This is a race I'm planning on putting in the next book of the "Chronicles of Xigmus" saga.

Scylles live in an area so hot that the land is constantly on fire, so they were forced to live underground (think Crematoria from Riddick). The air, heat, and magic of the area hyper-mutated the once humans into basically a weak version of what might be a Dire Human, if you're into D&D.

By Dire I mean that the mutated humans rapidly grew bone spurs all over their bodies (at least, where they have bones...I'm not sadistic.). These spurs quickly turned into a contest between competitive humans, resulting in a new social pattern that resembled a Native American tribe where the person with the most strength, smarts, speed, and, of course, spurs was the one in command.

As for the lack of skin, espescially on the crown of the head is a result of the mutation stopping halfway while forming a type of exoskeleton. Since then, Scylles have evolved slowly, if at all.