About 300,000 years ago, herds of rhino-sized creatures migrated across the floodplains of east-central Australia, mimicking the treks that zebras and antelopes make across Africa’s Serengeti today. But these migrants weren’t majestic, long-limbed grazers. Instead they were car-sized relatives of today’s short and stocky wombat. Evoked by a new analysis of a fossil tooth of the long-extinct animal, called Diprotodon, the scenario would be the only known seasonal mass migration among marsupials and their close kin.
The team’s findings are “pretty convincing to me,” says Anthony Stuart, a vertebrate paleontologist at Durham University in the United Kingdom. “For the first time, someone has demonstrated that Diprotodon migrated seasonally.” Similar analyses of fossils of other members of the same ancient ecosystem may reveal that multiple species participated in the ancient migrations, scientists say.
Like many parts of the world during the most recent ice ages (the last of which ended about 12,000 years ago), Australia had its share of weird giant animals, including a supersized relative of the Komodo dragon, today’s largest land lizard. But most of the continent’s so-called megafauna were marsupials. The largest was Diprotodon, 1.8 meters tall, nearly 3000 kilograms in weight, and named for its dentition—roughly translated from Greek, Diprotodon means “two forward teeth.” That pair of teeth, like the incisors of modern-day rabbits, never stopped growing, says Gilbert Price, a vertebrate paleontologist at the University of Queensland in Brisbane, Australia. As they grew, they incorporated trace elements dissolved in the water that the creature drank, as well as carbon, oxygen, and other elements from the food it ate. Because the isotopic makeup of those elements varies from place to place, Diprotodon’s steadily growing teeth became, in essence, a chronicle of its movements, in layers akin to tree rings.
“You’ve heard of the old saying ‘You are what you eat?’” Price asks. “As it turns out, you are where you eat, too.”
He and his colleagues took dozens of small samples from a 30-centimeter-long Diprotodon incisor. Using radioactive dating techniques, the team found that the creature lived about 300,000 years ago. Samples drilled from evenly spaced sites along the tooth also revealed cyclic variations in isotope ratios, including strontium, carbon, and oxygen, suggesting that this animal migrated about 200 kilometers each year, Price and his colleagues report today in the Proceedings of the Royal Society B. As a comparison, annual mass migrations of animals across the Serengeti plains of Africa cover about 800 kilometers.
The regular variation in isotope ratios suggests that rather than wandering at random from site to site, Diprotodon made the same round trip each year, following seasonal shifts in vegetation and rainfall. It’s the first time any marsupial living or extinct has been shown to migrate regularly, the team notes.
Modern-day marsupials such as red kangaroos do roam to find ephemeral food sources, says Stephen Wroe, a paleo-ecologist at the University of New England in Armidale, Australia. But those rovings are random, not regular, as Price and his colleagues propose is the case for Diprotodon. “That’s what makes this team’s results so interesting, and it suggests that the climate in the region at the time was more predictable than it is now,” Wroe says.
The new analysis “is an elegant example of how to use the geochemistry of a fossil to infer the behavior and movement of an ancient creature,” says Henry Fricke, an isotope geochemist at Colorado College in Colorado Springs. “Where an animal died doesn’t necessarily tell you where it spent its time while alive.”