Moreover, what looked like the skin of the creature in the space jockey's chair turned out to be just a shell as per Prometheus. So if Dallas deduced that the creature was fossilized on the basis of it being hard, he would be wrong. He also never took his gloves off..
What's interesting about that, is that there is the implication that somewhere on the derelict, there may be a live, dormant, hibernating, alien.