Welcome to the forum.
I suspect your FIL may argue with you because he's been given to think that reading the WT magazine and the Awake and all the other publications gives one the equivalency of a college education.(Yes, this was published in the Watchtower magazine quite some years ago) Since he dotes on every pronouncement of the WT Society and their publications, he feels that YOU are the mistaken one, and that the flood explains the sharks teeth in one location, and that they just haven't found them in another location yet. He probably left that conversation feeling that you were pathetically misinformed, and that the WT Society is so far above your puny college experience. He is content to wait for the day when the shark's teeth are found in other locations, and can smugly show you how wrong you are.