Mega marsupial: A fossil skeleton of Diprotodon optatum at the Queensland Museum stands next to a modern lion and emu for comparison.
Credit: Stewart Gould, University of Queensland
SYDNEY: A long-standing mystery concerning the number of species of the largest known marsupial, the ancient Diprotodon, has been resolved, say experts, and sheds light on its mating practices.
A herbivore that lived during the Pleistocene epoch until around 40,000 years ago, Diprotodon "was the biggest marsupial of its time … bigger than anything that came before it, and bigger than anything [that has come] after," said Gilbert Price a palaeontologist at the University of Queensland in Brisbane, Australia.
At two metres in height and three and half metres in length, the wombat-like animal has been compared to a small car in size. Though one of the most famous prehistoric marsupials, partly due to fragmentary fossils, details concerning it have been few and far between.
Classification confusion
Particular confusion has surrounded the number of species that existed. Estimates typically range from two to eight, though some researchers argue for as many as 20, said Price.
To settle the debate once and for all, he toured museums and research collections across Australia examining over 1,000 fossils and completing the most detailed analysis of the remains to date.
Comparing the features and geographical distributions of the fossils – and using a modern marsupial, the grey kangaroo, for comparison – Price found that, while Diprotodon varied widely in anatomy across its range, it is unlikely to have comprised more than a single species.
Instead he assigns some of the difference in size and anatomy of adult remains to the gender of the animals. The analysis, reported in the latest Zoological Journal of the Linnean Society has also revealed some intriguing clues about the species' behaviour and social structure.
"The bones have not only, very importantly, resolved the debate over how many species of Diprotodon existed," said Price, "but have also uncovered the sex secrets of this enigmatic giant."
Ancient sex secrets
Since the majority of Diprotodon specimens known from fossils are the smaller size class, and therefore female, it is likely "where large Diprotodon populations once existed, they were almost always dominated [in number] by females," he said.
As in many large animal herbivore species today where populations mostly comprise harems of females, battle scars on some of the larger fossils suggest that the males may have competed fiercely for access to mates.
It's interesting work, commented palaeontologist Robert Jones of the Australian Museum in Sydney. "While many of us have assumed that there was ultimately only one species of Diprotodon," he said, "now here it is … really solid work that confirms it, once and for all."
"There has been a long-running debate over what drove the extinction of Diprotodon and other megafauna. Was it climate change or human hunting…or both?" concluded Price. "But the thing is, in order to understand the extinction of these creatures, we need to understand the animals themselves."
Knowing precisely how many species of Diprotodon actually existed may be critical in this regard, he said.
A similar 2003 study of New Zealand's ancient moa birds revealed that what were thought to be the remains of many ancient species of different sizes were in fact the remains of just one species, which varied widely in size over time and between genders.
