This is sort of a follow-up to "Synapsid skin" (I wouldn't mind some more feedback on that question, by the way). I have seen a few websites* on gorgonopsids stating that their skin was likely smooth, hairless, scaleless, and covered with glands and pores (This is based on a skin fossil from Estemmenosuchus). Is there really enough evidence to suggest that gorgonopsids didn't have hairy and/or scaly skin? If I'm not mistaken, gorgonopsids were more closely related to the cynodonts, some of which were most definitely hairy, than to Estemmenosuchus. Seeing as hair is probably derived from scales (unless there's some other hypothesis I'm not aware of), wouldn't a more realistic explanation be that the ancestor of Estemmenosuchus, gorgonopsids, and cynodonts had hair, which was secondarily lost in Estemmenosuchus (as opposed to a hairless, scaleless ancestor giving rise to two smooth-skinned groups and one hairy group)? If we go with this hypothesis, then there's nothing to keep us from drawing hairy gorgonopsids, is there?

*www.alexfreeman.co.uk/skull/Skull.html

We know so little about the skin of non-mammalian synapsids that what you've just said basically summarises it all. However, while we know that fur was present in early mammals (we have it from multituberculates, for example), its distribution in non-mammalian cynodonts remains uncertain: it was probably present in these animals, but we can't be sure. Given that basal synapsids (ophiacodonts) and, apparently, basal therapsids (dinocephalians) had a scale-less dorsal surface (meaning that such animals were naked-skinned on their dorsal surfaces at least), it has been inferred that hairs did not evolve from scales, but are instead novel structures that started off as whisker-like mechanoreceptors. It is these proto-hairs that may have been the 'ancestors' of hairs of insulatory function. I can't pretend to be particularly up to date on this subject however and would be interested to know if better data has come in from competing models.

For a recent-ish review (2003) you should read Paul Maderson's review in _Experimental Dermatology_: available free here... http://www.blackwell-synergy.com/doi/pd … 03.00069.x

PS - why is this question in the 'reptiles & amphibians' section? It shouldn't be.