Pouched killers not poor cousins of carnivores

"We've looked at the deep-time perspective and shown that over evolutionary history marsupial carnivores have been every bit as varied in shape and habit as their placental counterparts," says Dr. Stephen Wroe, an author of a paper published in the journal Proceedings of the Royal Society B.

"Their ranks have included creatures as bizarre as the Argentinean pouched sabretooth, which sported monstrous, self-sharpening canine teeth that extended almost back into its braincase, and Australia’s own marsupial lion, which had teeth like bolt-cutters and the muscle power to match.”

"Throughout most of the Age of Mammals – a period extending back some 65 million years – warm-blooded furry predators have dominated most carnivorous niches on land. Today, these roles are mostly filled by placental meat-eaters, including such iconic species as the lion, tiger and grey wolf.”

Computer model of a marsupial lion skull used in the analysis.

"However, among the other major group of mammals, the pouched marsupials, predators are mostly small and few and far between. They display relatively little variation in shape or habit. Most are found only in Australia and the largest – weighing only around 7 kilograms - is the Tasmanian devil, which is now restricted to the Australian island of Tasmania and under serious threat."

The traditional explanation for this apparent lack of diversity has long been that the marsupial mode of development, where the young are born at a very early stage, had seriously constrained their ability to adapt to new habits and environments.

"But if we consider species back through the last 60 million years or so it is clear that this was not always the case. For most of this time the marsupial predator club contained a much wider range of species that included some real giants, and the marsupials dominated South America – and probably Antarctica - as well as Australia."

Working with Anjali Goswami of University College London, and Nick Milne of the University of Western Australia, Wroe looked at data collected from museums on five continents, including members of a wide range of living and extinct mammalian carnivore families. Applying an approach known as geometric morphometrics to map out and analyse whole skulls of living and fossil species, they showed that the variation in shape of marsupial carnivores was actually greater than that observed in placentals.

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Although superficially similar to the placental sabretoothed cats, like Smilodon fatalis, the extinct sabretoothed marsupial, Thylacosmilus atrox, shown here, is the only mammalian carnivore to have evolved ever-growing canines, making it one of the most formidable predators known in mammalian history. Image (c) UCL, Grant Museum of Zoology

“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,” says Dr. Wroe, an expert in mammalian carnivore evolution. “Our results reinforce my own conclusion that the lack of marsupial predators in the world today has more to do with bad luck than bad genes.”

More information: http://rspb.royals … 031.abstract

Provided by University of New South Wales