Researchers recently uncovered a new and remarkable animal in the fossil record in China: Rugosodon eurasiaticus, a chipmunk-like creature that was a member of the multituberculate family. The multitberculates, a sort of proto-rodent family, were eventually out-competed by the first rodents, and went extinct.
But Rugosodon eurasiaticus is remarkable for its longevity. It's been found in the fossil record with examples dated from both 170 million and 35 million years ago, making it the longest-lived (and, by some definitions, therefore the "most successful") mammal known to science.
There are lots of animals still around that are remarkably unchanged from millions of years ago. These are popularly called "living fossils," though that phrase is not usually used by scientists, and often give us insight as to the basal, or primitive forms and evolutionary history of other, more recently evolved animals. They also give us significant insight into the geologic history of the planet; animals survive when they can reproduce at a stable rate, simple as that. But these animals plateaued early, finding a way to breed, often in isolation (on islands, for example), at a stable rate, and remained successful without changing much, physically. We don't really understand why this happens to some animals and not others, but the facts are pretty clear: they were doing fine, so they pretty much just stayed the way they are.
And you might be surprised at a few of them. Check out the gallery to see seven of the most fascinating.