by Tim Flannery
We can trace an important trend through these abundant Miocene deposits, for in the oldest the creatures are rather small: the diprotodontids sheep-sized at best, the kangaroos rabbit-sized and the marsupial lion no larger than a tabby. Yet as we progress to those laid down nearer our own time, the average size for these lineages increases markedly, until by the Miocene’s end the largest diprotodontids, kangaroos and marsupial lions are ten times heavier, or more, than their ancestors. Because the largest species inhabited open habitats, it is possible that a drying climate, which reduced forest cover, was responsible for this trend. Curiously, arboreal species such as koalas and possums hardly change in size at all.
Following the abundant deposits of the Miocene age comes yet another frustrating gap in the record lasting three to five million years. Opening with the 4.46-million-year-old Hamilton site, many fossil localities follow, and this final phase in the evolution of Australia’s mammal fauna is dominated by kangaroos.
The fossil record of North America is remarkably complete and here I wish to compare it against Australia’s fossil record, determine what role climate change has had in shaping Australia’s fauna, and investigate whether our fauna has ever experienced a ‘golden age’. So firstly, how does Australia’s fossil record compare with that of North America? At the time of the extinction of the dinosaurs three types of mammals existed in North America—placentals, marsupials and a primitive group known as multituberculates. The marsupials dominated both in number of species and abundance, but many fell victim to the asteroid that carried off the dinosaurs. Thereafter the placental mammals—initially only the size of rats—began their rise, so that by around 18 million years ago they were the only mammal group present on the continent.
The Murgon site gives us an insight into a time when Australia’s marsupials had only just arrived and were in the process of ousting the competition—a little like the situation in North America 65 million years ago, only here the marsupials were in the ascendancy.
Within a few million years of the extinction of the dinosaurs, some of North America’s placentals were already pig- to cow-sized. Likewise, the cumbersome ilariids of Lake Pinpa may be Australia’s first experiment in producing pig-sized herbivores. By 55 million years ago in North America the first large, primitive herbivores had given way to the perissodactyls (rhinos, rhino-like creatures and their relatives), and the earliest artiodactyls (camels, sheep, cattle). These two lineages would come to dominate the large herbivore niche in most of the world. They evolved in response to the challenge of eating tough plant food, the perissodactyls being hindgut fermenters (the primitive placental condition) and the artiodactyls foregut fermenters. ‘Fermenting’ relates to the way plant materials are broken down by bacteria and other organisms.
Hindgut fermenters accommodate their microbes in either a caecum (the same structure, anatomically speaking, as the human appendix) or a colon. The koala’s caecum is proportionately longer than in any other mammal, and into it the most nutritious fraction of the food is diverted, the rest being passed as faeces. While serviceable the caecum has a flaw, which leads to ailments in humans and difficulties for marsupials—it is open at only one end. This means that it must be emptied through the same orifice from which it is filled and if a seed obstructs that opening appendicitis can develop. In herbivores the time required to fill and empty the organ dictates the rate at which food can be processed.
Hindgut fermenters that use the colon to host their microbes—such as the wombat and horse—are not so limited. They just keep shovelling the food into the tubular colon and out the other end. It’s a solution suited to very large creatures, which can exist on less nutritious yet abundant food-types, which is why perissodactyls (rhinos, horses and tapirs) tend to be large and not too fussy about food.
The artiodactyls (of whom the ruminants form a subsection) are foregut fermenters that process their food in a complex stomach, burping up portions to be re-chewed as ‘cud’. The stomach has the great virtue of being open at both ends and located close to the teeth, allowing bits of vegetation that were hastily swallowed to be broken down at leisure. This strategy is suited to creatures that feed selectively on difficult-to-break-down food and need to extract maximum calorific value from each mouthful. Although the artiodactyls arose around the same time as the perissodactyls, they stayed smaller for longer. Foregut fermentation is suited to a diet of grass, which benefits from prolonged mastication and breakdown by stomach microbes. And that is why the farms of the world are filled with pasture rather than forest. The artiodactyls developed one further feature; they have the equivalent of a gear-shift lever in their ankle. The central unit of this device is the ‘knuckle’ bone used to play knuckles with, and by altering its orientation a sheep, goat or gazelle can change from ‘low’ to ‘high’ gear, quickly increasing speed when required.
Australia’s hindgut fermenters, like the perissodactyls, flourished in the earlier part of the age of mammals, but are fewer in number today. They fall into two distinct catgories—those like the koala that have a caecum, and those that use the colon as a fermentation chamber. The caecum-bearing koala, and possibly its ilariid relatives, flourished very early on, and were replaced by the colon-fermenting diprotodontids and wombats. Present-day wombats have a relict caecum, indicating that their ancestors once had a functional caecum. I think that by moving the site of fermentation from the caecum to the colon, wombats and their relatives gained an advantage over the ilariids.
Clear parallels exist between the kangaroos and the artiodactyls; both are foregut fermenters that remained small compared with hindgut fermenters, yet in the end came to dominate. Furthermore, both developed highly successful and innovative modes of locomotion—the gear-shift ankle in artiodactyls and hopping in the roos. The parallels are not precise, of course, but it is a distinctly similar course of evolution that has played out on the two continents.
How much of the evolution of Australia’s mammals can be explained by climate change? In the light of the threat that human-induced climate change is bringing to biodiversity, this is a vital question. Frustratingly, however, our inability to date most of Australia’s animal fossil record means that we cannot relate it directly either to known changes in flora or changes in the world’s climate. This lamentable situation allows us to draw only general conclusions. Around 50 million years ago rainforests flourished in central Australia, but drying has since restricted them to parts of the east coast. The fossils from Lake Pinpa suggest that the ilariids lived in a well-watered environment, but whether the region was covered in rainforest or not is difficult to say. By the time the Lake Ngapakaldi deposit formed eucalypts were present, but possums and other arboreal creatures that appear to have been rainforest dwellers still flourished in central Australia. It is not until Bullock Creek times, around 10 million years ago, that fauna from an open plains environment appears and what we see here is very odd: instead of a new fauna which developed in response to the open habitat, we find only a few mammal types, such as the cow-sized Neohelos and the primitive balbarine kangaroos, which were present in earlier, forested habitats. Perhaps the climate changed so quickly that there was no time for new kinds of mammals to evolve and they were the only ones that could tolerate the new conditions.
By Alcoota times, around 8 million years ago, the Australian fauna was increasingly adapted to drier, more open conditions. Both macropodines and sthenurines (short-faced kangaroos) had appeared, and they thrived in the dry new Australia. It is tempting to think that they originated in dry habitats, but the fossil record contradicts this—the very oldest sthenurine is preserved in some of the last deposits laid down at Riversleigh, intimating a rainforest origin, while the Hamilton site indicates that the macropodines predominated in rainforest environments from an early stage.
Did a drying climate cause a diminution in Australasia’s rainforest diversity? The Riversleigh deposits contain a great diversity of mammals, yet it remains difficult to determine how many sp
ecies co-existed at any one time. The trouble arises from Riversleigh’s many separate fossil deposits, which were laid down over millions of years. Because of their small size many have been lumped together in analyses, giving the illusion that species co-existed when in fact they may have been separated by millenia. Despite the drying trend, some modern rainforests remain as diverse as any realistic estimate of Riversleigh’s biodiversity. The Upper Fly and Sepik rivers area in New Guinea, for example, is a mountainous region the size of metropolitan Sydney and home to at least 120 mammal (including many marsupial) species—more than one-third as many as recorded for all of Australia. Given the importance of the impact of climate change on Australia’s fauna this should be a focus for future research. Unless we can understand how climate affected Australia in the past, it will be difficult to manage future climate change.
Did Australia’s mammals ever experience a ‘golden age’—a period during which large mammals achieved great diversity? During the Miocene period (around 24 to 5 million years ago), as North America’s climate dried and warmed, the continent’s large mammals diversified spectacularly: some camels evolved to become giraffe-like, hornless rhinos evolved into hippo lookalikes, and up to a dozen species of horse co-existed. Today east Africa, with its magnificent megafauna, is clearly in the midst of such an age. The closest thing Australia ever experienced began in Alcoota times when four cow-sized marsupials (diprotodontids and their relatives) co-existed with several species of gigantic, flightless herbivorous birds (dromornithids).
By the ice age, paradoxically, diversity had increased, largely due to the evolutionary radiation of the short-faced kangaroos, which provided the greatest diversity of large mammals Australia had ever seen. I believe that we live at the end of this modest ‘golden age’, and its decline began with the arrival of humans. Ever since then, to quote Alfred Russel Wallace, we have lived in ‘a zoologically impoverished world, from which all the largest, fiercest and strangest forms have recently disappeared’.
22
The Groote Eylandt
I wanted to learn more about living Aboriginal cultures and their relationship with the land. So when the opportunity arose to undertake a survey of the mammals of Groote Eylandt in 1989, I seized it. The Dutchman Jan Carstensz, who saw the place in 1623, named it well, for Groote is indeed a ‘great’ island, lying in the western side of the Gulf of Carpentaria. The waters that surround it are a shade of aqua I’ve seen nowhere else, and where it meets the shore the water is fringed with dark green mangroves and silver beaches, behind which lie red sand dunes and the maze-like quartzite hills that form the backbone of the island.
To visit Groote you need either to be invited by the traditional Aboriginal owners or by the managers of a manganese mine known as GEMCO, which operates in the island’s northwestern corner. My entree came when GEMCO commissioned a survey of the animals inhabiting their mining lease. For once I was not going to wildest New Guinea, so I decided to take my children—David, six, and Emma, who was just four. Although liaison with Groote’s Aborigines had not been foreshadowed, I had requested contact. If they were willing to share their traditional knowledge, I believed I stood to learn a great deal.
GEMCO had organised for Murrabudda, an elder from the Alyangula settlement, to spend time with me. When I went to pick him up I found the settlement a sad-looking place—its dilapidated buildings and litter-strewn streets spoke of dispiritedness and decline. Murrabudda, in contrast, a dark-skinned man of medium height in his seventies, was full of life. When the missionaries came to Groote, they persuaded Murrabudda’s father to ‘put away’ one of his wives. The discarded woman was Murrabudda’s mother, and without the support of her husband she and her children had difficulty surviving. They had lived off the land, and so Murrabudda knew the resources of the island exceedingly well. For a few weeks we roamed Groote from end to end, Murrabudda explaining where the animals and plants were found and how Aboriginal people used them. He was clearly delighted to visit places he had not seen for years, and seemed anxious to show me as much as possible.
One day when resting in the shade of a quartzite hill, Murrabudda pointed out some rock art in a shallow cave. Along with the fish, crocodiles and other figures representing the coastal environment, there were two distinctly out-of-place animals—an emu and a Tasmanian devil. The depictions perhaps dated to when Groote was joined to the mainland over 10,000 years ago, and they underlined the fact that the fauna of Groote Eylandt is impoverished when compared with that of the mainland, for not only does it lack emus but also the euro and antilopine wallaroo, which are so significent to the people of Arnhem Land. Also absent was Arnhem Land’s nabarlek (Petrogale concinna), a tiny rock-wallaby unique among land mammals in having a limitless supply of molars that are replaced from the rear as those at the front wear out.
Only two species of the kangaroo family—the agile wallaby and a rock-wallaby—survive on Groote, but they are remarkably common, making them easy to study. Rod Strong, Groote’s police sergeant, took me to a place called Makbamanja to show me rock-wallabies. Its virginal beach with white sand and aqua waters looked idyllic, but Rod’s warning not to camp near it was reinforced by drag marks in the sand made by a crocodile of considerable girth. We unrolled our swags between rocks fringed with pandanus palms, and watched the stars blazing in the tropical sky. I rose as the first light of dawn stole across the horizon, and crept towards a rocky promontory at the far end of the beach which was already alive with rock-wallabies.
The species found on Groote Eylandt is the short-eared rock-wallaby (Petrogale brachyotis)—one of the most beautiful members of the most exquisite genus of kangaroos. Perhaps it is the creature’s habitat—that tropical north of sharp colours and clean edges—or perhaps it is my familiarity with the gentle, soft-furred creatures that makes me think so, for they are not the most brightly coloured or largest of rock-wallabies; but on that morning, as they sat grooming their young and warming themselves in the first light of day on rocks overlooking the sea, they appeared to be living a version of paradise.
Because of their preference for steep, rocky ranges, rock-wallabies are patchily distributed, although they can be (or could be) found in suitable habitats throughout Australia. This combination of a broad yet patchy distribution has provided frequent opportunities for isolated populations to evolve into new species, making the rock-wallaby genus one of the largest in the family.
Scientists are still unravelling exactly how many species there are, but since 1970 five new ones have been discovered—most from coastal Queensland—with one striking discovery made in the Kimberley in the 1970s. The creature had escaped detection because it was mistaken for the diminutive nabarlek, but the new species, known for many years as the warabi (Petrogale burbidgei), is even smaller. Weighing in at just a kilogram, it is one of the smallest members of the kangaroo family, and with its large eyes, chinchilla-like fur and big bushy tail it is also one of the most endearing.
The warabi was discovered by a survey team that was documenting the mammals of Western Australia’s remote north. Someone asked a local Aboriginal man if he knew the animal and was told, ‘That’s a warabi, boss.’ This was taken to be an Aboriginal name until zoologist Ron Strahan pointed out that ‘warabi’ was most probably a misheard ‘wallaby’; though not before ‘warabi’ had been given as the species name in a compendium of Australian mammals. Embarrassed mammalogists scrambled to rename the creature ‘monjon’, its proper Aboriginal appellation, by which name it is known today.
Rock-wallabies have redesigned the basic kangaroo body-plan to live in precipitous areas. They have much-reduced nails on their feet and large footpads that at their ends bear fingerprint-like patterns which are used to grip the tiniest irregularities in the rocks. Their tails are remarkably long and thin, and often bear a tuft at the end. Such tails are of little use for pushing the animals along; instead they are magnificent aerial rudders, invaluable for making precise adjustments in body position while in mida
ir. Rock-wallabies are astonishingly nimble: once in a room in the animal facility at Monash University, a pint-sized nabarlek circled effortlessly above me at ceiling level, ricocheting off the brick walls in an endless series of bounds by inserting the ‘fingerprints’ at the end of its toes in the cracks.
Rock-wallabies were once so common that a thriving trade in pelts existed. As late as the 1860s you could see rock-wallabies scampering over Sydney Harbour’s Middle Head. Now, despite their superb adaptations, many species are gravely threatened. Changed fire regimes and the spread of cats and foxes have seen them vanish from most of Victoria, New South Wales, the southwest of Western Australia and the arid centre.
On the drive back to Alyangula, Rod revealed a little of what a hell this seeming paradise can be for the indigenous people of Groote. The initiation of young men, he said, had long ago been replaced by a stint in Darwin’s Fanny Bay Jail. And ‘Black Christmas’ on the island, when the Aborigines received royalty payments from the mine, was utterly demoralising. If a European had a clapped-out car to sell, this was the time to do it—at a handsome profit; the occasion also marked the high point of alcohol-fuelled violent crime.