the early ufc was used to help promote gracie jujitsu, it was to show America that the traditional martial arts and our style of fighting was nothing compared to gracie jujitsu, and as ken shamrock said the us fighters did not know submissions.
top rear mount(never heard of it, but guessing standing with him applying rear naked?)= i would jump backward landing on top of them. rolling to the side killing on leg with my body weight. pulling their top leg off me, and walk out turning into north south or side control. my other option would be to dive forward tucking into a forward break fall hoping to catch his head on the ground.
bottom rear mount(laying down rear naked) roll to one side, kill the bottom leg with my body weight, pull top leg off and walk out. other option would be is bait them in for the choke(with out the leverage of the feet the choke is near imposable to complete, that is why you'll heard "get your hooks in")by splitting their feet not letting hooks plant, by using slapping motion, its causes 98% of the people that try to land the choke still to cross ankles. when this happens i will step over their top foot with my opposite leg catching their foot with my inner thigh, and apply pressure, causing their feet to crush.
mount= opposite hand to their opposite hip, rolling to the side of their hip and shrimp out to and guard(butterfly, half, closed, etc.) or control their left hand with my left hand, reach deep across their back with my right and umpa over my right shoulder and into their guard.
knee on belly= roll away from them to my side, take my top arm and press against their knee, and roll back into them, catch their leg and drive for a single.
north south= swing legs from hip down pendulum action, giving space for a half guard. or from belly down, turtle to knees, control one of his wrist, same side post out and step through under his arm using your head to push on his pit to shoulder sitting out, giving you his back upon release of his wrist
side control= outside forearm to his throat, inside palm of hand to his hip, roll toward him placing top knee inside on his hip, shift body and slide bottom leg under him to half, butterfly,feet on hips, any type of guard. or far side arm slide under his far side arm pit, sole of outside foot planted on ground, bridge while shooting hand in pit to the sky, and step backward with close foot under far side foot and bridge, bringing you to turtle, grab his pant legs at knees for control, and shoot for a single or double.
turtle=roll to your back, suck a leg in in a headlock motion, with your legs bend at knees cris-crossing ankles at his opposing knee, this is the X-guard, good to keep him off balance. or grab a leg rolling forward keeping him leg between you two legs, landing him in a knee bar..
against someone trying to ground and pound=you must break their posture, using high guard or rubber guard. i went to a dave camarillo seminar and he taught me triangle hold that is very good against someone attempting to posture up on you.