Why are computer scientists obsessed with wombats



The scene takes place in northeast Australia 15 million years ago. A mother bag badger ventures out of the shelter of the dense vegetation and carefully leads her young to a shallow pool. The early morning fog still covers the rainforest. The ears of the little nasal sacs move incessantly as they carefully dip their snouts into the water, because any cracking or rustling in the undergrowth can mean danger.

Suddenly a big, dark, shaggy thing shoots out of the misty thicket. With one jump, the muscle pack grabs one of the young animals, pierces it with its protruding toothed daggers and drags it into the bushes.

Animals are eaten by their predators every day. But for Australia in the geological epoch of the Miocene, around 25 to 5 million years ago - in the Young Tertiary - many would not have expected such a scene. Because the predator is a closer relative of the kangaroo: It is a "strong-toothed giant rat kangaroo", scientifically Ekaltadeta ima (picture on page 72 top right).

Few species of larger warm-blooded carnivores live in Australia today, and most of them are rare. The largest native species are the spotted-tailed sacred marten (Dasyurus maculatus; sometimes also called spotted sacred marten or giant sacred marten) and the marsupial devil (Sarcophilus harrisii, pictures on page 74), which, however, only survived in Tasmania; on the mainland it disappeared 600 years ago. The dingo widespread in Australia is a feral dog, not a marsupial; he came to the continent with humans, perhaps only 4,000 to 5,000 years ago. The spotted-tail pouch marten can weigh around seven kilograms, the marsupial devil a little more at nine kilograms. Australians of European descent call the group of pouch martens "native cats". The squat marsupial devil - also known as the Tasmanian devil - which looks like a lap dog with a hyena head, is likely to be the least gourmet of all the predators in the world: it eats the carcasses with skin, hair and teeth.

Scientists place both species in the Dasyuridae zoological family. In addition to various pouch martens, this group also includes pouch mice, many small insectivorous species that resemble shrews.

Until recently, some researchers doubted that Australia had ever given habitat to an extensive fauna of warm-blooded large carnivores. Tim Flannery from the Australian Museum in Sydney recently made this point. In his opinion, nutrient-poor soils and a changeable climate would not have allowed any particular evolutionary development of large warm-blooded predators, at least in the last 20 million years or so - no continent today has so much sterile soil as Australia. Under these conditions, the plants simply could not have produced enough biomass for many large herbivores. But that would have meant that large predatory mammals would have lacked the food source. Rather, the niche of the large predators in the ecosystem, so assume Flannery and some of his colleagues, would have occupied reptiles: about the seven meter long monitor, Megalania prisca, which became extinct about 25,000 years ago (picture on page 76; see also Spektrum der Wissenschaft, May 1999, p . 48), and giant snakes like the up to five meters long, thick Wonambi species. Cold-blooded predators, so the argument of the paleo-zoologists, could have survived shortages better because they got by on much less food.

Recent finds, especially from Riversleigh in Queensland, make this thesis appear questionable. The British researcher W. E. Cameron found the first fossils in the remote location near the Gulf of Carpentaria as early as 1900. But he thought she was "quite young", barely two million years old. And because the area only allows excavations in winter, since it is practically inaccessible in summer due to the heat and monsoon rains, the paleontologists have happily ignored it. It wasn't until 1963 that Richard Tedford of the American Museum of Natural History in New York and Alan R. Lloyd of the Australian Soil Minerals Office ventured there to look for fossils. The bones found were more interesting and also older than the researchers had expected, but fragmented and difficult to recover.



Unexpected competition



Even so, their reports caught the interest of other experts. Michael Archer, the current director of the Australian Museum in Sydney and my doctoral supervisor, provided the highlight in 1983. During a break from work, he suddenly noticed a large boulder full of fossils right in front of his feet. Inside were the remains of about as many new mammalian species from the Tertiary as the paleontologists had described for Australia since the European discovery.

Since then, an abundance of other species has come to light, including an unexpectedly large number of large carnivores. Many of these fossils are extraordinarily well preserved - at first glance, some of them can actually be confused with bones of decayed carcasses.

Most of the animals probably perished in limestone caves. The water, which is rich in calcium carbonate, quickly penetrated the bones and thus perfectly preserved them. The range of larger predators is particularly impressive: in terms of their dangerousness as predators, they are certainly not inferior to today's predatory mammals, even if they seem extremely strange to us today. Since 1985, the researchers have described nine new species of great predators from Riversleigh, none of which were smaller than the spotted-tailed sacred marten, but some were significantly larger. The number of now extinct Australian large carnivore species from the epoch more than five million years ago has more than doubled. According to current knowledge, this fauna included two giant rat kangaroos, nine pouch wolves, five marsupial lions and a pouch marten.

The scientists place the giant rat kangaroos in the subfamily Propleopinae. They have a living relative, the musk rat kangaroo (Hypsiprymnodon moschatus; see family tree on page 75). This species from the rainforests of Queensland, a "living fossil", with its 350 to 700 grams, in many ways really resembles a rat (picture on page 74). The animals feed on small animals and all kinds of plants. As the only ground kangaroo, they cannot hop on their hind legs the kangaroo way. The musk rat kangaroo is the last and smallest member of a family with once very defensive, muscular species: the giant rat kangaroos weighed between 15 and 60 kilograms and could probably only hop on all fours.

The marsupial wolves (family Thylacinidae) and the marsupial lions (Thylacoleonidae) get their names from superficial similarities with dog or cat-like representatives of the "placental mammals" or "placentalia". (This is the name of all "higher" mammals that feed the fetus via a highly developed placenta; in contrast, most marsupials give birth to their young at a very immature stage and then care for them in a brood sac.) which still occurred up to this century, even if last only in Tasmania (bottom right picture), was shot by man around 1930. He wiped out the species because of its largely unjustified reputation as a sheep hunter. (On the Australian continent, the pouch wolf had to give way to the dingo, with which it apparently could not compete.) The marsupial lions are already rather extinct, the last species probably not until after humans had already penetrated into Australia. Comparable to the feline species, they had short, broad, strong skulls, and they are likely to have had similar ecological functions. The smallest species was about the size of a house cat, the largest was the weight of a lion (picture above).

Despite the superficial similarities with placental mammals, all of these were clearly marsupials, or scientifically, marsupials (from the Greek and Latin words for pouch). Of course, a belly pouch does not fossilize as a pure skin structure. It is therefore not possible to say with certainty whether the individual species had one and what it looked like. But in the zoological system, the Marsupials are not classified according to the brood pouch anyway (even some of today's opossum rats do not have one), but according to certain bone and dentition characteristics - and according to this, the Riversleigh predators clearly belong to the marsupials.

During the rest of the Miocene, which lasted from around 25 to 5 million years ago, Australia was green from coast to coast: At that time there was rainforest on many of today's savannah and desert areas. These forests must have been an Eldorado of evolution - in any case, they supported more life forms than any Australian region today.

The strong-toothed giant rat kangaroo, which lived in that era, is one of the oldest rat kangaroos. (Five other types of giant rat kangaroos are known from recent deposits.) Weighing 10 to 20 kilograms, it was the smallest of all the giant rat kangaroos. The two almost completely preserved skulls of his give us the best picture of the feeding habits of this group of predator groups so far.

Because these animals descended from herbivorous to omnivorous ancestors, their way of life is still debatable. Scientists today agree, however, that the diet of the giant rat kangaroos included meat: This is attested by tooth and skull features, and traces have even been found on the teeth that look as if they came from biting bones.

Many believe that predatory mammals can be recognized by their powerful canine teeth. Often this is also the case - but not always. Quite a few people consume more meat than many so-called predators. Nevertheless, our canine teeth are small, whereas those of gorillas, which are practically entirely vegetarian, are very pronounced. (Their canines serve a social purpose.) The truly reliable anatomical feature of a land-dwelling predatory mammal are molars with incisor edges. (Many placentalists use the canines as "fangs", while the sharp-edged molars serve as "fangs" and also as crushing scissors from the upper and lower teeth.)

In the species of placental carnivores, which are less specialized in carnivores, the last two to four molars in the lower and upper jaws have characteristically wide crowns, with which the animal mainly grinds or, better, crushes plant parts. The fangs for severing muscles, skin and tendons sit right in front of it. Some giant rat kangaroos and some marsupial lions have such teeth. The species of these groups of placental animals such as marsupials, which are particularly specialized in carnal nutrition, are different: They have extremely enlarged fangs. The pinch teeth behind it, on the other hand, have become smaller or have completely disappeared. The house cat, for example, has extremely adapted predator teeth. Thus, the characteristics of these two types of teeth show how high the meat content of the food of a species is.

In this respect, the giant rat kangaroos were similar to foxes, which, as pronounced food opportunists, are also good at chopping up plants (picture on page 74 above). In addition, the skull of the species Ekaltadeta ima shows typical characteristics of a carnivore: It looks so robust that the animals must have had strong neck and jaw muscles, so they were probably able to hold onto wriggling prey. However, E. ima did not develop long canines in the lower jaw. Instead, the lower incisors turned into protruding daggers.

From all this, some colleagues and I conclude that the giant rat kangaroos were probably generalists who ate meat whenever the opportunity arose, but also a variety of vegetable foods. For at least 25 million years these unusual, carnivorous "kangaroos" have unsettled the Australian continent. They only died out sometime in the last 40,000 years.

They shared the predatory niche with marsupial species from other kin groups. The marsupial lions, for example, lived in the trees, four species in the Miocene. Like the giant rat kangaroos, the marsupial lions had herbivorous ancestors. The most primitive species had the omnivore characteristic dentition with crush and fangs. In others, the back molars have receded to a greater or lesser extent for grinding, and the sharp fangs in front of it have become overpowering (picture on page 75 above).

Scientists have formally described a total of at least eight species of marsupial lions; two others are currently being studied by Anna Gillespie of the University of New South Wales in Sydney. In the past, the way of life of the thylacoleonides was quite controversial among experts, because, according to current opinion, they belong to the same family group as the wombats and the koala, and these are herbivores. The advocates of a vegetarian diet, however, had to muster some imagination: The most extreme thesis was that marsupial lions had eaten melons - after all, hard plants could not chew them with their molars.

Today, however, there is a consensus that the marsupial lions feed on predatory food. Many researchers classify the youngest species, Thylacoleo carnifex, as the most specialized mammal that, to our knowledge, ever lived. The animals have practically given up their molar teeth, but instead have extremely enlarged fangs (picture on the right). They too did not have large canine teeth and may have used the long, protruding incisors for killing in their place.

Paleontologists have only found a complete skeleton of T. carnifex. Many of the experts estimate that the animals have reached the size of large wolves or perhaps even leopards. However, other researchers, including myself, believe that this does not do justice to the extremely robust skeleton: In our opinion, the weight of the predator could have been the equivalent of a lion today. The last marsupial lion had incredibly muscular front legs (picture on page 72). His strong body was certainly not designed for endurance performance. But the animal had teeth like bolt cutters and thumbs with retractable dagger-like claws that could be splayed out on the front feet. With these weapons it would have made an impression outside of Australia. T. carnifex could undoubtedly hunt down fairly large prey, probably also animals that were much larger than itself. It is not entirely clear how it used its large thumb claws. One can imagine, however, that hardly a victim escaped him once he was clutching it.

Marsupial lions of the genus Wakaleo were smaller, perhaps the size of a leopard. They too were not so much sprinters as real powerhouses. It is possible that these species, like Thylacoleo, attacked, preferably from above, down from trees - similar to leopards. The smallest marsupial lion, Priscileo roskellyae, was only about the size of a domestic cat. Perhaps this species was looking for its prey directly in the branches. In my opinion, the great marsupial lions should have been at the top of the Australian food pyramid.

If you consider that T. carnifex lived until at least 50,000 years ago, then the first human settlers in Australia might still have to be careful about it. On the forest floor, on the other hand, the pouch wolves once dominated, which even the Europeans encountered when they came to Australia over 200 years ago. At that time they found only two families of marsupials with larger predators: the tylacinids with a single remaining species, the tassel wolf, which was already pushed back to Tasmania, and the dasyurids with over 60 described species: the marsupial devil, several martens and numerous pouch mice, some of which were pronounced are tiny and eat insects and other small animals.

Because the bag martens in recent times clearly surpassed all other predatory mammals in biodiversity, the paleontologists had expected a comparable number ratio for earlier epochs. But the pouch wolves had underestimated them. Since 1990 the researchers have come across seven new species of them in Miocene layers: including the Tasmanian wolf, nine species have now been described and four more are in progress. On the other hand, we only know one species from the Miocene that is clearly a bag marten. There could be a few more if the fragmentary fossil record at hand is found to belong to species in this group. But even then, the species relationship between the two predatory families in the Miocene would be completely different from what it is today.

The last bag wolves in Tasmania are said to have been shot around 1930. Allegedly animals or their tracks were spotted from time to time later, but this has never been proven with certainty. A thylacine lived in London Zoo until 1934.Nevertheless, due caution should be exercised with regard to the information on the biology and behavior of this species. At least this much, however, should be fairly certain: The Tasmanian pouch-wolf resembled the majority of the canids (the placental dog-like) in that it was a purely ground-living, long-nosed hunter who probably mainly caught prey that was smaller than it; but he differed from the canids in that he was probably not very adapted to running - and he was probably no pack hunter like the wolf. And unlike most canines, he had molar teeth, which identified him as a pure carnivore.

The teeth of the thylacinids and dasyurids are, by the way, specially adapted to meat-eating. In contrast to the marsupial lions and giant rat kangaroos, each individual molar has both a vertically cutting edge and a squeeze surface. The extreme carnivores of these two groups then have molars with a reduced squish area and an enlarged cutting area.

Indeed, all pouch wolves have largely predated vertebrates, although the smaller, less specialized species may also eat insects. A large number of them resemble the typical placental canine even less than the last thylacine: some of the Miocene species were small compared to their Tasmanian cousin, and one, Wabulacinus ridei, had a short, more cat-like skull. It is not even certain whether all Miocene thylacinids were living in the ground, since most of them only survived as fragments of teeth and jaws. A beautiful exception is a 15 million year old fossil from Riversleigh with a skull and largely complete skeleton. At least this animal must have lived its life on the ground.

One more exciting note on the fringes of my topic: In the past few months, Henk Godthelp of the University of New South Wales in Sydney, Archer and I have re-described a mouse-sized marsupial that was made from deposits of around 55 million years old from Murgon in Southeast Queensland originates. The dentition of this animal is so unspecialized and primitive that the exact relationship with certain other lines of the marsupials is very difficult to determine. The new species could be an ancestor of the Thylacinids and Dasyurids - but perhaps even one of all Australian marsupials, the Australidelphia, as taxonomists call today's marsupials of Australia as a whole. It is possible that the animal does not even belong to the closer relationship of this subgroup of Marsupialia, but to that of the predominantly South American Ameridelphia, today mostly quite small and sometimes quite primeval species. (One species of opossum has penetrated as far as North America.) The land masses of South America and Australia once formed the southern continent of Gondwana together with the Antarctic, and according to some scientists only close relatives of Australidelphia got to Australia before Gondwana finally broke up. Perhaps this will give a different picture of the early history of the typical Australian mammal fauna.

What later happened to the many great Australian predatory baggers of the Miocene? The last species of marsupial lions or the giant rat kangaroos - Thylacoleo carnifex and Propleopus oscillans - died out not long ago. They were probably still roaming around when the first humans arrived in Australia 50,000 years ago or more - which is why some scholars believe that the early Australian Aborigines killed the last of the great predators.

So far this thesis has not been proven, but human influence remains a hotly debated topic during the long period before Europeans appropriated the continent. It is at least certain that the natives practically helped to drive the Tasmanian wolf out of the Australian mainland by introducing the dingo there, which was superior to the thylacine; The Australian predator was only able to assert itself until recently in exile in Tasmania. Human involvement in the extinction of the other species is less clear and will probably never be fully clarified. In any case, the fossil record shows that the biodiversity of large predatory baggers reached its peak in the early to middle Miocene and fell rapidly long before the arrival of humans. In the Middle Miocene, for example, there were at least five pouch-wolf species, but only two left in the Late Miocene, and only one species ever came into contact with humans.

Obviously, the loss was caused by other, not man-made conditions. The indigenous people would then have only accelerated the extinction that had started long before their arrival. The most likely factor responsible for species loss is dehydration. From the Middle Miocene onwards, the ice age conditions in Australia became increasingly stronger, which was accompanied by declining precipitation and falling sea levels. This process culminated in the last two million years or so: Australian wildlife had to endure around 20 ice ages. The last, very hard, wasn't even the hardest.

Many researchers believe that both climatic changes and pressure caused by human immigration contributed to the fact that the majority of the larger herbivorous marsupials died out, and that the food sources of the large predatory mammals dwindled. Of the dozen species of great predator that once unsettled Australia, only the spotted-tailed sacred marten and the Tasmanian devil - the Tasmanian devil - have survived. The white Australians bear full responsibility for the loss of the last thylacine. It would have serious consequences if the last two species also had to share his fate.

Bibliography


Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. By M. Archer, S. Hand and H. Godthelp. Reed Books, 1994.

Killer kangaroo. By S. Wroe in: Australasian Science, Volume 19, Issue 6, pp. 25-28, July 1998.

The Geologically Oldest Dasyurid, from the Miocene of Riversleigh, Northwestern Queensland. By S. Wroe in Palaeontology, in press


From: Spektrum der Wissenschaft 8/1999, page 70
© Spektrum der Wissenschaft Verlagsgesellschaft mbH

This article is included in Spectrum of Science 8/1999