The extinct Tasmanian tiger, or thylacine, bears an uncanny resemblance to today’s canines.
Like dogs, wolves and dingoes, it was a carnivore with a svelte body, long, narrow snout and strong hind legs. But the Tasmanian tiger was a marsupial, meaning it had a pouch like a kangaroo. And despite the similarities, the Tasmanian tiger last shared a common ancestor with the placental pack some 160 million years ago during the Jurassic period.
“One of the things we were interested in was how come they look so much like dogs even though they are distantly related?” said Andrew Pask, a developmental biologist at the University of Melbourne who sequenced the thylacine genome last year.
He said it was one of the most apparent cases of convergent evolution, where two unrelated organisms evolve to look or function alike because of the similar niches they fill in their environments.
The Tasmanian tiger was wiped out by hunters in Tasmania during the early 1900s. Scientists have had to examine rare museum specimens in order to better understand the characteristics the creatures shared with canines.
Now, by studying baby thylacines preserved in jars, Dr. Pask and his colleagues have pinpointed when those similarities began to develop. Using CT-scanning, the team revealed, for the first time digitally, the developmental stages of the extinct thylacine.
The findings, which were published Wednesday in the journal Royal Society Open Science, provide insight into when the Tasmanian tigers left their marsupial inheritance to become more dog-like. They also show the three-dimensional growth of the joeys’ internal organs and skeletons as they prepared to exit their mother’s pouches and enter the world.
Marsupials don’t have a placenta, so when they give birth their young are born premature. All of these babies, from koalas to wombats, look like pink jelly beans.
“They are alien-like with a little head and no eyes yet,” said Dr. Pask. “But they’ve got these forearms that have impressive muscles and little claws and paws they use to climb into the pouch.”
As thylacines nurse, a change occurs that distinguishes them from other marsupial species, the team found. Somewhere from week five to week eight, their hind legs and faces elongate, giving them a puppy-like appearance.
“The bones are growing much more like a big canid or other large body predators, than they do in other marsupials,” said Christy Hipsley an evolutionary biologist at the University of Melbourne, and an author on the paper. “Knowing when that happens can lead us to understanding why that happens and under what circumstances.”
After about twelve weeks, the striped joeys are ready to leave their mother’s pouch.
The team constructed the Tasmanian tiger’s developmental timeline after studying all of the 13 known thylacine pouch pups in the world from collections in Australia, Tasmania and the Czech Republic. But after conducting their CT scans, they discovered that two of the specimens were mislabeled and actually belonged to some other marsupial.
Because it’s hard to gain access to the rare specimens, the team made their digital scans available online for other researchers to use.
“The data are enormously valuable to a wide variety of scientists and incredibly hard to obtain,” said John Hutchinson, an evolutionary biomechanist at the Royal Veterinary College at the University of London who was not involved in the study, “so this is a very important effort on the grounds of obtaining rare data alone.”
Verity Bennett, an expert in marsupial evolution at Cardiff University, who was not involved in the paper, said the research added another important piece of data to the growing understanding of marsupial development.
“It shows despite its relatively ‘unique’ ecology and skeletal morphology, the early development of the thylacine is generally consistent with that of other marsupials, ” Dr. Bennett said.
She added that she’d be interested to see if the thylacine’s developmental pattern was shared among its carnivorous pouched cousins, like the thylacoleo, or marsupial lion, and the thylacosmilus, or saber-toothed marsupial.