my bearded dragon died with no apparent cause. He was eating all threw the week and looked perfectly fine. The only thing different was his beard turned black a day or so prior to this. He lives in a screen cage with ground walnut shells mixed with playsand as a substrate. He climbed down from his basknig spot on night and lay on the ground. He was alive before breakfast when i checked up on him and then when i came back abou 30min later he was dead. Does anyone know why he might have died?