You have 3choices
1, epoxy alone (gorilla glue is good too, dampen one side before aligning after letting it partially set for 5 mins, but only put the glue on either antler or skull, not both before jointing). it has the gap filling properties and will probably work perfectly well so long as you are particular with alignment and clamp with elastic or something similar. don't be tempted to touch it until at least 24 hours after.
2, threaded bar, brass or stainless not steel, it can stain . you have the choice in diameter's and can cut to exactly the correct length. slightly over bore the holes to a push fit. you obviously won't be able to screw it on because the mating surfaces do not allow you to screw it in as they won't be level. bear in mind that alignment will be very difficult this way and can throw the project.
3, wooden dowel. you can cut to pretty much any length and make any diameter you want. you can even make it with a kink in to get the alignment spot on depending where the heaviest part of the mating surfaces are if a straight line is not the best. the dowel will swell a little taking up any inaccuracies if you soak it first and use gorilla glue for the job. just remember to not overdo the glue or you'll have a mess on your hands. if you do, let it set and then carefully cut off the overspill with a craft knife.
Stick at it !
Ade