HOLY CRAP!
I never thought I'd live to see this. Hell, I didn't think it was possible, but...OH MY GOSH!
In today's issue of the journal Nature, Zheng et al. reported on a heterodontosaur (basal ornithischian)...with feathers. Now, feathered dinosaurs are a dime a dozen these days. Raptors, therizinosaurs, troodontids, basal tyrannosaurs, oviraptorosaurs, and compsognathids all had feathers. But they're all theropods (and saurischians).
Dinosauria is divided into two branches: the Saurischian (sauropods and theropods) and Ornithischia (everyone else). Heterodontosaurs are dog-sized bipedal ornithischians who have canine teeth and were probably omnivorous. Anyway, the fact that this new animal...Tianyulong, has feathers which are likely homologous to those of theropods means one thing:
ALL DINOSAURS HAD FEATHERS.
Look at it this way: Humans have hair, and chimps have hair. That means the common ancestor of chimps and humans had hair. Hair didn't pop up twice (convergently). The simpler answer is to say that hair was present in the common ancestor, and we both inherited it. So it is with dinosaurs and feathers!
Now, it's obvious that many dinosaurs did NOT have feathers. Skin impressions from sauropods, ceratopsians, and ornithopods all show that these animals had scaley skin, which means one of two things:
1) Feathers were ontologically important. That is, babies and juveniles had feathers (probably to keep warm), but those feathers were shed when a certain size was attained to prevent overheating.
2) Feathers were secondarily lost (like hair is in elephants and rhinos) in many dinosaur groups. Right now, both scenarios are equally plausible.
What's even more interesting is that we've known for ages that pterosaurs had their own kind of "fuzz" that covered their entire bodies. This "fuzz" is usually thought to be different from feathers, although it's constantly compared to baby chicken fuzz (which it probably resembled). If the fuzz of pterosaurs really is a kind of feather, that means ALL OF THE ORNITHODIRA, which includes a sizeable number of non-dinosaurian, non-pterosaurian groups, had feathers. WHOA.
So yeah. If you want to see the fossil, Google for a blog called "The Dragon's Tales." There's a really wonderful photo up there, and the feathers are clearly defined on the slab. This is really unbelievable stuff, and its implications cannot be understated.