Marsupial carnivores 'as diverse as other mammals once'

image captionMany of the marsupial carnivores have long been extinct

They are an extraordinary and now rare group of animals but Earth has had some formidable marsupial carnivores.

These pouched killers have included lions, wolves, and even sabretooths.

Today, the only large marsupial carnivore left in existence is the Tasmanian Devil, and that is on the brink of extinction.

These animals' past success though is illustrated by a new skull study that reveals the creatures to have been just as diverse as their cousins, the placental mammals.

An international team examined the skulls of some 130 carnivores - marsupial and placental, living and extinct - from the past 40 million years.

Dr Anjali Goswami and colleagues used a technique known as geometric morphometrics to map the objects.

Their analysis shows the variation in shape in marsupial carnivores' skulls is actually greater than that observed in placentals, such as "ordinary" lions and tigers - even though the marsupial sample was smaller.

The findings are published in the journal Proceedings of the Royal Society B.

The team says the research gives the lie to the idea that marsupial carnivores' method of reproduction, wherein the young are born at a very early stage, somehow limited their ability to adapt to new habits and environments.

"A straightforward example is with the forelimbs," explained Dr Goswami.

image captionAn extinct sabretooth marsupial, T. atrox, evolved ever-growing canines

"Because marsupials have to crawl really early - they have to develop these crawling, grasping hands to get into the pouch. Once you have to have that kind of structure, it's really hard to then develop a flipper or a bat wing.

"And while it's been shown marsupials do have less diversity in their forelimbs than placental carnivores, our study has shown that's not true of the face.

"I think you can argue that marsupials have gone way beyond what placentals have done in terms of modifying their face and their dentition to be able to eat meat," she told BBC News.

"There was a marsupial sabretooth from South America, for example, that had ever-growing canines, and the roots of these teeth went up over the [eye socket]. There's nothing like that in placental carnivores. It's really very extreme."

The reasons for the loss of marsupial carnivores must therefore be more complex than some have recognised, the researchers argue. The group cites competition with placentals during the fusion of North and South America three million years ago, and more recent human hunting as both likely reasons for the creatures' decline.

Co-author Dr Stephen Wroe from the University of New South Wales said: "It seems likely that the diversity in skull shape among marsupial carnivores reflects a diversity in lifestyle that once was quite comparable to that of placentals.

"Our results reinforce my own suspicion that the lack of marsupial predators in the world today has more to do with bad luck than bad genes."

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