Some species have chitin as Armour.
We Humans have interior Bones and Skin.
But what If an Alien species has developed an exterior Skin of Bone ? -- Anon Guest
There are very few examples of osteodermis extremis in cogniscent lifeforms. For Deathworlds, and the Deathworlders who dwell on them, it makes more evolutionary sense than -say- being small, mobile and squishy. Though not common, it's generally expected for Deathworlders of level four or above to at least have some osteoderms.
The Kraakinok were one such species with heavy osteoderm coverage. Having heard the stories of the Alliance's most impossible Deathworlders, they followed the usual rules of egocentricity and assumed that the Humans would, like them, be covered in bony plating.
Their first sight of a Human out of their livesuit was more than a little shocking. The Human Bob was small, apparently squishy, and not even slightly armoured with any kind of bony plating. Indeed, their exposed skin displayed several contusions and lacerations that had managed to reach their soft flesh despite the livesuit protecting them.