During the 65 million years following the extinction of the dinosaurs, the success story of the mammals has been more than a little imbalanced. Eutherians (placental mammals like dogs, horses, you and I) had an evolutionary rager, exploding in diversity and filling vacant ecological roles across the Northern Hemisphere. Metatherians (including marsupials like kangaroos and koalas) only got a modest foothold in the smaller, southern continents of South America and Australia. For tens of millions of years, everything north of the equator seemed to be a land of total placental mammal dominance -- but the fossilised remains of a cat-sized metatherian carnivore in Turkey are now challenging that story.
A reconstruction of the Anatoliadelphys maasae. Image: Peter Schouten
The fossilised skull and skeleton were unearthed and collected from the Uzunçarşıdere Formation in Turkey's Central Anatolia region, and described in a new paper in the journal PLOS ONE by researchers from the University of Washington and the UK's University of Salford. The remains represented a new species -- Anatoliadelphys maasae -- which, due to its status as a metatherian mammal, certain features of its skeleton, and its age (about 43-44 million years old), was quickly recognised as an incredible find.
Anatoliadelphys was unlike any of its northern contemporaries, which are tiny, bug-eating critters. For one, it was big -- weighting roughly as much as a honeydew melon, Anatoliadelphys was an order of magnitude more massive than any other northern metatherian mammal. It was also carnivorous, armed with burly jaws lined with rugged premolars, allowing for a powerful bite that could have splintered bone or crunched the shells of armoured invertebrates. Based on analysis of Anatoliadelphys's skeleton, the cat-sized animal was probably an excellent climber and grasper, and would have been at home in the trees. In life, Anatoliadelphys would have resembled something akin to one of Australia's quolls crossed with a possum. However, instead of the lovely, wet-eyed pile of fluff staring out at you from a Qantas brochure, Anatoliadelphys would have been more like a mini marsupial tree-hyena, hungrily licking its bone-crushing chops while stalking prey in the canopy.
The discovery of Anatoliadelphys rewrites a portion of the palaeontological story of metatherians, showing that they occupied a position in the food web that was thought to be invariably the domain of placental mammals in that half of the world. This reality is typically explained by a possible competitive advantage in placental mammals. Indeed, the metatherian-rich southern continents were conveniently isolated from the roiling cauldron of northern, placental competition, and when these two guilds finally met, it was the metatherians that lost out.
Consider what happened in the aftermath of the "Great American Interchange" -- the movement of animals separately unique to North and South America three million years ago across the then-newly-formed Isthmus of Panama. Before this, South America harbored fearsome metatherian carnivores known as "borhyaenoids", which included wolf-like animals and Thylacosmilus, which was basically an off-brand sabretooth cat. But soon after North American placental carnivores like dogs and cats migrated across the isthmus, these metatherians went extinct. Notably, Australia too has suffered extinctions of endemic marsupials (like giant wombats and immense, flat-faced kangaroos) suspiciously close to the timing of first contact with placental invaders.
Suggestions as to why metatherians always seem to fall apart in these encounters include developmental constraints in their shoulders and skull, making them less evolutionarily flexible -- with these limitations, marsupials and their relatives might be less able to adapt to new challenges. Unburdened by these anatomical constraints, placental mammals could utilise a larger evolutionary toolbox to acquire food, flee predators, and exploit their environment.
But Anatoliadelphys probably wasn't a battle-hardened exception, beating the odds and rejecting the notion of metatherian competitive "inferiority". The secret to Anatoliadelphys's existence may come from geological and palaeontological evidence that 40 million years-ago, its Anatolian home was an island. This island would have acted as a refuge, allowing Anatoliadelphys to evolve to fill a predator niche with zero competition from placental mammals on the mainland. The fossil find is an amazing snapshot of the past, illuminating what was likely an ephemeral hold-out, a protected relic reigning behind enemy lines.