They may have been vegetarians, but the ancient wombats that roamed Australia were a frightening lot. Up to 9 feet (3 meters) long and 70 inches (180 centimeters) tall, some of the marsupials weighed as much as a pickup truck and stood as tall as a person. Others were much smaller, about the weight of a compact car.
This size variation has led paleontologists to debate just how many ancient wombat species existed, with estimates ranging from 2 to 20.
But a new study, published in the current issue of the journal Zoological Journal of the Linnean Society, suggests that—despite their vastly different sizes—ancient wombats all belonged to the same species, and that gender differences accounted for the huge size gaps.
Today's wombats, found throughout much of southern Australia, are more modest in appearance—short-legged, plant-eaters about 3.2 feet (1 meter) long. They hardly resemble their giant Ice Age ancestors, the largest marsupials to roam Earth from about two million to 10,000 years ago.
Researchers analyzed fossil teeth of giant wombat specimens.
"I suspected that just looking at teeth might give a much clearer picture of who was related to who," said study author Gilbert Price, a paleontologist of the University of Queensland in Brisbane, Australia.
"I figured the study would reveal that two, maybe three species once roamed the continent," Price said.
[Related: "Tooth Study Suggests Humans Caused Australian Ice-Age Extinctions" [January 24, 2007].)
In humans and other mammals, males and females often diverge drastically in size—a trait known as sexual dimorphism. Analyzing fossil teeth can thus prove an effective strategy to study ancient species.
Unlike the rest of the body, which is subject to the demands of sexual display, back teeth such as molars tend to only be involved in eating. Since both sexes of a particular species usually eat similar foods, their teeth should look the same.
Price leveraged this fact while comparing more than a thousand ancient wombat teeth held in museums around the world. He discovered that the fossils all showed similar patterns.
This indicated just one giant wombat species existed and that paleontologists were mistaking the differently sized male and female giant wombats for separate species.
The discovery helps explain why the bones of different-size wombats—male and female—are often found together.
Andy Currant is a paleontologist at the British Natural History Museum in London who was not involved in the study.
"Most large fossil herbivores tend to tune down to a smaller number of taxa [biological classifications] when you look at them closely," he said.
But, he added, the mistakes made by early investigators are easy to forgive. Researchers used to be hypersensitive to variation in specimens and did not really understand how much variation there can be between males and females within a population, he said.
Marcelo Sánchez, a paleontologist at the University of Zurich in Switzerland, said: "Solving such a specific taxonomic problem may not seem important. But individual-species research like this is ultimately what major claims about evolution depends[s] upon."
"Now we need to figure out why these giants went extinct," he added.