A small mammal older than T. rex with reptilian features and long poison spurs may be one of our most ancient relatives, scientists say.
Its fossil remains, unearthed in China, show that early "proto-mammals" existed 100 million years before the king of the dinosaurs, Tyrannosaurus rex, appeared on Earth.
The well-preserved skeleton found in Inner Mongolia still shows signs of the animal's fur.
By studying its rodent-like teeth, scientists established that the creature was able to chew tough plant material as well as feed on insects and worms.
Scientists believe Megaconus lived on the ground, moving a little like an armadillo, and was unable to evade predators by fleeing up trees.
The creature may not have been as defenceless as it looked, however. On each heel, it sported a long keratinous spur which was probably poisonous.
A similar weapon can be seen in the present-day duck-billed platypus, from Australia.
Although Megaconus had fur and mammalian teeth, it also had a number of reptilian features.
Its primitive middle ear was firmly attached to the jaw, reptile-style, and its anklebones and vertebral column were similar to those of previously known mammal-like reptiles.
Professor Zhe-Xi Luo, from the University of Chicago, who led the description of Megaconus in the journal Nature, said: "We finally have a glimpse of what may be the ancestral condition of all mammals, by looking at what is preserved in Megaconus. It allows us to piece together poorly understood details of the critical transition of modern mammals from pre-mammalian ancestors.
"We cannot say that Megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165 million years removed. These features are evidence of what our mammalian ancestor looked like during the Triassic-Jurassic transition.
"Megaconus shows that many adaptations found in modern mammals were already tried by our distant, extinct relatives. In a sense, the three big branches of modern mammals are all accidental survivors among many other mammaliaform lineages that perished in extinction."
Before the find, scientists had only been able to study a few teeth from Megaconus.
They were puzzled by pronounced cusps on the molar surfaces, now known to be adapted for crushing hard vegetation. The name Megaconus means "large cusp".
"Paleontologists have been wondering for over a hundred years what the animal that went with these teeth might have looked like," said co-author Professor Thomas Martin, from the University of Bonn in Germany.
