A team of researchers believe a new technique will help to dispel the idea that marsupial carnivores have been less diverse and less evolved than their placental cousins.
Throughout most of the last 65 million years, the so-called 'age of mammals', warm-blooded furry predators have been the dominant land-based carnivores. Today, aside from a few small marsupial carnivores, these roles are filled by placental meat-eaters.
That has led many scientists to regard marsupial carnivores as the poor cousins of their placental counterparts.
But in research published today in Proceedings of the Royal Society B, a team of Australian and English scientists argue pouched carnivores have been every bit as specialised as their evolutionary competitors.
Co-author Doctor Stephen Wroe of the Computational Biomechanics Research Group says marsupials have been underestimated.
"Traditionally it has been assumed that marsupials were somehow inferior. It's as if they didn't quite get to the level of near perfection that placental mammals have achieved. They're seen as more primitive," says Wroe.
He says if we look at marsupial carnivores over the last 60 million years the evidence contradicts that view.
"If you go way back to around the end of the dinosaur era and the beginning of the so-called age of mammals, marsupial carnivores dominated through South America, Antarctica and Australia. So that was a fair whack of the world's surface area."
Comparable diversity to placentals
Wroe says scientists have postulated that marsupials had an inferior reproductive method to placental mammals.
"It's been assumed that this would put constraints both on the morphology of the forelimbs, and also the face, because for a considerable period the animal's got to be able to lock onto the mother's nipple," he says.
"But what our analyses are showing is that if you take the diversity in the fossil record into account, the marsupial carnivores are quite comparable to the placentals."
In their determination to rescue the honour of marsupial carnivores, Wroe and his colleagues combed museums on five continents. They measured 130 skulls from 62 species of both placental and marsupial carnivores, extinct and living.
The measurements were then digitised to produce three-dimensional computer models.
"This approach allows us to map points on the skull in three dimensions," says Wroe.
"Then we can run what we call multi-variant statistical analyses. So it's really a way of getting good quantitative comparisons.
"We've shown that there were clear similarities between marsupial and placental carnivores."
Self-sharpening sabre teeth
But he says some marsupial carnivores, like the marsupial lion and sabre tooth Thylacosmilus, have had extremely specialised morphologies, probably to suit quite narrow environmental niches.
"The South American Thylacosmilus is just weird, and far more specialised than any of the placental sabre tooths that have existed over the last thirty million years or so," says Wroe.
"The canine teeth go right back almost into its brain case. The teeth themselves are thought to have been ever-growing and self-sharpening. The arrangement of bones in its skull is bizarre."
Wroe says the reason none of these large marsupial meat-eaters exist today has more to do "with bad luck than with bad genes".
"South America, Antarctica and Australia all split apart," he says. "All of the other continents have been more or less in contact with each other, as one large land mass.
"When you're talking about extinction, one of the most important variables to factor in is the land mass area that the species are constrained by.
"If you've got a species that ranges across 30 million square kilometres it's going to be a whole lot harder to send that species to extinction than if it's living on five million square kilometres. So it doesn't necessarily mean that one or the other is quote unquote superior."