When European settlers first arrived in Tasmania, they were surprised to find a large canine marsupial that had stripes resembling a tiger.
These features gave the animal a common name – Tasmanian tiger and scientific name, Thylacinus cynocephaluswhich translates as “dog’s head in a bag”.
Despite the striking resemblance of the Tasmanian tiger to large dogs such as the gray wolf, they are very distant relatives and had no common ancestor from the Jurassic period, more than 160 million years ago.
Their striking resemblance is the result convergent evolution, the process by which different animals evolve to look alike because they occupy similar places in the ecosystem, sharing certain lifestyle factors such as hunting.
However, how animals evolve to become closer, especially the forces driving their early development, is a question that still baffles scientists.
A new joint study by researchers at the University of Melbourne used state-of-the-art technology such as micro-CT scans and digital reconstructions to compare Tasmanian tiger and wolf skulls during their early development and into adulthood, finding them to be even more similar to first glance.
TASMANIAN TIGER IN THE MOUNTAIN
After reconstructing the early development of the thylacin sac, we still wanted to understand when the Tasmanian tiger established its canine skull shape during its growth.
We know that thylacine and wolf are similar to adults, but we did not know when they began to show their extraordinary resemblance during development.
Through collaborations with Australian museums and the Northern Museum in Alaska, USA, our team has borrowed tilacin and wolf skulls of all ages, stages and sizes, from infants to adults.
We then used micro-CT scans on turtles to create digital models that could be compared to determine when there was a similarity between tilacin and wolf during development.
CT is a technique similar to medical CAT-scanning, which can generate high-resolution digital reconstructions of complex shapes such as skulls and bones. We can then make detailed statistical comparisons between structures such as the shape of the nose and mouth.
Using these comparisons, we found that the Tasmanian tiger not only resembled a wolf as an adult, but was extremely similar to both juveniles and newborns.
Notably, Tasmanian tiger puppies were more like wolf cubs than other closely related marsupials, such as civilization.
This was a truly unexpected discovery, given that the thylacine and the wolf last had a common ancestor more than 160 million years ago, when dinosaurs still roamed the ancient world.
This finding is even more striking when we think about how different marsupials reproduce, giving birth to tiny young that continue most of their early development in the sac.
We thought that the need to suck at such an early stage of development, compared to dogs, could make the development of their skulls very different.
Our results also showed that some parts of the skull, such as the face and cerebral sheath, were more similar than other areas, such as the mouth and jaw.
A fellow Dr. Vera Weisbecker of Flinders University says all marsupials – including thylacine – are born with extremely well-developed jaws relative to the rest of the head.
“Scientists believe that this reduces the potential of marsupials to develop some extreme skull shapes. However, this clearly did not prevent the evolution of the unusual wolf-like skull of thylacine, ”says Dr. Weisbecker.
Another finding was that these regions develop from known groups of early embryonic cells.
These observations complement our other findings that thylacine and wolf created similar instructions in their genome that affect cranial stem cells during development.
Together, these findings underscore that tilacin and the wolf developed similar genetic plans and developmental strategies to create a skull-like resemblance that occurs in the earliest stages of development.
Co-author Dr. Christie Hipsley of the Victoria Museum, who specializes in CT scans and previously worked with our team to sequence the thylacin genome, notes that this is another example where 3D images can reveal hidden diversity in nature.
“Comparing the entire series of growth from newborns to adults, we were able to visualize tiny differences in development that pinpoint when and where in the skull adaptations to the predator occur at the cellular level.
“This discovery was made possible only by museum loans of surviving specimens, in this case from faraway Alaska.”
Banner: Left, adult thylacine (sample C5744), right: adult wolf (sample UAM101206), supplied