PARIS, Jan 12 (AFP) - Two remarkable finds in a fossil treasure trove in northeastern China may smash perceptions about early mammals, according to a study published on Thursday in the British weekly journal Nature.
The conventional view is that in the long reign of the dinosaurs, mammals were small creatures no bigger than rats and weighed a kilo or two (a few pounds) at most.
They probably scurried around in the undergrowth in mortal fear of the dinosaurs, tucking into nocturnal insects for their food.
The mammals' career prospects changed dramatically when Earth was hit by an asteroid or comet about 65 million years ago, causing climate change that wiped out the dinosaurs forever.
With no competition from the dinosaurs, mammals inherited the planet, gradually becoming bigger and more diverse to exploit habitat niches and food opportunities.
But two newly-discovered mammal fossils may change this version forever.
They imply that some early mammals were much larger and more diverse than previously thought and were tough enough to compete with the dinosaurs.
The finds, dating from at least 128 million years ago, were found in layers of sediment in Liaoning province, where dozens of preserved dinosaur bones and early birds have been found in recent years.
The first fossil, the size of a large cat and dubbed Repenomamus robustus, was found to have the remains of a young parrot-faced dinosaur, a psittacosaur, in its stomach. This is the first direct evidence that early mammals ate small vertebrates.
The other find is arguably even more impressive -- a relative of R. robustus called Repenomamus gigantus: a dog-sized mammal more than a metre (3.25 feet) long and probably weighing 14 to 15 kilos (28-32 pounds).
"A full-grown Repenomamus gigantus ... could hold its own against small dinosaurs," says the American Museum of Natural History, whose curator of palaeontology, Meng Jin, led the Chinese-US expedition.
The creatures had large incisors, for grabbing and ripping prey, but their molars were flat and blunt, which suggests that the food was swallowed without much chewing. That implies that chewing among mammals evolved later.
Neither creature looks like any mammal alive today.
Indeed, the term "Repenomamus" ("reptile mammal") was coined to express the odd aspects of these creatures, which had large, sharp and pointy teeth and reptile-like limbs that stuck out from their bodies at an angle.
However, they were far more limber than any reptile, thanks to more flexible joints.
A Repenomamus could waddle, although not run, and could stand on its hindlimbs and walk effectively enough to stalk small prey.
"Mesozoic mammals had a much greater range of body sizes than previously known," the authors say. "(They) occupied diverse niches and ... some large mammals probably competed with dinosaurs for food and territory."
01/12/2005 16:58 GMT - AFP