Pre-existing clause for dental work?

Have I really been out of the loop? My DH, who I married this May, has great dental insurance and it's time for me to take advantage of it and get two crowns done. When I called our family dentist for an appointment to get things started I was told about a pre-existing clause with dental insurance. Since my two molars didn't have any fractures they will be covered, but otherwise dental insurance would not cover the crowns if they did. Is this the norm these days? Do insurance companies ask for a copy of your dental x-rays to decide if they are going to pay the claim? Call me the clueless one here. Thanks in advance, Carol