http://www.springerlink.com/content/365408287l2v655x/

Voigt et al. (2008): Feather-like development of Triassic diapsid skin appendages. Naturwissenschaften (in press)

With some interesting ideas (arguably).

Once again I’ve nicked a drawing from your blog (credited of course!). Hope it’s okay!

cheers,
Neil

Only a handful of small endothermic form lack integument of some sort, such as naked mole rats. Clearly, anyone that would benefit from insulation already has it.

If you look at the “transitional” or “protofeather” covering on dinosaurs like Dilong or Sinosauropteryx, it resembles nothing so much as the fur that nearly every mammal is making good use of today. That’s not to say that it was actually the same as hair, but it seems to have some similarity of form and probably offered similar benefits for warmth, display, etc.

As to why currently scaled critters aren’t modifying their scales to similar effect, it is hard to say, since we don’t exactly know why the first feathers (or hair, for that matter) evolved. It may be that today’s reptiles are outcompeted by already-outfitted birds and mammals in niches where such coverings would provide a distinct advantage. It may also be that certain groups have modified their scales for other purposes that make evolving fur or feathers (or something else) less likely in the absence of strong selective pressures–turtles, for instance, have set up their scales for specific, protective purposes, and lizards and snakes are characterized by overlapping scales that are quite different from those of dinosaurs or other reptiles. Crocodilians, whose scales are probably most like those of ancestral dinosaurs, specialize in an aquatic lifestyle where extraneous coverings would put them at a distinct disadvantage.