Australian study discovers how a marsupial and a mammal have nearly the same skull shape

A new study has discovered that two distinct orders of the animal kingdom, the extinct Tasmanian tiger and the gray wolf, have nearly identical skull shapes, a unique example of convergent evolution, the independent development of similar features in non-related animals or plants.

The Tasmanian tiger (thylacine), which became extinct around 1930, looks like a dog with a striped back, but bears its young in a pouch like kangaroos and other marsupials. The gray wolf, a large canine native to Eurasia and North America, bears its young in a uterus like other placental mammals.

Remarkably, the thylacine and the gray wolf last shared a common ancestor more than 160 million years ago, but their skulls look and function the same. The study shows that the thylacine young were more similar to wolf pups than to pups of closely related marsupials.

A team of Australian scientists used X-ray computed tomography (CT) scans of thylacine and wolf skulls from museums to make digital 3D reconstructions of the animals at different ages. Comparing the skull reconstructions enabled the researchers to analyze the similarities of the skulls and the different stages of the animals' development.

Their research appears Jan. 8 in Nature Communications Biology.

The authors wrote, "The extinct marsupial thylacine and placental gray wolf represent one of the most extraordinary cases of convergent evolution in mammals, sharing striking cranial similarities despite 160 million years of independent evolution."

The researchers were able to compare individual cranial bones at different stages of development, from infancy to adulthood, giving them a window on the evolution of "shared molecular pathways underlying their convergence." They also compared the thylacine cranial development to that of existing marsupial species.

The researchers were looking for patterns in the cranial development of each species in each stage of development. What they found surprised them. They had hypothesized that the cranial shapes of the neonate thylacine and the wolf would be different and only converge later in the juvenile to adult stages. Instead, the thylacine and the wolf "displayed parallel, nearly overlapping patterns"  throughout cranial development," the study noted.

The cranial development of the thylacine was more similar to the wolf than it was to "any of its close marsupial relatives," the authors wrote.

How did this extraordinary convergence of cranial development occur in two very different carnivorous animals? The authors write: "These similar developmental trajectories are likely an adaptive response to shared ecological niches and biomechanical demands of predation, i.e., the development of robust cranial bones and musculature to generate high bite force quotients."

"Importantly," they add, "the thylacine appears to have evolved unique developmental mechanisms to facilitate its adaptive convergence, making this comparison an extraordinary evolutionary model to scrutinize the developmental processes underlying convergent evolution."

The authors conclude that their findings suggest "exciting new research avenues to examine how molecular changes within tissue-specific regulatory networks may influence craniofacial shape disparity and convergence." Further, they note that their findings provide "a novel framework for defining the causative factors underpinning skull evolution and the remarkable cranial convergence seen between the thylacine and wolf."