Thylacine (4 page)

There are also theoretical proponents for marsupials originating in Australia, not colonising it. Why not? Might there even have been primitive placentals on the Australian continent, banished by their more successful marsupial relatives?

Whatever the direction of migration, tectonic plate activity governed much of evolution. Features of continental movement include orogenesis (mountain building), the creation and extinction of oceans and seas, and great subterranean upheavals causing mass volcanic activity and resultant atmospheric change with huge increases in carbon dioxide levels. Such an increase in carbon dioxide occurred during most of the Mesozoic era, when Pangaea was breaking apart. It created a greenhouse effect which was ideal for large cold-blooded reptiles.

During the earlier Palaeozoic era, plants had become the earliest land colonists. They pumped oxygen into the atmosphere, thus offsetting the danger of ultra-violet radiation and enabling some of the fish to evolve safely into amphibians. These in turn became the earliest reptiles, which split into two lines, the petrolacosaurs (becoming dinosaurs and birds) and the pelycosaurs (leading to protomammals) of the warm Carboniferous and Permian periods.
²

Dinosaurs were the dominant life form for 200 million years. Their sudden demise led directly to mammal domination. What happened? The catastrophic extraterrestrial theory involving asteroid or comet impact seems to provide the most likely answer. An indentation clearly visible near Mexico's Yucatan Peninsula, in the Caribbean Sea, as well as the presence of much extraterrestrial iridium dating to this point in rock strata around the world, is evidence for the theory. Such an impact would have caused great floods and firestorms and blocked out sunlight to large parts of the Earth for months. Loss of warmth, and the sunlight required for plant photosynthesis, suggests considerable immediate devastation. Unlike the large dinosaurs, the small, mostly nocturnal, heat-generating mammals then present were less vulnerable.
³

Another theory holds that plate tectonic activity put so much volcanic particle debris into the atmosphere that a nuclear winter took hold. Sunlight would have been blocked and the food chain disrupted, terminating plant life and therefore the lives of the herbivorous dinosaurs upon which the carnivorous ones preyed. Whatever the truth, this momentous evolutionary event at the end of the Mesozoic left a world for the taking.

With the reptile dinosaur line now reduced to a pool of lizards, snakes, tortoises and crocodiles, mammals were in a prime position to diversify. By intriguing coincidence, this was round about the time that Australia set off on its long journey of isolation with its cargo of early marsupials and established monotremes. Isolation may well be a key term in explaining how the thylacine came to occupy its niche at the top of the food chain, as one of only a few carnivores.

By comparison, the South American experience was one of isolation interrupted. It seems that all of its mammals—the small herbivorous marsupials, the larger but slower carnivorous marsupials Borhyaenidae and Thylacosimidae, and the placental ungulates (hoofed herbivores)—fell victim to what is known as the Great Interchange. This occurred when the sea level dropped and the Panama isthmus once again created a land bridge with North America. Faster, more efficient placental predators flooded in and decimated the South American endemics. That, of course, did not happen to Australia's evolving marsupials.

Reference has been made to confusion over the exact lineage of the thylacine. Carnivorous groups moving through the super-continents left empty of dinosaurs included cat-like oxyaenids, dog-like mesonychids and mongoose-like hyaenodonts, either or all of which might lay claim to being remote thylacine progenitors.

Unfortunately, the Australian fossil record doesn't extend back beyond about the halfway point in the continent's isolation. This means that the thylacine's true origins may always remain unknown; the gap in the record is too great. The Thylacinidae family has so far been dated back about 30 million years.

Australia's combination of flat topography and prolonged subterranean chemical activity have worked against the preservation of fossils. Before the 25 million year mark, the continent's terrestrial vertebrate fossil record is meagre. Many assumptions have had to be made. But there is a spectacular exception with the presence of the monotremes, the egg-laying, milk-suckling mammals, represented today by the terrestrial echidna and the aquatic platypus. Their singular ancestral line bypasses all other mammalian branches as it leads directly back to the protomammals. To call monotremes ‘primitive' may be taxonomically correct but, like the crocodilians, they have survived and flourished remarkably, surely a true test of Darwinian fitness.

As noted, marsupial mammals generally have been considered ‘less advanced' than placentals. The logic supporting this—that placental gestation operates at a higher evolutionary level— ignores the circumstances of marsupial evolution. Once cut adrift, marsupial herbivores, carnivores and omnivores adapted in two ways, to each other and to the constantly changing island continent, from the extremes of its Antarctic position, which included months of perpetual southern darkness, to the periods of great inland seas, when the continent was like a patchwork of islands, to the relatively recent aridification of the centre contrasting with the humid forests fringing the north and north-east.

True marsupial diversification and radiation
⁴
is first evident in the Oligocene. But because fossil evidence of
Thylacinus
cynocephalus
doesn't predate this, there is at present no way of determining its Australian lineage—let alone a link to the South American Borhyaenidae, to which it bears a superficial resemblance.

What is clear is that the thylacine became widespread across the continent, which includes New Guinea. Its predecessor by some 4 to 6 million years was the considerably larger
Thylacinus
potens
, whose fossil record is very meagre. The modern thylacine outlived both the marsupial lion,
Thylacoleo carnifex
, and the giant predatory kangaroo,
Propleopus.
It would also have competed to some extent with reptilian carnivores like the giant goanna and the terrestrial crocodile
Quinkana
. Even so, these represent a paucity of major predators, given the abundance and variety of herbivores. Down in size and occupying different niches are the Tasmanian devil, also once widespread on the mainland (and much larger than its modern-day descendant), and the tiger cats.

The Riversleigh fossil collections of Queensland provide a comprehensive thylacine picture back to a point: ‘Five species ranging in size from that of a big domestic cat to that of a dingo thrived 25 million years ago—a sure indication that they had been evolving for a long time. But by 15 million years ago they were reduced to two species, and by 8 million years ago there was only one.'
⁵

It is possible that the niche-specific thylacine developed relatively free of competition even from the much larger carnivorous megafauna, although rather than being an evolutionary bonus this may have spelled long-term problems once dingoes, men and dogs came on the scene. Although the theory of convergent evolution suggests that its canid-like features and some behavioural characteristics gave it a dominant position equivalent to that of the Northern Hemisphere wolf, some critical features may have been absent.
⁶
The thylacine may not have needed to develop genuine speed to go with its stamina when hunting wallaby and kangaroo. It may also have enjoyed the luxury of being able to depend exclusively upon these and similar prey, to the extent of feeding selectively from fresh kill. Perhaps it lacked the ability to scavenge and to learn to adopt an omnivorous diet in the face of later competition.

That the thylacine inhabited much of the Australian continent is further evidence of a lack of major competing predators; it also reveals an ability to adapt to dissimilar environments. This was particularly the case from about 6000 years ago when the interior began to dry out. There is no real evidence that thylacines were affected by the increasing aridity. On the contrary, Aboriginal rock and sacred cave art dating back to that time, in such areas as the Kimberley and Arnhem Land, vividly portrays the animal. There is also a famous mummified thylacine (see plate section), found in 1966 at the base of a shaft on the dry Nullarbor Plain, that fell in and died there about 4000 years ago. It would have lived in very different circumstances to the thylacines of Tasmania, by then separated from the mainland and affected in large areas by cold, moist Antarctic winds and cold ocean currents.

Predator follows prey (never the other way around!), and as the kangaroos and wallabies radiated into most parts of the continent, it's entirely logical that the thylacine did likewise. Aborigines are known to have hunted them for food, but this would have been in a strictly utilitarian manner, in no way threatening the species' existence. Ancient Aboriginal rock-cave art depicting thylacines exists in at least eight locations, scattered geographically and over time. There are about 30 known thylacine fossil sites, out of a total of about 70 significant Quaternary mammal sites in Australia. In an evolutionary sense, therefore, there is evidence that the thylacine evolved as a successful marsupial, unchallenged as the continent's pre-eminent modern terrestrial carnivore.

3 AT THE END:
EXTINCTION

During our conversation, he told me that the last Tasmanian Tiger taken alive was caught at Adamsfield in the 1930s by his uncle. The method they used to trap the animal was to make a large box out of bush timber with a snare at the door and a baited release mechanism. The tiger had been around the camp for some time so it was not too long before it was caught in the box. To remove him from the trap they placed a rope noose, attached to the end of a long pole, around his neck. After that it was a simple job to hobble him and muzzle his mouth. Placed on the back of a horse he was delivered to Westbury Zoo where he was put on exhibition to the public.

J
OHN
G
OULDING,
G
EILSTON
B
AY

E
xtinction has always been integral to the life process, but when, as in the case of the thylacine, it is both manifestly unnatural and recent, it becomes our ineluctable duty to learn from the experience. Many have already done so. Alongside its positive allure and mystique as a tourism and quality-brand symbol, the loss of the thylacine glaringly symbolises wanton, careless destruction of the natural world.

Too late, Tasmanians realised that their champion marsupial carnivore occupied a vulnerable niche in the island's ecosystem. Thylacines were neither pest nor vermin but simply a perceived obstacle to nineteenth-century progress. And, whether snared or shot, they were also a source of bounty income.

Prior to European settlement, thylacine numbers may have been around the 5000 mark. While the true figure can never be known, this is a reasonable estimate, based on predator/ land-requirement studies. Tasmania is approximately the same size as Ireland or Sri Lanka, or West Virginia or Hokkaido, with over 30 per cent of its land surface permanently locked away as World Heritage Area and protected parkland. Much of this area is inaccessible.

Five thousand is not a great number. What would it take to decimate this population in just one century, to the extent that it could not even find refuge in those wild areas of the state? Human predation, at a minimum, accounted for over two thousand thylacines presented for government bounty between 1888 and 1912. Many hundreds had been killed long before then, in response to a sheep-protecting private bounty and by hunting, already disrupting the population's stability. Habitat alteration simultaneously compacted the animal's range. On top of this, at the beginning of the twentieth century a virulent, possibly bacterial, infection may have fatally attacked many marsupial animals, including the thylacine. But there has been no conclusive evidence to support this as a contributing factor to its decline.

Naturalists, scientists and others, such as the artist John Gould, had been warning of the thylacine's possible demise since the middle of the nineteenth century. Such was the animal's lot, although sightings continue to this day and, feasibly, it is in those locked-away and inaccessible areas of Tasmania that conditions may allow for a remnant population to have survived. It would have to be a remarkable throw of the dice:

Top carnivores, including eagles, tigers, and great white sharks, are predestined by their perch at the apex of the food web to be big in size and sparse in numbers. They live on such a small portion of life's available energy as always to skirt the edge of extinction, and they are the first to suffer when the ecosystem around them starts to erode.
¹

Despite the odds against its survival, the thylacine's extinction is not automatically proven. Two primary points are at issue. The first is that a species will become extinct if its numbers drop below the critical mass needed to sustain the gene pool, if the gene pool is limiting. The second is whether a species can survive once forced into a relatively difficult, increasingly shrinking wild environment. After millions of years of evolving as a specialised predator, what mechanisms would it have for ‘adapting' to rapid, deleterious change?
²

Scientists distinguish between mass extinctions and ‘background' or ‘random' extinctions, the latter occurring as a normal evolutionary feature, as first set out in Charles Darwin's 1859
The
Origin of Species
. The thylacine belongs in a third category, of human-induced extinctions, and so joins vast numbers of terrestrial, arboreal, aerial and aquatic species which have become extinct in just that way. However, it would be dangerous in the extreme to ascribe positive Darwinian notions of survival of the fittest (that humans need to eat) to justify extinction of, say, the dodo or the passenger pigeon.