So... feathered dinosaurs have been coming out of China for a while now, due in part to the preservational bias and the extremely fine ash-based rock. However, to date they had always been from only one (albeit a very diverse and successful) branch of the therapod group, which is part of the saurischian root amongst dinosaurs. I've said for a while now that all we need to do to strongly suggest (or heck, even prove) that feathers and warm-bloodedness were a pre-dinosaurian (to say nothing of pre-avian) feature would be a feathered dinosaur on the ornithischian side of the fence. That would make it a dinosaur as far away as possible on the dino family tree from the therapods previously discovered with feathers, and one that hadn't shared a common ancestor with therapods since prior to the saurischian/ornithischian split. Which would mean, essentially, the first dinosaur ever, or there-abouts.
The reason this works out that way is because feathers are seen as a synapomorphy, that is, a complex evolutionary development that two species (or groups of species) have that is unlikely to have been convergently evolved more than once, and one that therefore must have been inherited from a common ancestor between the two.
I've long suspected that some kind of ornithischian dinosaur with feathers would turn up. My money was on a "hypsilophodontid", but Tianyulong is even better because it's a heterodontosaur; an even more primitive creature, and one, therefore, that is even more likely to pound away at opposition to the idea of feathered dinosaurs across the board. Suggestive facts that led up to this discovery (and made me believe it was eventually inevitable) were 1) the frill-tailed Psittacosaurus which, sadly, were not definitively proto-feathers, but were highly suggestive of them, 2) using parsimony to link dinosaurs to pterosaurs, a closely related sister group amongst the ornithodirans that also had an obvious warm-blooded nature and a hair or down-like integuement of some kind, 3) lots of evidence of varied nature to suggest that warm-bloodedness came about amongst at least some of the archosaurs quite a while before the dinosaurs actually came on the scene, 4) dinosaurs such as Leaellynasaura who lived close to the poles were too large to dig a burrow and hibernate, but too small to migrate or manage the winters in Dinosaur Cove, where the fossils were found. This suggests warm-bloodedness and some kind of adaptation to deal with the cold (i.e., feathers or down.)
Anyway, this isn't really surprising to me, or to some of the other more bold folks in the dino-biz (Greg Paul's been illustrating feathered ornithischians and proto-dinosaurs for over twenty years in anticipation of this discovery, for instance) but it'll turn the more conservative element amongst dino-guys on their heads. However, one point: if all dinosaurs had a feathered ancestry, why is it that most of the skin impressions we have found are for naked, scaly skin? I think that's an easy one to answer; large-bodied mammals have extremely short hair or no hair as well, unless they live in cold climates where they need it to stay warm. Look at elephants, rhinos, or even horses, zebras and lions. Male lions have a big hairy mane, but that is primarily a display feature and one for protection in fights with other male lions over breeding and territorial rights. Otherwise, they are short-haired animals. Hair (or down, or feathers) is actually maladaptive in warm climates for large-bodied animals. It helps retain heat and in warm climates, large animals are liable to overheat... if they're warm-blooded especially.
I suspect, for future illustrators, that feathers should be put only on smallish dinosaurs and proto-dinosaurs. Guys like the little Tianyulong which was about house-cat sized. Also; it's not unreasonable to assume that hatchlings and other younger dinosaurs may have sported a downy coat that they shed as they matured.
Anyway, here's the illustration of Tianyulong that's been floating around the internet.
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