The last known Tasmanian tiger in footage digitised by the National Film and Sound Archive of Australia.
Photograph: National Film and Sound Archive of Australia.
The thylacine, or Tasmanian tiger, has long been a textbook example of convergent evolution: a marsupial predator whose resemblance to a wolf evolved independently in a distant branch of the mammalian family tree. For creationists, such similarities offer an opportunity to invoke a designer reusing a successful design. But that explanation overlooks the more revealing question: how far does the resemblance go, and why do similarities in some features coexist with substantial differences in others?
New research led by Flinders University’s Professor Vera Weisbecker, published in Nature Communications, shows why describing the thylacine as a “marsupial wolf” can be misleading. Its skull combined features resembling those of different carnivores with an unusual specialisation of its own. As the university’s account explains, this animal was more closely related to kangaroos than to dogs, despite its superficially canine appearance.
Three-dimensional comparisons revealed a striking mixture. The back of the skull resembled that of larger canids, including wolves, while the long, slender snout looked more like that of smaller foxes and jackals. Yet the whole skull was disproportionately large for the animal carrying it. The researchers suggest that this combination supported rapid, forceful snapping bites at relatively small prey. Some features even invite comparison with crocodiles and other vertebrates that catch agile prey with elongated jaws. These are inferences from anatomy, rather than direct observations of hunting behaviour, but they undermine the familiar image of a predator equipped to overpower large animals in the manner of a wolf. The authors explain these findings in their article for The Conversation.
This is precisely where convergent evolution becomes more informative than a superficial resemblance. Natural selection acts on inherited anatomy, modifying structures already present in a lineage. Similar demands can favour similar features in distantly related animals, without producing identical bodies or requiring identical lifestyles. Each lineage brings its own evolutionary history, developmental constraints and available variation to the process. The result can be a mosaic of resemblance and difference.
Invoking a designer who sometimes copies a wolf, sometimes a fox and sometimes produces something quite different adds no testable explanation for that pattern. Descent with modification, by contrast, provides a framework for investigating both the shared features and the departures from them. The thylacine’s distinctive skull makes that investigation richer, while reminding us that evolutionary convergence need never amount to a complete anatomical duplicate.
The article in The Conversation is reprinted here under a Creative Commons licence, reformatted for stylistic consistency:
3D scans of Tasmanian tiger’s skull reveal it had crocodile‑style snapping jaws
A scanned Thylacine skull from the South Australian Museum.
Douglass Rovinsky
Some 90 years after the death of the last documented thylacine – better known as the Tasmanian tiger – the species remains a global symbol of human-made extinction.
The carnivorous marsupial fell victim to 19th-century colonists’ perception that unfamiliar creatures were primitive, vermin-like imitations of more familiar European animals. The species was later exterminated due to sensationalist claims that it was a ferocious, wolf-like predator of sheep.
However, our new study, published in Nature Communications, suggests the thylacine defied the colonialists’ assumptions in every way. It was unique among modern predatory mammals by subduing its prey with high-impact snaps of slender, oversized jaws.
Our focus was on one of the most widely cited cases of evolutionary convergence – the dog-shaped skull of the thylacine, which inspired its species epithet cynocephalus (“dog-head”). This is a striking similarity as thylacines are separated from dogs by more than 120 million years of evolution.
So we set out to understand how deep the similarity really goes.
Overview over the mammalian tree of life with thylacine in pink and wolf in orange.
We used 3D shape analysis to compare scanned thylacine skulls from museums around the world with those of other predatory mammals. This revealed that different regions of the thylacine’s skull resemble those of several other animals.
The thylacine’s long and thin snout looks pretty fragile, most like that of small foxes and jackals. By contrast, the back of the thylacine’s skull resembles that of wolves, dingos and dholes (also known as the Asiatic wild dog).
But the standout feature of this odd patchwork was the thylacine’s enormous head. Its relatively puny 17 kilogram body carried a skull as large as that of wolves, which are more than twice the thylacine’s weight.
So why this skinny jaw on an outsized head?
We suspect this combination allowed thylacines to subdue their prey with powerful snapping bites. Thylacine jaws are clearly built for speed: the longer a jaw is, the faster its tip moves during biting.
Combined with the animal’s famously large gape, this should increase the impact of the strike. Long jaws like this are usually quite weak, but the huge size of the thylacine’s skull would have added enough substance to withstand lethal snaps into potentially unsuspecting bandicoots.
A yawning thylacine at Beaumaris (now Hobart) Zoo taken in 1933.
But it is the features that are not found in other predatory mammals that may have taken the thylacine’s snaps to a new deadly level.
The thylacine’s upper jaw is exceptionally tall, with a distinctive “pinched” hourglass shape at the tip. This combination is perfect for resisting the stress of high-impact, upwards-directed bites. It also matches earlier results that thylacine canine teeth seem most suited to puncturing, crushing bites (rather than pulling, tearing, or holding).
Thylacines were probably the last modern predatory mammals to sport such a tall, mace-like snout. But similar jaw types are found across other vertebrates, such as crocodiles, and are usually interpreted as adaptations for hunting fast-moving prey.
But some extinct mammals also had thylacine-like snouts, most notably in the gigantic extinct predator Andrewsarchus.
Our results are particularly tantalising in view of earlier genomic work which suggested that natural selection acted on regulatory genes in some shared developmental pathways of wolves and thylacines.
It is possible these genomic similarities are behind some of the shared aspects of skull shape between wolves and thylacines. However, as we know wolves and thylacines did not fill the same ecological niche, we might ask how these genomic convergences produced such a uniquely specialised marsupial predator.
Even though wolves, thylacines and red foxes are separated by millions of years of evolution, their skulls show striking similarities.
Vera Weisbecker
Our results highlight – yet again – that the thylacine’s eradication targeted an animal that only existed in the early colonial imagination. But they are also a timely reminder we cannot afford to repeat this mistake through negligence.
Australia has the worst mammal extinction rate in the world. Its wildlife faces escalating threats from environmental degradation and climate change, as well as new threats such as bird flu.
Museum-based research like ours is needed to illustrate how these statistics reflect irreplaceable biodiversity lost with every extinction.
But we also need to remember Western science is only part of the story – respectfully requesting knowledge from Indigenous custodians about the unique Australian wildlife should be key to our future research.
After all, the people who lived alongside thylacines for tens of thousands of years knew exactly what kind of animal it was.
Vera Weisbecker, Professor in Evolutionary Biology, College of Science and Engineering, Flinders University; Andrew Pask, Professor, School of Biosciences, The University of Melbourne, and Douglass S Rovinsky, Associate Research Scientist, School of Biological Sciences, Monash University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The thylacine illustrates why convergent evolution involves much more than two animals looking alike. Its skull combined similarities to several other predators with a distinctive set of proportions and probable feeding adaptations. The familiar description “marsupial wolf” concealed much of that complexity. Looking more closely has revealed the sort of mosaic that descent with modification leads us to expect: inherited anatomy reshaped by natural selection, with different features evolving under different constraints.
Evolution has no obligation to reproduce an entire wolf simply because some wolf-like features are useful. Nor does it anticipate a finished product. Variations that improve reproductive success under prevailing conditions can spread, while the accumulated consequences remain dependent on a lineage’s history. Similar mechanical demands can therefore produce comparable adaptations in distant relatives without erasing their ancestry or making their ecological roles identical.
For creationism, invoking a designer who reuses some features while changing others merely accommodates whatever scientists discover. Unless it specifies independently testable reasons for those choices, it explains neither the similarities nor the differences. Evolutionary biology, meanwhile, provides questions that can be investigated: which structures converged, which retained ancestral characteristics, and what might their combination reveal about feeding behaviour? Even the remaining uncertainty about the thylacine’s hunting technique becomes an opportunity for further research.
There is also a poignant human lesson here. The thylacine was persecuted under a reputation for wolf-like livestock killing that this research further calls into question. Its extinction deprived us of the opportunity to observe the behaviour that scientists must now reconstruct from museum specimens. We destroyed an animal before we properly understood it. The least we can do is allow evidence to correct the assumptions that helped condemn it—and apply that lesson to the species we still have time to protect.
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