Nimbacinus peterbridgei (top right) would have been a predator of small animals, while Ngamalacinus nigelmarveni (bottom right) was a specialised hunter. © Peter Schouten
The thylacine is an icon of extinction. But when the last one died in 1936, it marked the end of a diverse lineage that had existed for over 25 million years.
Now, researchers have unearthed three new relatives of the extinct marsupial, revealing more about this poorly understood family of mammals.
The earliest relatives of the thylacine have been discovered in northern Australia.
The trio of newly described thylacinids lived around 25 million years ago in the cool and wet forests which once covered the country. While the largest of these animals was only the size of a small dog, they would have been important scavengers and predators in this ancient ecosystem.
Timothy Churchill, a PhD student who led the research, says that the new fossil discoveries highlight the diverse roles the thylacines once played in Australia.
“These new species exhibit very different dental adaptations, suggesting there were several unique carnivorous forest niches available during this period,” he says.
“All but one of these lineages – the one that led to the modern thylacine – became extinct by eight million years ago. That lineage ended with the death of Benjamin, the last Tasmanian tiger, in Hobart’s Beaumaris Zoo on September 7, 1936.”
The findings of the study were published in the Journal of Vertebrate Paleontology.
The fossils were found in the Riversleigh World Heritage Area in Queensland, northern Australia. © UNSW Sydney/ Henk Godthelp
Around 30 million years ago, the continent of Australia split away from Antarctica. Cut off from the rest of the world, its wildlife evolved into new and distinctive forms, from ‘giga-geese’ to the duck-billed platypus.
Australia’s mammals are particularly unusual. Most of them are marsupials, meaning they give birth to live young and raise them in a pouch.
When Australia became isolated, these marsupials filled the ecological roles occupied by placental mammals elsewhere in the world. In the case of the thylacinids, this saw them gradually transition from small, fox-like animals into apex predators that, due to the process of convergent evolution, were strikingly similar to wolves.
But as Dr Neil Adams, the Natural History Museum’s Curator of Fossil Mammals, explains, the early years of these animals are shrouded in mystery.
“We still know relatively little about the earliest thylacinids,” Neil says, who was not involved in this new research. “This is because they are only known from a small number of rare fossils, almost all of which represent just the jaws and teeth.”
“What we can say with reasonable confidence is that they are likely to have been relatively small-bodied, because tooth and jaw size are correlated with body size in living mammals. Our measurements suggest that the earliest thylacinids were roughly between two to 11 kilogrammes.”
These new species, discovered in the Riversleigh World Heritage Area on Waanyi Country, are now helping to clarify the lives and relationships of these ancient mammals.
Badjcinus timfaulkneri (top right) was a scavenger, and probably ate the flesh and bones of kills left behind by larger predators. © Peter Schouten
The oldest of the new species is Badjcinus timfaulkneri, which at an estimated 11 kilogrammes is also the largest. Not only is this around the same weight as a modern Tasmanian devil, but Badjcinus probably had a similar lifestyle.
“Badjcinus timfaulkneri has very heavily worn teeth, which suggests it had a hard diet,” Neil says. “It also has very deep lower jaw bones, which provide enhanced strength and resistance to the jaw bending that occurs while eating hard foods.”
“This suggests it could have been scavenging the big, hard carcasses abandoned by larger predators, like marsupial lions, which would have lived in the same areas as these early thylacinids.”
Meanwhile, the jaw of the second new species, Ngamalacinus nigelmarveni, suggest it was living a very different lifestyle. Rather than scavenging, Ngamalacinus appears to have been adapting for a hypercarnivorous diet, meaning it ate almost nothing but meat.
“Hypercarnivory has evolved in many different groups of mammals independently, with similar features developing to help these animals process meat more effectively,” Neil explains. “The front molars become simpler, forming steep planes and sharp edges that are excellent for slicing and cutting.”
“The rear molars, which are used for crushing and grinding, become smaller and can eventually be lost altogether in hypercarnivores because slicing up flesh is much more important. We can see these sorts of changes through the evolution of thylacinids, with some of the early stages represented in Ngamalacinus.”
Jawbones are often all that's left of thylacinids like Nimbacinus peterbridgei. © UNSW Sydney/ Tim Churchill
The last of the new species, Nimbacinus peterbridgei, is also the smallest. However, it might be the thylacinid with the greatest legacy. The researchers believe it is the oldest direct ancestor of the modern thylacine.
“Nimbacinus peterbridgei was a more generalised predator that probably focused on small mammals and other prey species that lived in Riversleigh’s ancient forests,” says co-author Professor Mike Archer.
“We think it may have been on the direct line to the only other species of Nimbacinus, the larger Nimbacinus dicksoni, which lived 15 million years ago. This group of thylacines appears to be the one that led directly to the thylacine.”
What happened to all the other thylacinids in the interim is not certain, but a combination of ancient climate change and increasing competition could have been to blame.
“From around 15 million years ago, a period of climate change known as the middle Miocene climatic transition resulted in cooling and drying globally,” Neil explains. “In Australia, this saw the wet rainforests contract and become replaced by more open forests with more seasonal rainfall.”
“At the same time, other groups of marsupials were diversifying. One of these was the Dasyuridae, the group which includes the living quolls and the Tasmanian devil. These factors could both have put the thylacinids under pressure, ultimately leading to their extinction.”
The last surviving group of these animals, Thylacinus, lived on in mainland Australia for millions of years. However, it too was wiped out around 3,200 years ago through a combination of climate change, human hunting and the introduction of the dingo.
While thylacines continued to live on in Tasmania, they were hunted to extinction by European colonisers in the eighteenth and nineteenth centuries. There have been suggestions the animals survived into the 1980s, but sightings have remained uncomfirmed.
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