The thylacine, or Tasmanian tiger, had an unusual shape for a marsupial, much more closely resembling a dog in basic structure than its closer relatives. Even though the species has been extinct for almost a century, scientists at Museums Victoria in Australia have managed to reconstruct the thylacine's growth from birth and found they started off with far more usual marsupial features.

The finding is not only a masterpiece of reconstruction for an extinct creature but sheds light on the process known as convergent evolution, where similar ecological niches cause distantly related animals to come to resemble each other more than the species from which they evolved.

Thylacines exert something of a fascination for zoologists. They were a large animal that went extinct at humanity's hand at an exact point we can identify. The ones we encountered were also the last of a family containing at least 14 species that once filled many ecological niches across Australia.

Reinforcing the interest is their resemblance to animals to which we are much more familiar. Like other marsupials, thylacines had pouches, with their young being born much less developed than placental mammals, and growing from helpless to independence tucked against their mother's skin. In other ways, thylacines were much like members of the canid family, including the dingo that displaced them from mainland Australia.

We also have an unusually rich store of specimens for an extinct species, including 15 initially thought to represent thylacine joeys at different ages. Dr Christy Hipsley applied CT scanning to all these, creating digital models of the species development.

Among the things the study revealed was that two of the supposed thylacine joeys were not thylacines at all, more likely being quolls or Tasmanian devils. Museums weren't always as rigorous in cataloging their specimens as today.

More importantly, Hipsley reveals in Royal Society Open Science, we can see how similar the thylacine's body structure initially was to other marsupials at the same age, with front legs and mouthparts overdeveloped compared to the rest of the body. These are the essential items marsupials need to crawl into the pouch after birth and start sucking on the mother's teats, even when the rest of the body resembles a jellybean.

“But by the time it left the pouch around 12 weeks to start independent life, it looked more like a dog or wolf, with longer hind limbs than forelimbs,” Hipsley said in a statement. Hipsley told IFLScience, “Convergent evolution has been a fascination for evolutionary biology for a long time, wondering how it occurs and under what circumstances.” Her team sequenced the thylacine genome last year, and think the combined approaches will shed light on these questions, as well as the improbable idea of "de-extincting" the tiger.