Furry like a squirrel but bearing a reptilelike jaw structure, with the middle ear bone fused to the lower jaw bone, little Megaconus mammaliaformis walked among (or ran in terror from) the feathered dinosaurs of the Jurassic period. It’s a new species among the primitive relatives of early mammals, previously only known from a jumble of fossil teeth. But an international team of scientists has now discovered a complete fossilized skeleton of this rat-sized creature  (artist’s conception above) in the Inner Mongolia region of China. The 165-million-year-old skeleton (inset) reveals a curious animal: Its fused, inflexible leg bones gave it an armadillolike gait, and it was neither a good climber nor a fast runner. But it was armed with clawlike spurs sticking out of its heels, which the scientists believe had poisonous glands like in a platypus. The omnivorous Megaconus foraged by night and is named for the large mounds on its premolars, which could grind tough plant material at a point when other mammal lineages were still largely insect-eaters. It “was already an old-timer,” evolutionarily speaking, says paleontologist Thomas Martin of the University of Bonn in Germany, a member of the research team, which presents its results online today in Nature. Megaconus was among the first to try out mammalian features before falling into extinction while modern mammals rose.