Not a Wolf After All: How the Thylacine Bit More Like a Croc and Spinosaurus Than a Canine

You may have heard of convergent evolution, the idea that unrelated animals evolve similar features to deal with similar problems. In high school, we covered this, but since it was not important for the exams, we never went into much detail (to my dismay). One of the most classic examples is bats and birds; both evolved wings to fly, but one is a mammal, and the other is an avian.

Another popular example is the thylacine (Tasmanian tiger) and the wolf or fox, due to their similar-looking skulls despite one being a marsupial mammal and the other being a placental mammal, and both being separated by around 160 million years of evolution.

But how deep do those similarities go, and are thylacines and foxes/wolves really an example of convergent evolution, or are they far less similar than we had once assumed?

To figure this out, a new study by Vera Weisbecker and colleagues compiled new morphometric evidence (measurements of size and shape) from the thylacine skull and compared it with that of living carnivores. What they found was that it had a smattering of traits, some similar to crocodiles and Spinosaurus, others found in no living carnivores.

The Mysterious Thylacine

The extinction of the world's largest modern hypercarnivorous (meat-only diet) marsupial, the thylacine, has captured public imagination for generations, with people wondering how it lived, what it was most similar to, and some even speculating whether it could have survived in remote parts of its former habitat to this day.

Thylacines were sandy brownish-yellow to gray-coated marsupials typically sporting 15 to 20 stripes across their back and rump. Like all marsupials, they gave birth to premature young, which they nursed in a back-opening pouch. Interestingly, males also had a partial pouch.

The exact reason for its extinction in mainland Australia is speculative at best, though theories suggest that competition with dingoes and perhaps human hunting led to their decimation at least 3000 years ago.

In Tasmania, they continued to exist until the introduction of dogs and human hunting; due to the perceived danger they posed to livestock, the last thylacine died in the Hobart Zoo in 1936.

Its name, Thylacinus cynocephalus, means dog-headed pouched-dog, while its other common name, the Tasmanian wolf, all point to its common comparison to canids, based mainly on its skull, and have been used as a basis for many studies trying to understand the thylacine's feeding and sensory ecology.

Many studies have found various canid-thylacine similarities, which are striking considering the two mammals represent two very different clades (branches on the evolutionary family tree).

Some differences do occur in the skull, teeth, and other skeletal features, but these have often been attributed to evolutionary constraints such as phylogenetic contingency (being limited by what your ancestors already had) or the very different reproductive and developmental biology of marsupials and placental mammals.

Comparing Skulls

To better understand the similarities and differences, the researchers examined 60 faunivore species (carnivores, piscivores, insectivores, etc.) totaling 225 individual skulls to determine whether the thylacine's cranial shape differs from that of its marsupial relatives and to compare it with canids and metatherians (marsupials and their extinct relatives).

What they found initially seemed to confirm the canid similarities, with the thylacines' overall shape comparable to that of mid- and small-sized prey specialists like foxes and jackals, but a closer look revealed something surprising: thylacines had rostrums (snouts) similar to those of most canids but neurocrania (braincases) similar to those of metatherians.

Looking more closely still, the canids they resembled depended on which part of the skull you were looking at, with foxes and South American canids sharing similar snouts with thylacines, but African wild dogs, wolves, and dholes (Asiatic wild dog) having braincases more similar, though not the snouts.

In other words, the thylacine had a unique combination of snout and braincase features that do not occur together in any living canid or marsupial.

But it is in size where the results of this study diverge most from previous studies. Looking at a thylacine, something always seemed a bit off to me, as it turned out something was 'off', namely their skull size. The head of a thylacine is far larger than expected for its body mass.

A living thylacine weighed around 17 kg (37.5 lbs). Yet, its cranial size is more similar to that of species weighing between 24.5 and 66.7 kg, on par with much heavier animals like the gray wolf, African wild dogs, pumas, and leopards. Meanwhile, the skulls the thylacine is most similar to are those of jackals and foxes, which weigh around 6 to 14 kg.

This massive head, not necessarily its shape, may be one reason the creature was once described as wolf-like.

The researchers wondered whether the big skull and shape were simply a result of its larger body size. Bigger animals often have their body shapes change in predictable ways, such as having thicker leg bones than smaller animals. But when they plotted the skull size against skull shape against other species, they found that bigger animals didn't just start falling into similar skull shapes. On top of that, each family — canids, felines, and marsupials — all followed their own unique trajectory of how skull shape changed with size, that the thylacine simply did not fit.

So the resemblance isn't a by-product of size.

Bite Like a Croc, Look Like a Canine, Born a Marsupial

Now we get to the weird and wonderfulness that is the thylacine. Based on its gracile (slender) snout, the thylacine could not chomp down and hold onto struggling prey, or shake and pull its target. Think of it like a long, slender stick vs a thick, chunky one; the other would simply snap. Instead, its longer jaw was built for fast snapping, something usually associated with fast-moving, small prey.

This is where that large head comes in handy because its cranium is so big it counteracts the weaker structure of the snout. This is because a larger head also allows you to carry more muscle and is more resilient under greater loads, enabling the thylacine to capture prey with fast, high-impact bites without its weaker snout breaking.

Two other traits support this type of hunting, namely the snout's 'terminal rosette', which describes how the snout flares out at the canines and pinches again behind them. That puts more bone, and thus more mass and strength, at the canines, the main tools being used when the animal bites.

Additionally, the snout is unusually tall rather than wide (a rare trait in living mammals), which is great for an up-and-down biting force, but not exactly great when dealing with twisting and turning prey.

The second trait concerns the infraorbital foramen (IOF), an opening that allows the maxillary [upper jaw] nerve to connect to the face. It's much larger than in any living marsupial; its exact function is unclear, but some research on saber-toothed cats' large IOF suggests it may have helped in precision placement of the canines, basically aiming the teeth exactly right for a deadly blow.

This weird combination of traits is not seen in a single other modern mammal: a large cranium, a gracile and tall snout, and terminal rosette.

But similar morphologies are seen in some very unlikely places, such as fast-snapping crocodiles, Spinosaurus, and pterosaurs (yeah, our extinct friend has more in common with reptiles, including dinosaurs, than canids or marsupials! Well, at least in terms of this combination of traits). This combination of morphologies is also seen in some extinct mammals like sparassodonts, entelodontids, hyaenodonts, and mesonychids.

That leads us back to something I mentioned in the beginning. Some researchers suggest the weird skull of the thylacine may have been a result of being constrained by the reproductive and developmental biology of marsupials. Because they are born basically premature, some things just never quite develop as well as in placental mammals. But thylacines' skulls don't seem to be a compromise — where evolution went, yeah, we wanted to be like a wolf but didn't quite get it right. Rather, their design is simply one that has no other living mammalian equivalents.

In terms of convergent evolution and shape, the answer seems to be yes: the thylacine and canids share more physical traits than the thylacine does with other marsupials.

In terms of function, according to this paper, no. If we take convergence as meaning two species independently got to the same solution to the same problem, then canids and thylacines were facing different problems. Wolves' jaws are built to grab large, struggling prey, hold them down, shake them, and pull at their flesh. A thylacine's skull is built for speed and high impact; for small, fast prey, one deadly bite and it's game over.

No living animal can show us how this skull worked, but maybe its genome can at least explain how it came to be.

Wolves and thylacines independently evolved changes in the same parts of their DNA, basically the control switches for certain genes. Understanding how they compare to each other, when one is turned on and off, and what the results are, can help us better understand the thylacine's evolution and how it came to be such an oddity even among the already rare marsupials.

The romantic in me wants to believe these weird creatures survive in some remote parts of New Guinea or Tasmania, but the scientist in me knows they are extinct. And thus, to find out how they lived and hunted, and what forces shaped them into what they were on their evolutionary path, all we can rely on are museum collections and fossils.

The study shows us that even textbook examples of convergent evolution can be a lot more complicated.

It means we may not be able to figure out how it lived simply by studying its closest living relatives or look-alikes, which is exactly what a lot of thylacine research has done, but instead we may need to dig deeper into the genetic history of these animals to better understand them.

The research also highlights that marsupials aren't just constrained second-rate versions of placental mammals, just as the thylacine wasn't just a second-rate wolf.

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References

Australian Museum. (2021, January 27). Thylacine. The Australian Museum; The Australian Museum. https://australian.museum/learn/australia-over-time/extinct-animals/the-thylacine/

Weisbecker, V., Pask, A.J., Newton, A.H. et al. Skull morphology of the extinct Tasmanian tiger suggests unique biting style. Nat Commun 17, 8729 (2026). https://doi.org/10.1038/s41467-026-76614-0