where tree-dwelling creatures will come to ground—indeed, during field work in the region I have caught many a tree-dweller taking a nocturnal wander across the grassland. Only the droughts seem to be lacking, but El Niño brings periods of dryness and frost—if not absolute drought—even to New Guinea’s wettest mountains. The ancient Australia that gave birth to kangaroos may have been as cool and predator-free, but it was surely far flatter, and its open spaces more likely to have resulted from infertile soil or rocks. Thus, although we can say nothing definitive about the colour or pride of the first kangaroo, perhaps Kipling was correct in placing him upon an ‘outcrop’. And he is far more likely to have danced than hopped.
Trying to determine just when and where the first kangaroos lived has been an obsession throughout my professional career. In 1981, when I began my doctoral studies on kangaroo evolution, so little was known that searching for kangaroo origins in the vastness of Australian evolutionary time was akin to looking for a needle in a haystack. Then we had no idea even of when the first marsupials arrived in Australia. Was it over 100 million years ago, when marsupials first show up in the fossil record in Asia and North America, or after 65 million years ago, when they first appeared in South America?
Back then there was but one hypothesis to guide me—an estimate published by molecular biologists that all kangaroos whose DNA they had examined shared a common ancestor 50 million years ago. But this estimate proved to be dubious because it was based upon the rate at which kangaroo DNA changes, and in order to establish that rate well-dated fossils were needed to calibrate the molecular ‘clock’. As such fossils were lacking, it was a classic catch-22. Based on my construction of the ancestral kangaroo, two possibilities seemed worth investigating. Perhaps the lineage was a very ancient one and its first member had come to live on the ground in the wake of the extinction of the dinosaurs 65 million years ago. I thought this was possible because those extinctions had carried off so many creatures that a multitude of ecological niches were left vacant, and perhaps an opening at ground level was sufficient to induce one of the surviving marsupials to forsake the trees for life on the ground. Or perhaps the ancestral kangaroo came to ground 55 to 33 million years ago, when worldwide changes in the Earth’s climate broke up the rainforests that had dominated the planet for millions of years, creating a new evolutionary frontier in the form of open woodlands
and plains. With these two possible explanations in mind, I began searching through fossil deposits from 120 million to 10 million years in age, looking for evidence of early kangaroos.
7
Dead-end in the Inland Sea
One of my first opportunities for field work came courtesy of my friend and mentor Dr Tom Rich. One day in 1977, while I was cleaning fossil bones in his lab, Tom let slip that a team of scientists from the British Museum of Natural History was coming to western Queensland. They intended to spend two months searching for dinosaurs in sediments that had formed along the margins of Australia’s inland sea over 100 million years ago. Tom was going to join the group because he thought that the region might yield Mesozoic (dinosaur age) mammal fossils.
Back then, the record of fossil mammals in Australia was, as Tom put it, a truly ghastly blank, without a single mammal bone having been discovered that was older than about 30 million years. Quite frankly, many of us thought Tom was chasing a phantom, for we felt that such mammals may never have existed in Australia.
It was to be an epic quest. Australia’s federal research funding body had other priorities, so Tom turned to the land of his birth, and more specifically to the National Geographic Society, for aid. The fossils he was chasing proved exceptionally elusive, yet for over two decades the society kept the faith, during which time their dogged palaeontologist found so many dinosaur bones that he became a world authority on the Dinosauria; and such an abundance of fossils from the later Cainozoic era (the age of mammals) that Tom is considered an expert on these as well.
When he asked if I would like to join him in Queensland I felt as if I’d died and gone to heaven, especially when I learned that among the expeditioners were some of the gods of my teenage reading: people like Alan Charig, whose book on dinosaurs entranced me, and Barry Cox, whose publications on fossil mammals I had spent hours puzzling over. The chance to meet such scientists and to look for dinosaurs in western Queensland seemed too good to be true. And what exciting rocks! They had their origins in deltas, swamps and floodplains that had surrounded Australia’s inland sea. Between 120 and 100 million years ago this long-defunct ocean had occupied much of what is now inland Australia. Then, Uluru (Ayers Rock) and Kata Tjuta (the Olgas) must have stood as lonely sentinels in an expanse of azure, home perhaps to nesting pterodactyls, turtles and Loch Ness monster-like plesiosaurs, while along the eastern margins of the sea, great rivers coursed through the Australian uplands, laying down the rocks that we were to visit.
The stories of Aborigines and the westward flow of many rivers convinced Australia’s pioneers that an inland sea existed, prompting many an explorer to set out in search of it. Some, such as Charles Sturt, even dragged boats over the plains, turning an arduous venture into a total nightmare. But now, we were off to prospect for evidence of life from around that sea—an adventure only slightly less quixotic, perhaps, than that of the ill-fated explorers.
The British had been in the field for several weeks by the time Tom and I arrived at the dusty outback town of Winton. The place resembled the set of a Wild West movie: endless plains, wide verandahs and the lure of dinosaurs induced a state of high excitement in my young heart. Our rendezvous was half a day’s drive down the road in the big sky country out towards Birdsville, through a landscape dotted with cattle, and every now and then a barbed-wire fence festooned with the carcasses of eagles and hawks, testimony to a hatred of predators then felt by so many rural people. To me this was a revolting sight, for the graziers had hunted down the very animals that were their best allies in the war on rabbits. Yet there they were, from the rarest to the most common, and in varying states of decomposition, laid out with military precision, victims of a blind hatred that had come with the first European settlers to this land.
Southwest of Winton the Diamantina River has cut a wide valley filled with bountiful black-soil plains that in a good season are waist-high with grass. To the east they are bounded by a plateau around 200 metres high known locally as the ‘jump-up’. Its margin has been eaten into by creeks and gullies for millennia, forming a rough jumble of bluffs, mesas and narrow valleys whose white, yellow and pink sandstones and chalks give it a distinct ‘badlands’ feel. These were the rocks we had come to investigate and I hoped that somewhere out there, on a wide wash-out or nestled amid the tufty grass were dinosaur and mammal bones—maybe even entire skeletons. The British, however, had other ideas, for they began their survey not on the plains but in outback pubs.
Alan Charig turned out to be a boyish man with a wild laugh and a wicked sense of humour, whose bright blue eyes, peering out between a haphazard straw hat and an unruly black beard, gave him the appearance of a pirate of the sort found in Treasure Island. I immediately liked him, and was eager to learn from him, but when he explained his team’s strategy to find dinosaurs, I thought he was pulling my leg.
‘Queensland cattle-men,’ he explained, ‘are a curious and thirsty people. While out mustering cattle they must come across plenty of fossils which they occasionally bring into their pub to show their friends.’ Charig reasoned that these fossils sometimes get propped up behind the bar, at least until a palaeontologist wanders by. And he proved to be spectacularly correct, for in a couple of weeks of pub-crawling the British turned up more fossils than we would see in a month’s intensive on-the-ground survey. One of their most remarkable discoveries had been made while enjoying a cool beer in the airconditioned comfort of the Waltzing Matilda Hotel in Winton. A member of the team spied a particularly fine bone from an armoured dinosaur propping open a door. If it turned out to be a new species, he explained, the British would call it Matildasaurus in honour of its place and method of discovery.
Charlie, the owner of the station we used as base camp, had agreed to allow our expedition to occupy an outbuilding. We would rise at first light and drive to where we had left off the day before. It was necessary to cross country to do this, and we would frequently encounter low-slung ‘party line’ phone wires, then the only means of telecommunication for many rural Australians. While they offered little in the way of privacy (anyone picking up a phone could listen in on the conversations), they were a vital lifeline in this remote country. The jerry-rigged wires were strung on crooked old mulga posts just high enough for a Holden ute to pass under. When we misjudged their height or just did not see them our Land Rover would snag the wire on a roof rack, perhaps cutting a conversation mid-sentence. Then we’d spend hours repairing the break, stretching the wire back into place, all the while hoping that we had not interrupted an emergency call and that the posts would hold.
Having arrived at the outcrop we would fan out and walk over the stark countryside, converging at a predetermined pick-up point in the afternoon to return to base camp before dusk. Although dinosaur bones were scarce, the region fascinated me, for you never knew what lay around the corner. One spectacular fossil site was already known. In the late 1960s a local, Ron McKenzie, had come across some strange marks in a purplish-pink rock. They were the tracks of chicken-sized dinosaurs—thousands of them. When the layer containing them was excavated it was discovered that a massive carnivorous dinosaur, whose tracks were also beautifully preserved, had scattered a herd of small herbivores. The tracks had been formed in mud beside a waterhole, and probably extended over several hectares, only a small part of which had been unearthed.
Charlie told us that ‘black wallabies’ had once inhabited the jump-up. They may have been rock-wallabies, but along with all of the smaller members of the kangaroo family they had vanished from the region at least a generation earlier. All we saw in the hills were roan-coloured euros. They were in abundance, not because the country had been overgrazed, but because these rocky ranges were an ideal habitat. Caves were full of their droppings, and their footprints through the rocky pinnacles often served as my pathway. Sometimes, in the early morning or afternoon, I would spy the silhouette of a euro resting under an overhang. If I approached quietly I could get quite close before the
creature bounded away, breaking the silence of the bush with the clatter of dislodged stones. Occasionally I would surprise groups—a mother and young, and perhaps a male waiting nearby as well. Wandering alone in such country, day after day, was a dizzying experience for this twenty-year-old, for at last I was exploring the wild outback. After hours of absorbed walking with my eyes fixed on the ground I would look up to find myself surrounded by a labyrinth of bluffs and mesas and realise I had no idea where I was. The temptation was to panic, but if I could suppress the urge and sit quietly on a high spot, the sun and the lie of the land would invariably give some direction to the rendezvous point.
Truly unexpected things occasionally burst upon us. A furious squeal and wild rushing through waist-high grass would indicate that we had roused a sounder of feral pigs from their midday rest. It took some nerve to stand still as the unseen creatures, the males armed with razor-sharp tusks, rushed by our legs. Once, on the rubble-strewn shoulder of a mesa, a glint of deep-ocean blue caught my eye. I picked up the cobble, spat on it and rubbed away the dirt with the tail of my shirt, revealing a double seam of opal glistening like a vein of tropical seawater in the red sandstone—all the more glorious for the desolate surroundings.
One morning we located a canyon leading deep into the jump-up. The sunlight was all but cut off by the steep walls, and lusher vegetation grew along a sandy wash-out with traces of moisture at its base. As we penetrated further the water began to flow, then figs appeared perched on a rocky precipice high above. Finally the gully narrowed, becoming dark and ferny. On the higher walls was Aboriginal art—hand stencils, boomerangs, kangaroos—and under the deepest overhangs lay bundles of paperbark. The remnants of Aboriginal burials in bark coffins. Why, I wondered, had I not seen any Aborigines around Winton? Western Queensland was, it transpired, a particularly bleak place to be an Aborigine in the nineteenth century with one squatter, C. B. Dutton, summing up the situation in a phrase: ‘You are black, and you must be shot.’ No one there could tell me anything much about the local tribes.