"The first known mammals lived about 220 million years ago. But we don’t know much about the mammals that existed from then until 65 million years ago, since we have very few intact skeletons from the Mesozoic era. Instead, scientists rely mainly on scattered teeth and jaw scraps to understand the earliest stages of mammalian evolution.
Now, Nature reports that paleontologists have unearthed a great find in Argentina: a pair of relatively intact skulls from a previously unknown mammal the size of a shrew. Estimated to be about 100 million years old, these skulls are beginning to fill in the giant gap in the mammalian fossil record.
The species has been named Cronopio dentiacutus, and is a member of the extinct superorder Dryolestoidea, the ancestor group to modern-day marsupials and placental mammals. Unlike other dryolestoids, however, the saber-toothed squirrel skulls are surprisingly specialized. They have very long, narrow snouts, well-developed musculature, and perhaps most interestingly, long dagger-like canines. "