I keep antlers (deer, elk, moose), and carapaces in my tanks. The antlers are all in african tanks to help keep the water on the alkaline side. The AST carapace is epoxy sealed to preserve it and make it easy to wipe algae from it.
You can keep skulls in tanks but it's best to epoxy coat them for the reason stated above. Without the sealer, the only way to clean the algae completely from it would be to soak it in warm bleach. Then, you'll have to neutralize the bleach before putting it back in the tank.
BTW, antlers (dried and with the velvet removed) are also great for anyone keeping the larger varieties of snails. They'll grind small surface particles off to glean the calcium they need for shell development.