News from National Geographic this week: a fossil "pygmy" panda was discovered, a species that lived 2-3 million years ago. The species, named Ailuropoda microta, was only about half the size of modern pandas (Ailuropoda melanoleuca). Shown below is the skull of A. microta compared to A. melanoleuca
Interestingly, this early species already displayed adaptations for a diet consisting primarily of bamboo, including certain cranial/dental features and the infamous "false thumb" that aids pandas in stripping the leaves from bamboo. Pandas appear to have depended upon bamboo for millions of years now, I don't know if they have shown any evidence of co-evolution but it would be an interesting thing to look into.
I've always thought pandas were interesting, because they're an example of an animal that has a slightly oxymoronic taxonomic classification, being found in the order Carnivora despite their herbivorous lifestyle. Also, few people are aware that there are actually two extant subspecies of giant panda, the classic Ailuropoda melanoleuca melanoleuca that we all know and love, plus the less famous A. melanoleuca qinlingensis. This subspecies, known from the Qinling mountains, has brown fur and a slightly smaller skull.